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Keywords:

  • Indian plate;
  • neodymium isotopes;
  • paleoceanography;
  • tectonic

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

[1] We have analyzed four sediment cores from the Southern Indian Ocean (ODP sites 757, 758, 1135 and 762) with high carbonate content, in order to reconstruct the neodymium isotopic composition (εNd) of ancient intermediate South Indian seawater from Late Cretaceous (90 Ma) to Early Eocene (40 Ma). The εNdvariations are highly consistent and exhibit reproducible patterns over a very large geographic area, confirming the seawater origin of the signal. Combining geochemical constraints with paleogeographic reconstructions, we highlight the respective roles of (1) large-scale tectonic events, (2) continental weathering from surrounding Precambrian terrains (90–65 Ma), (3) oceanic circulation changes (50–40 Ma) and, possibly, (4) local volcanism of the ultra-fast spreading South East Indian Ridge (SEIR) (60–50 Ma) on the Nd isotopic composition of South Indian seawater. Between 60 Ma and 50 Ma, the regional Nd isotopic variations closely mimic changes in SEIR spreading rate. We suggest that the Nd isotopic composition of seawater could be influenced by Nd of volcanic origin in the vicinity of ultra-fast spreading ridges (>13 cm/yr). The India-Asia collision closed the Equatorial Seaway between Asia and India and drastically changed oceanic circulation patterns in the Indian Ocean: warm and more radiogenic Pacific equatorial seawater was diverted to the South by the East Indian coast. A stronger mixing of this Pacific seawater with South Indian seawater would explain the rapid shift ofεNd from 50 Ma (−11) to 40 Ma (−8).

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

[2] The 50 Ma time period between 90 Ma and 40 Ma has been crucial for the history of the Indian Ocean. Some of the largest tectonic changes took place in this region during this period, with significant consequences for Indian Ocean paleoceanography. In the South, the last step of Gondwana super-continent breakup occurred around 90 Ma, with the final separation of modern continental blocks; and the Indian plate started to drift northward. This drift accelerated drastically from 60 to 50 Ma then slowed down with the collision of India with Asia at ∼50 Ma [Murphy and Kennett, 1986; Patriat and Achache, 1984; Rowley, 1996]. As India moved to the North, from 90 to 50 Ma, the contours and boundaries of the Indian Ocean were continuously modified. After 40 Ma, the continental configuration continued to change in the region with the separation of the Australian block from Antarctica. The northward drift of Australia induced the opening of the Tasman Gate at 37 Ma [Stickley et al., 2004]. As a consequence, the global oceanic pattern gradually changed from a wide open equatorial Indo-Pacific Ocean [Bush, 1997; Pucéat et al., 2005; Stille et al., 1996] to a southern Indo-Pacific circulation with the establishment of the Antarctic Circum Current (ACC) during Late Oligocene [Kennett, 1977; Pfuhl and McCave, 2005; Scher and Martin, 2004].

[3] We use Nd isotopes as a geochemical tracer in order to improve our knowledge of the oceanic changes that occurred in the South Indian Ocean over this period. We performed analyses of neodymium (Nd) isotopes on four sediment cores that were located in the southern Indian Ocean at the time. The Nd isotopic composition of seawater (143Nd/144 Nd, expressed as εNd) is not uniform in modern oceans [Albarède and Goldstein, 1992; Frank, 2002; Goldstein and Hemming, 2003; Piepgras and Wasserburg, 1980] because: (1) Nd has a shorter residence time (500–1500 years) [Bertram and Elderfield, 1993; Tachikawa et al., 1999] than the global oceanic mixing time [Broecker and Peng, 1982] (2) the distribution of oceanic Nd isotopes is linked to the variable nature of its sources and their isotopic compositions and to ways in which Nd is transported. Nd in the ocean is a product of continental and volcanic erosion and is transported to the ocean by surface rivers, groundwater, or winds. In the ocean, Nd is mixed both vertically and horizontally by water currents. Thus the Nd isotopic ratio can be used as a paleoceanographic tracer [Bertram and Elderfield, 1993; Elderfield et al., 1990; Frank, 2002; Goldstein and Hemming, 2003; O'Nions et al., 1978; Piepgras and Wasserburg, 1980].

[4] The most remarkable result in our study is the existence of a coherent Nd isotopic pattern of intermediate seawater during this 90 to 40 Ma period in an area of approximately 3000 km in diameter in the Southern Ocean (from 30°S to 60°S and from 60°E to 120°E; see Figure 1). This pattern is interpreted in connection with tectonic and oceanic changes occurring in the Indian Ocean at the time.

image

Figure 1. Paleo-locations of sites cited. The paleogeographical maps show the geographical positions of continents from 90 Ma to modern time and the tectonic plates limits (ridges and subduction zones) in the region [O'Neill et al., 2003; Ricou, 1995]. The successive locations at (a) 80 Ma, (b) 60 Ma, (c) 50 Ma, (d) 45 Ma, and (e) 0 Ma are shown in a context of drifting plates: Sites ODP 1135, 762, 758, 757 (this study), Site DSDP 213 [Thomas et al., 2003] and Site 766 [Robinson et al., 2010]. No sediment was deposited at site 758 from 50 to 20 Ma, therefore Site 758 is not shown in Figure 1d.

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2. Materials and Methods

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

2.1. Samples

[5] We selected four long sedimentary cores that sample the 90–40 Ma period in our target area in the Southern Indian Ocean (Figure 1). The sediment was deposited at all sites between 2000 and 1300 m of water depth in intermediate water masses. ODP site 1135 (59°54′S, 84°15′E), located South of the Kerguelen Plateau, and ODP site 762 (19°53′S, 112°15′E), drilled on the Western Australian Margin, were both located on the Antarctica plate before 40 Ma. In contrast, the two ODP sites 758 (5°23′N, 90°21′E) and 757 (17°01′S, 88°10′E), located on the Kerguelen hot spot track (i.e., Ninety East Ridge), were on the Indian plate (see Figure 1 and section 2.2 for more details).

[6] Sedimentation at ODP sites 1135 (water depth = 1566 m) and 762 (water depth = 1360 m) was continuous respectively from 90 Ma to 40 Ma [Coffin et al., 2000] and from 80 to 35 Ma [Haq et al., 1990]. At ODP Site 758, the sedimentation started around 80 Ma [Weissel et al., 1991], but a 30 Myr sedimentary gap occurs after 50 Ma. Although this site was cored 2900 m below sea level (mbsl), Peirce et al. [1989a]suggest that it was located at the average and constant water depth of the Ninety East Ridge (between 2000 to 1500 m) in Late Mesozoic-Middle Cenozoic times. According to these authors, its present-day depth results from bending of the northern Indian plate caused by its subduction under Asia during the last 10 Myr. Sedimentation at ODP site 757, which is presently located at 1661 m below sea level, began only at ∼50 Ma [Peirce et al., 1989b] and was continuous since. Taken together, ODP sites 757 and 758 can be used to reconstruct a composite Nd record from 80 to 40 Ma on the Indian plate side of the ocean.

[7] The four ODP cores have homogenous mineralogical contents with high pelagic carbonate content, mainly composed of foraminifera and coccolithophoridae [Coffin et al., 2000; Haq et al., 1990; Peirce et al., 1989a]. Precise dating was obtained by biostratigraphy, with an error of +/−0.5 Ma using the biostratigraphic chart of Berggren et al. [1995a, 1995b]. Sedimentation rates range from 1 to 10 m/Myr.

[8] Forty-seven 2 cm thick samples, each spanning a duration of 2 to 8 Kyr, were chosen on the basis of shipboard core photos [Coffin et al., 2000; Haq et al., 1990; Peirce et al., 1989a] in order to avoid disturbed sediment. No ash layers were identified during the interval of time we studied. At ODP site 1135, twenty samples were collected from 39.2 Ma (9.72 mbsf) to 89.4 Ma (498.23 mbsf), with a carbonate content ranging from 92 to 97%wt after 65 Ma (except for the first sample at 72%wt) and from 77 to 95%wt before 65 Ma [Coffin et al., 2000] (see auxiliary material). For ODP site 757, nine samples were selected from 38.6 Ma (137.9 mbsf) to 51.2 Ma (198.5 mbsf) with a carbonate content from 93 to 97%wt [Peirce et al., 1989a] (see auxiliary material). For ODP site 758, eleven samples were analyzed between 49.8 Ma (257.98 mbsf) and 79.3 Ma (309.37 mbsf) with a carbonate content higher than 85%wt after the K/T boundary and from 77 to 81%wt before [Peirce et al., 1989b] (see auxiliary material). For ODP site 762, only seven samples were selected between 209.8 mbsf (35 Ma) and 700.4 mbsf (77 Ma). These samples have a carbonate content that ranges from 72 to 93%wt [Haq et al., 1990]. From 63 Ma to 65 Ma a low sedimentation rate and a small number of biochronological markers prevented us from performing higher resolution sampling. At site 758 as well as at site 1135, the sedimentation rate is very low at around 1 to 2 m/Ma [Coffin et al., 2000; Peirce et al., 1989a] over a few Ma after the K/T boundary. In addition, the exact K/T boundary is poorly constrained at site ODP 758. It is presumed to lie between the core catcher of core 121-758A-31X and the first 25 cm of core 121-758A-32X-1 and thus has probably not been sampled. At ODP site 1135, the boundary appeared to be marked by a disconformity in section 183-1135A-28R-2 between 124 and 125 cm.

2.2. Plate Tectonic Setting and Evolution of the Paleo-Positions of the Sites

[9] In order to precisely evaluate Indian plate motion and determine the successive paleo-positions of all four sites, we show paleogeographic reconstructions at 80 Ma, 65 Ma, 50 Ma and 40 Ma (Figure 1). These were performed using the PaleoMac application [Cogné, 2003] based on ocean floor magnetic anomaly patterns using rotation parameters from Besse and Courtillot [2002], which are based on Müller et al. [1997]isochrons corrected using the GOS04 time-scale [Gradstein et al., 2004]. The rotation parameters of the relative motion of adjacent lithospheric plates were calculated by Müller et al. [1997]. The reconstruction of the global synthetic apparent geomagnetic polar wander path for each continental block based on paleomagnetic data and Müller et al.'s rotation parameters [Besse and Courtillot, 2002] provided accurate paleopositions of the tectonic plates both in longitude and latitude. The apparent polar wander paths of India was obtained with great precision by Besse and Courtillot [2002]. Müller et al. [2008b] introduced isochrons in marginal seas that do not affect the relative movement of the continental blocks considered in our study. The present calculations are in very good agreement with previous results deduced from ocean floor isochron spacing [Cande et al., 2010; Cogné and Humler, 2004, 2006; Müller et al., 2008a, 1997; Patriat and Achache, 1984].

[10] In the upper Cretaceous, as the Gondwana Supercontinent was finally breaking up, the continental landmasses of South America, Africa, India and Australia/Antarctica split into separate plates (Figure 1a). Australia remained attached to Antarctica long after 90 Ma [Fisher et al., 1971; Hall, 2002; McKenzie and Sclater, 1971] and remained almost stationary at very low latitudes over the 90–40 Ma period. In contrast, the Indian plate drifted northward over very large distances. This drift took place in three main steps: (1) first, the separation of the Indian plate from Gondwana, starting ∼120 Ma ago and followed by moderate ∼6–8 cm/yr northward drift, circa ∼2000 km, of India from 90 to 60 Ma (Figures 1a and 1b); (2) The second step corresponds to one of the most important changes in Indian plate dynamics. At 60 Ma, activity on the South East Indian Ridge (SEIR) increased considerably, resulting in rapid northward drift of India at more than 13 cm/yr [Cande et al., 2010; Cogné and Humler, 2006; Patriat and Achache, 1984]. From 60 to 50 Ma, the SEIR was an ultra-fast spreading ridge, at least as rapid as the East Pacific Rise now (Figure 1c); (3) The third episode started at ∼50 Ma when SEIR spreading rate fell back to values similar to upper Cretaceous ones at 4–7 cm/yr. Following the initial interpretation of Patriat and Achache [1984], this sharp decrease of plate velocities was a consequence of the collision of India with Eurasia.

[11] ODP sites 1135 and 762, which were then on the Antarctica-Australia plate, remained almost at the same latitude and longitude throughout the entire period under study. ODP site 762 moved with the Australian continent later, when this continental block started its northward drift between 33 and 35 Ma [Pfuhl and McCave, 2005; Stickley et al., 2004] and became separate from Antarctica. Because formation of the Ninety East Ridge accompanied northward Indian plate motion, ODP cores 758 and 757 successively occupied different positions in the Indian Ocean. From 90 to 60 Ma, ODP 758 remained in the target area in the Southern Indian Ocean (highlighted in blue in Figure 1). But at 50 Ma the older ODP Site 758 was located at 18°S, 82°E closer to the Equator, more than 4000 km away from ODP site 1135 (Figure 1c). In contrast, the younger ODP site 757 (paleolocation at 50 Ma: −40°S, 77°E) was in a much more southern position. Consequently, to evaluate the εNd composition of South Indian Ocean seawater we analyzed Nd isotopic composition of ancient seawater in ODP Sites 1135 and 762 and in Site 758 from 80 Ma to 50 Ma and, from 50 to 40 Ma, in the younger and southern ODP Site 757.

[12] In order to describe the variations of plate velocities through time, we used the PaleoMac software [Cogné, 2003] and computed the time evolution of distances between pairs of sites pertaining to Antarctic plate (ODP 1135) and Indian plate (ODP 757, 758) and a reference point on the India continental block (presently at 22°N, 71°E). In Figure 2b, the time derivatives of these increasing distances (i.e., velocities) are presented as a function of time. Because the Indian plate rotated slightly as it moved northward, the movement of three distinct points on the Indian plate relatively to ODP 1135 is more complex than a simple translation. This explains the differences in the “velocities” calculated for the three sites. Figure 2 emphasizes the maximum in SEIR activity between 60 and 50 Ma.

image

Figure 2. (a) Nd isotopic composition of Southern Indian Ocean seawater from 90 Ma to 40 Ma. The signal of ancient seawater (expressed as εNd(0)) was measured on sediment from the four ODP cores located at paleo-depths between 2000 to 1500 mbsf (ODP 1135 in red diamonds, ODP 758 in blue circles, ODP 757 in green circles and ODP 762 in orange squares). The similarity of records from sites distant by more than 3000 km shows thatεNdvaries in a consistent way at an ocean-wide scale. The age of India-Asia collision was determined fromPatriat and Achache [1984] and Rowley [1996]. The age of Deccan Traps eruption is based on studies from Courtillot et al. [1988], Chenet et al. [2007] and Duncan and Pyle [1988]. (b) Kinematics of three cores. We computed distances between ODP site 1135, on the Antarctic plate, and a reference point on the India continental block (presently at 22°N, 71°E), together with ODP sites 758 and 757 on the Indian plate using the PaleoMac software [Cogné, 2003]. The time derivatives of these increasing distances (i.e., velocities) are presented as a function of time. In both figures, the yellow zone underlines the maximum of Indian ridge activity. The ages 80, 60, 50 and 45 Ma underlined by squares corresponds to the ages of the paleogeographical maps (Figures 1a, 1b, 1c and 1d).

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2.3. Chemical Method

[13] Samples were dried and crushed with agate mortar and pestle to avoid neodymium contamination. For each sample, around 400 mg of powder were weighted in a Teflon beaker. Since the samples exceeded 70 wt% carbonate content, we extracted ancient seawater Nd using the acetic acid chemical leaching technique described in Gourlan et al. [2008, 2010]. Acetic acid (1.6 N, 8 mL) was used in excess, thus carbonates were completely dissolved. The residues was dried at 40°C during 48H and weighted.

[14] Neodymium was separated using two ion chromatography columns [Caro et al., 2006]. The first extraction step uses Eichrom TRU-Spec™ resin (in a HNO32N solution) in order to extract REE from other chemical elements. The second step consists in using Eichrom Ln-Spec™ resin (in a HCl 0.25N solution) in order to separate Nd from other REE. The total procedure blanks were always smaller than 2 pg of Nd and thus negligible compared to the Nd content of acetic acid extraction (1μg of Nd on average).

[15] Nd isotopic ratios were measured in the laboratory at IPGP using a Neptune MC-ICP-MS (Thermo Scientific). Average external reproducibility of measurements was monitored by multiple analyses of NIST standard 3135A (143Nd/144Nd of 0.511411; 2σ = 7.10−6; n = 206). Mean 143Nd/144Nd is within the analytical precision of 0.511418 ± 6.10−6, as monitored by TIMS (NIST 3135A was anchored to AMES by [Caro et al., 2006]). Reproducibility expressed in εNd notation is ±0.2 (2σ). Final results are expressed as: εNd = [(143Nd/144Ndsample/143Nd/144NdCHUR) − 1].10000 with a 143Nd/144NdCHUR value of 0.512638.

[16] The REE concentrations were measured on Thermo-OPtec Series X7 ICP-MS at high resolution. The samples were diluted with HNO3 0.5 N by a factor of 2 and In (1 ppb) was added as internal standard to correct for instrumental drift. External calibration was monitored using the SLRS4 standard and multiple elements solutions at various concentrations (100 ppt, 1 ppb, 10 ppb). Reported elemental concentrations were normalized to the initial sample weight. No blank correction was needed.

[17] To confirm the seawater origin of the εNd signal extracted by the acetic acid chemical leach, we measured the Rare Earth Elements (REE) concentrations in the acetic acid leach (Table 1 and Figure 3a). They all show a characteristic seawater pattern with a typical Ce anomaly and enrichment in heavy REE [Elderfield and Greaves, 1982]. These results are compared in Figure 3b with other REE patterns measured on various leaches by Martin et al. [2010]. In Figure 4, Nd isotopic results obtained with acetic acid are compared with measurements (results quoted with “**” in Table 2) performed on the same sample using the Hydroxylamine Hydrochloride (HH) leaching method described by Bayon et al. [2002]. These confirm that the two techniques provide the same εNd values within statistical error for samples with carbonate content higher than 70%wt (Figure 4) and allow us to extract a seawater signal (Figure 3).

Table 1. REE Values in ppm for Acetic Acid Extractions From Site 1135a
SamplesLaCePrNdSmEuGdTbDyHoErTmYbLu
  • a

    All samples have a high carbonate content. All values were normalized to the initial sample weight.

Q005A43.06114.105.7722.464.121.014.640.835.631.254.290.674.510.68
Q006B45.43316.296.5325.114.411.236.040.915.941.625.380.906.171.01
Q011A48.7317.816.0324.583.931.045.730.855.571.534.960.775.390.90
Q014A49.4115.366.0824.214.441.226.180.876.341.495.210.765.510.90
Q016A16.885.031.948.001.430.361.840.271.770.451.640.231.730.26
Q018A52.2614.995.9823.973.941.055.620.855.781.504.450.774.730.70
Q021A37.6311.204.4418.553.330.804.130.604.251.063.590.543.620.56
Q024A75.5725.168.4836.066.001.467.971.299.082.157.351.218.111.40
Q027A63.0721.868.1733.785.561.497.561.178.051.956.691.047.211.16
Q029A33.6412.303.9116.272.610.683.810.623.890.913.120.463.540.64
Q032B42.2211.746.2025.344.921.045.630.824.801.244.270.654.440.72
Q033A71.6120.0310.8341.567.311.778.771.378.291.946.390.986.781.06
Q034B109.9526.4713.2554.799.212.2212.421.9013.083.3111.611.7211.681.89
Q035A81.9919.909.8240.326.941.689.141.3210.202.448.471.379.141.51
Q036A109.7227.1313.8656.409.682.3813.202.0813.333.3511.451.7412.071.89
Q039A126.4231.4015.6663.4011.002.4013.242.0213.893.3211.531.7712.191.92
Q042A38.6413.074.8518.513.060.813.880.633.981.023.500.604.490.74
Q047B103.8926.6411.6644.197.571.769.011.379.962.277.731.289.271.38
Q049B68.0230.359.1734.525.861.556.301.036.961.495.250.846.090.97
image

Figure 3. REE diagrams for acetic acid extraction ODP 1135 samples. (a) PAAS normalized REE patterns of ODP 1135 samples. (b) PAAS normalized HREE/LREE (Tm + Yb + Lu)/(La + Pr + Nd) versus MREE/MREE* (Gd + Tb + Dy)/((HREE + LREE)/2) for ODP 1135 samples. Our acetic leaches performed on our samples are compared with different chemical leaches obtained by Martin et al. [2010].

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image

Figure 4. Comparison of εNd signal provided by two different chemical techniques on the same sample. The εNdA-A (respectivelyεNdH-H) values were obtained with the Acetic Acid (resp. Hydroxylamine Hydrochloride) leaching technique described byGourlan et al. [2008] and Bayon et al. [2002]. Both methods give identical values within statistical uncertainty.

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Table 2. The εNd(0) Data for Carbonated Fraction of Samples From ODP Leg 183 Site 1135, ODP Leg 121 Sites 758 and 757, and ODP Leg 122 Site 762a
LabSiteHCorTScTop (cm)Bot (cm)Depth (mbsf)Age (Ma)εNd(0) ± 2σ143Nd/144Nd ± 2σ (106)
  • a

    H, Cor, T, Sc, Top, Bot refer to ODP sample characterization: Hole, Type, Core, Section, Bottom; *, analytical duplicates; **, chemical duplicates shown in Figure 3.

Leg 183, ODP Site 1135 (59°54′S, 84°15′E)
Q001A1135A2R121239.7239.19−8.54 ± 0.080.512200 ± 4
Q005A1135A5R1656737.9643.78−8.58 ± 0.100.512198 ± 5
Q005A**         −8.58 ± 0.080.512199 ± 4
Q006A1135A5R6979945.7844.78−8.68 ± 0.120.512193 ± 6
Q006A**         −8.69 ± 0.110.512192 ± 6
Q011A1135A8R6959773.5647.50−9.27 ± 0.130.512163 ± 7
Q011A*         −9.23 ± 0.120.512165 ± 6
Q014A1135A11R3858797.7648.11−9.61 ± 0.150.512145 ± 8
Q016A1135A12R657110.9648.44−9.67 ± 0.150.512142 ± 8
Q018A1135A15R14547132.6648.99−11.08 ± 0.090.512070 ± 5
Q018A*         −11.08 ± 0.100.512067 ± 5
Q021A1135A17R16567152.2649.48−10.10 ± 0.120.512120 ± 6
Q024A1135A19R37072174.5150.02−10.61 ± 0.130.512094 ± 7
Q024A*         −10.48 ± 0.160.512101 ± 8
Q027A1135A21R73335199.3450.59−10.26 ± 0.100.512112 ± 5
Q027A*         −10.21 ± 0.110.512115 ± 5
Q027A*         −10.12 ± 0.070.512119 ± 4
Q029A1135A25R23436230.3554.15−8.97 ± 0.080.512178 ± 4
Q032A1135A27R445.547.5252.1555.86−9.48 ± 0.130.512152 ± 6
Q033A1135A28R19597258.2661.83−9.30 ± 0.080.512161 ± 4
Q034B1135A28R71719266.4865.76−10.68 ± 0.110.512090 ± 5
Q034B*         −10.55 ± 0.090.512097 ± 5
Q035A1135A29R2110112269.5166.06−10.37 ± 0.070.512106 ± 4
Q036A1135A30R1120122277.7166.88−10.08 ± 0.060.512121 ± 3
Q036A*         −10.10 ± 0.060.512120 ± 3
Q036A*         −9.96 ± 0.070.512127 ± 4
Q039A1135A33R29698307.8470.28−10.10 ± 0.080.512120 ± 4
Q039A*         −10.04 ± 0.060.512123 ± 3
Q042A1135A40R14547373.0673.85−9.49 ± 0.080.512151 ± 4
Q042A**         −9.52 ± 0.090.512150 ± 5
Q047A1135A46R23436432.2577.77−9.46 ± 0.070.512153 ± 4
Q047A**         −9.53 ± 0.070.512149 ± 4
Q049A1135A53R1112114498.2389.39−8.65 ± 0.110.512195 ± 6
Q049A**         −8.78 ± 0.140.512188 ± 7
 
Leg 121, ODP Site 757 (17°01′S, 88°10′E)
Y90A757B15H66466137.9438.60−7.98 ± 0.200.512229 ± 10
Y97A757B16H6911147.0941.14−7.80 ± 0.120.512238 ± 12
Y100A757B17H33739152.4742.64−8.23 ± 0.100.512216 ± 5
Y113A757B19H13739168.8746.22−9.00 ± 0.300.512177 ± 15
Y120A757B20X45355179.7348.29−8.70 ± 0.290.512192 ± 15
Y124A757B21X15658183.2649.03−9.50 ± 0.150.512151 ± 8
Y126A757B21X35456186.2449.66−9.27 ± 0.080.512163 ± 4
Y126A*         −9.17 ± 0.130.512168 ± 7
Y131A757B22X19092193.3050.53−8.34 ± 0.120.512210 ± 6
Y135A757B22X51517198.5551.17−8.20 ± 0.170.512218 ± 9
 
Leg 121, ODP Site 758 (5°23′N 90°21′E)
Z149758A28X1108110257.9849.84−8.54 ± 0.100.512200 ± 5
Z151758A28X3145147261.3553.01−8.50 ± 0.090.512202 ± 5
Z156758A29X2125127269.3557.11−8.52 ± 0.170.512201 ± 4
Z158758A29X41214271.2257.47−7.94 ± 0.070.512231 ± 4
Z159758A30X15860276.8858.12−8.75 ± 0.220.512189 ± 3
Z160758A30X25860278.3858.29−8.56 ± 0.120.512199 ± 6
Z166758A31X46466291.0459.70−8.66 ± 0.120.512194 ± 6
Z168758A32X1110112296.7066.65−11.45 ± 0.090.512051 ± 4
Z170758A32X3111113299.7169.66−10.63 ± 0.080.512093 ± 4
Z170**         −10.54 ± 0.150.512097 ± 6
Z173758A32X66264303.7273.67−10.07 ± 0.110.512122 ± 5
Z175758A33X3107109309.3779.32−9.91 ± 0.120.512130 ± 6
 
Leg 122,ODP Site 762 (19°53′S 112°15′E)
R028762C6X235.539.5209.8635.01−7.86 ± 0.130.512235 ± 6
R031762C13X19599275.4541.80−9.35 ± 0.190.512159 ± 10
R031*         −9.36 ± 0.130.512158 ± 7
R033762C16X25357305.0347.01−9.95 ± 0.110.512128 ± 5
R035762C21X162.566.5351.1351.67−8.82 ± 0.110.512186 ± 5
R035*         −8.76 ± 0.160.512189 ± 8
R041762C41X2107111538.0764.07−9.67 ± 0.120.512142 ± 6
R042762C43X548.552.5560.9965.10−11.21 ± 0.130.512063 ± 7
R042*         −11.08 ± 0.100.512070 ± 5
R045762C58X340.544.5700.4177.28−9.37 ± 0.160.512157 ± 8

3. Results

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

[18] The Nd isotopic data are given as εNd(0) in Table 2. εNd values of ancient seawater in the South Indian Ocean appear to vary in the range εNd = −12 to εNd = −8 over the 90–40 Ma period. The variations are highly consistent in all four ODP 1135, 757, 758, and 762 sites and exhibit reproducible patterns over a very large geographic area. The synchronism of the signals with identical ranges of εNdvariations from ODP 757–758 (Indian plate), ODP 1135 and ODP 762 (Antarctica-Australia plate) clearly demonstrate that the same information has been preserved at these sites, though they were widely separated (1000 to 4000 km) from each other. This also confirms that the acetic acid leach has indeed extracted the Nd isotopic signal from ancient seawater.

[19] Nd isotopic values are not corrected for 147Sm in situ decay because the correction is small, and also because overall, if isotopic values were corrected for in situ decay, we should also correct the εNd values of the sources, the 147Sm/144Nd ratio of which we do not know precisely. It is thus appropriate to use uncorrected εNd values.

3.1. ODP Site 1135

[20] Nd isotope data from ODP site 1135 sediment (Table 2 and Figure 2a) trace the evolution of seawater Nd isotopic composition from 90 to 40 Ma at a fixed location in the South Indian Ocean. The record describes four episodes of successive decreases and increases of εNd. The first step is a decrease of εNd from −8.7 to −11 over the interval 90–65 Ma. Then εNd seawater values sharply increase by 1.5 εNd units and remain stable for the next 10 Myr. After 55 Ma, εNd values decrease again, with a rapid shift of −2.5 units over a 5 Myr interval. The last step of the record is characterized by a rise from −11.5 to −8.5 εNd, between 50 Ma and 40 Ma. This positive shift of 3 εNd units in 10 Myr is larger than the first increase after 65 Ma.

3.2. ODP Sites 758 and 757

[21] Since both ODP 758 and 757 sites started at different times according to the development of the Ninety East Ridge and drifted northward with the Indian plate through time, we built a εNd composite record from the two ODP sediment cores, which thus provides a second record of the evolution of the εNd Southern Indian seawater. This composite curve strongly mimics the pattern from ODP Site 1135 (Figure 2a). An identical εNd decrease is observed from 80 to 65 Ma with a lower value of −11.5 reached at 65 Ma. From 65 to 55 Ma, the εNd seawater value stabilizes around −8.5 εNd. During this time interval, εNd values are 1 εNd unit higher than those from ODP site 1135. Then, for the next 5 Myr, εNd values decrease by about 1.5 εNd units to reach −9.5 around 50 Ma. For the last 10 Myr of the record, from 50 to 40 Ma, the εNd trend reverses and εNd values increase up to εNd = −8. During 3 Myr, just before 50 Ma, sediment was deposited simultaneously at both sites. Even though ODP 757 and 758 sites were 2000 km apart from each other and at very different latitudes, they present similar εNd values over this short time period.

3.3. ODP Site 762

[22] The seven Nd isotope data from ODP site 762 sediment confirm the range of εNd variations described in the three other cores between 80 and 35 Ma as well as the global pattern. They start with a εNd decrease before the K/T boundary with a minimal εNd value of −11 reached around the boundary. After the K/T boundary, the εNd value rises to −8.7 at 51.6 Ma, corresponding to the εNd range of values previously observed at ODP sites 1135 and 757–758 from 65 to 50 Ma. As previously seen in the other cores, the same εNddecrease is observed synchronously with the India-Asia collision and the sameεNd increase is recorded after this collision.

3.4. Comparison With Other Studies

[23] Two other studies, based on fish teeth analyses, provide εNd data for the time period between 90 and 40 Ma in this region. The ODP site 766 studied by Robinson et al. [2010] is located near ODP site 762 on the Western Australian Margin (19°55′S, 110°27′E). However, it was cored 2 km deeper than our sites, at 4008 mbsl. Site 766 data are available from 90 Ma to 70 Ma. Before 80 Ma, the Robinson et al. [2010] εNd(0) data are about 3 εNd units higher than data from our sites and between 70 and 80 Ma all data are in the same range (between εNd(0) = −11.5 and εNd(0) = −10). The second site located in the Southern Indian Ocean is the DSDP site 213 studied by Thomas et al. [2003] located at 10°12′S, 93°53′E and drilled at 3000 mbsl. This site was studied at a very high resolution around the Paleocene/Eocene Boundary and shows variations from −6 to −10 within 0.2 Ma (between 55.25 and 55.45 Ma). At a larger oceanic scale, numerous studies have been performed in the modern South Atlantic, indicating a range of εNd values for the Southern deep water between −10 and −9 during this interval of time [Scher and Martin, 2004; Thomas et al., 2003; Via and Thomas, 2006]. However, as we will discuss in the next section, these studies were performed on sediments deposited below 3000 m water depth and have therefore recorded the εNd signal of very deep water masses.

4. Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

4.1. Neodymium as a Paleoceanographic Proxy

[24] Palmer [1985] and Palmer and Elderfield [1986] showed that formation of MnO2coatings around foraminifera occurs at the ocean surface. However, they also showed that these oxides, transferred at the bottom in less than a year, were affected by several dissolution re-precipitation processes at the sediment/water interface. They showed that, even if these processes change the Nd concentration in the oxides, their isotopic ratio is almost totally preserved. Indeed, we think that the finalεNd seawater signal is incorporated in oxides mainly at the sediment/water interface, but mainly results from the mixing of Nd isotopes that come from the surface via erosion processes [Burton and Vance, 2000; Gourlan et al., 2010; Horikawa et al., 2011; Le Houedec et al., 2012; von Blanckenburg and Nägler, 2001] and also from bottom currents [Piotrowski et al., 2004; Robinson et al., 2010; Scher and Martin, 2004; Thomas, 2005]. Thus, the final εNd signal depends on the relative influences of both of these processes which vary from place to place. These variations are illustrated by von Blanckenburg [1999] for present seawater or by Le Houedec et al. [2012] for the last 30 Ma. Le Houedec et al. [2012] show that at the same location the εNd signal of seawater at intermediate depth can be influenced by oceanic transport as well as weathering inputs.

[25] All samples studied here come from cores drilled between 2000 to 1300 m and the measured εNd signal is a proxy of intermediate water masses. We therefore consider that the εNd seawater signal extracted from our samples reflects a mixture of Nd isotopes that come from products of surface erosion and from oceanic intermediate currents flowing at ∼2000 m depth.

4.2. Regional Nd Sources

[26] During and after the Cretaceous, the Indian Block broke up from Gondwana, separated from the African and Antarctic continents and migrated northward. The drift rate of India increased tremendously between 60 and 50 Ma and slowed down upon full frontal collision with Asia at ∼50 Ma [Dyment, 1998; Fisher et al., 1971; McKenzie and Sclater, 1971; Müller et al., 2008a; Patriat and Achache, 1984; Rowley, 1996; Schlich, 1982; Sclater and Fisher, 1974]. Indian motion strongly affected the location of Nd sources and the oceanic circulation pattern in the Tethys and in the Indian Ocean. These will be now discussed in detail dealing first with the Indian block.

[27] Between 90 and 65 Ma, erosion of India transferred unradiogenic neodymium (typical values of εNd = −20) from the Precambrian platform to the ocean [Beckinsale et al., 1980; Naqvi and Rodgers, 1983]. Sixty five million years ago, the activity of the Reunion hot spot brought a huge amount of basaltic lava to the Earth's surface, forming the Deccan traps: most of the volume was erupted in (possibly much) less than 1 Ma [Chenet et al., 2007; Courtillot et al., 1988; Duncan and Pyle, 1988]. During this short period, the Deccan volcanism probably contributed to the isotopic composition of seawater in the region inducing a Nd radiogenic spike, but the resolution of our records across the K/T boundary does not allow the detect this signal. This effect was very well documented for Os isotopes [Peucker-Ehrenbrink and Ravizza, 2000; Ravizza and Peucker-Ehrenbrink, 2003].

[28] After 65 Ma, evaluating the contribution of Nd from India is not straightforward. Even though lavas have an εNd isotopic signature between −5 and +5 [Allègre et al., 1982], the present river waters draining this plateau on both the eastern and western sides carry unradiogenic Nd due to the large extent of Precambrian rocks surrounding the basalt [Colin et al., 1999; Kessarkar et al., 2003]. We could thus consider that the Deccan Traps have had little influence on the εNd value of the Indian Ocean. Note that the Precambrian terrains to the north of the Greater India have now disappeared in the subduction zone. Therefore, the proportion of Precambrian rocks versus volcanic rocks before the collision was higher that now. However, Kent and Muttoni [2008] suggested that erosional products and intensity changed with the transit of the Indian continent over different climatic belts. The equatorial humid belt was successively crossed over by the northern Precambrian plateau of the Greater India from 65 to ∼50/∼55 Ma and by highly weatherable Deccan Traps [Dessert et al., 2003; Louvat and Allègre, 1997] from ∼50/∼55 and 38 Ma. These authors suggest that after ∼50/∼55 Ma, the Deccan rocks must have suffered intense weathering. This effect could have added radiogenic Nd in seawater after ∼50/∼55 Ma.

[29] After 50 Ma, the Himalaya uplift did not occur immediately after the Indian-Asian collision, but only some 30–25 Ma ago after extrusion of Indochina to the Southeast [Tapponnier et al., 2001] and thus did not contribute to the εNd seawater signal during the 90–40 Ma interval.

[30] Between 90 Ma and 50 Ma, the erosion of the Australian and Antarctica Precambrian platforms brought very unradiogenic neodynium to the ocean [DePaolo et al., 1982; McCulloch, 1987]. Note that the separation of Gondwana in multiple pieces most likely enhanced the transfer of erosion products to the ocean, as has been shown to be the case for Rodinia [Donnadieu et al., 2004].

[31] The Southern Indian Ocean was also characterized by active submarine volcanism over this period of interest. It was initiated by the emplacement of the Kerguelen LIP with eruption ages ranging from 119 Ma in the southern part of the Kerguelen plateau to 34 Ma in its northern part [Coffin et al., 2002; Frey et al., 2002]. The peak of the Kerguelen LIP magmatic output occurred prior to 90 Ma (with the Rajmahal traps at ∼115 Ma; 0.9 km3/yr). Between ∼82 and 38 Ma, the Kerguelen hot spot generated the Ninetyeast Ridge (and the small Skiff Bank) at a much smaller rate of 0.1 km3/yr [Coffin et al., 2002; Frey et al., 2002]. The volume of volcanic rocks per 10 Myr period was evaluated by angle seismic data, gravity modeling and 40Ar/39Ar dating [Frey et al., 2002]. The Ninety East Ridge does not show large morphologic changes from South to North and thus Frey et al. [2002] suggested that the activity of the Kerguelen hot spot was regular between 82 Ma and 38 Ma. Therefore, we will consider that the Kerguelen hot spot contribution to the regional Nd budget has remained constant over the time period studied here. This is likely true at first order only and the Kerguelen hot spot activity has probably fluctuated through time around the proposed value.

[32] At circa 60 Ma very active submarine volcanism was caused by a sudden increase of the spreading rate of the South-East Indian Ridge (SEIR) from 7 cm/yr to more than 17 cm/yr [Cande et al., 2010; Cogné and Humler, 2004, 2006; Müller et al., 2008b; Patriat and Achache, 1984]. This ultra-fast spreading continued between 60 and 50 Ma before returning to a lower rate of 7 cm/yr after the India-Asia collision.

[33] All these tectonic changes had consequences on paleogeography and paleoceanography. The connection between the Indian and Pacific oceans remained open between 90 and 50 Ma in the equatorial area [Bush, 1997; Cogné and Humler, 2004; Poulsen et al., 1998]. An intermediate current flowed westward through this oceanic pathway, joining the Pacific and Atlantic oceans via the Equatorial Indian Basin [Bush, 1997; Pucéat et al., 2005; Stille et al., 1996]. However, the intermediate oceanic circulation between these Pacific and Indian basins occurred only near the equator, far away from our target region in the South which was, in addition, isolated from this equatorial current given the position of the Indian continent at the time (Figure 1). The situation changed after 50 Ma with the closure of the oceanic passage induced by the India-Asia collision. At that time,εNd equatorial intermediate seawater values from the Pacific Ocean ranged from −5 to −4 [Ling et al., 1997, 2005; Meynadier et al., 2008].

4.3. Interpretation

[34] The Nd composition of seawater in the southern Indian Ocean fluctuates between non-radiogenic (−10 to −11) and slightly more radiogenic (−9 to −8) composition. These variations are consistent at the scale of an entire oceanic basin and are interpreted as the result of regional weathering inputs as well as water mass mixing. At first order, we note that the ocean in this region was certainly well mixed (at least the first 2000 mbsl) over a large geographic distance since it generated a similar pattern in all four studied sites for each time period. The observed pattern leads us to divide the Indian Ocean Nd isotopic history into three episodes: (1) 90 to 65 Ma, (2) 65 to 50 Ma and (3) 50 to 40 Ma (Figure 5).

image

Figure 5. Schematic reconstructions of paleo oceanic currents and paleo Nd sources. (a) For the Late Cretaceous, the main intermediate oceanic current was the Equatorial Tethys Circumglobal Current (Tethys-CC). (c) For the Middle Eocene, after the India-Asia collision, we suggest a main oceanic influence of Pacific Equatorial water in South Indian Ocean. The Equatorial Circum Current was diverted by the Eastern coast of India to the Southern Indian Ocean as a result of India-Asia collision and the closure of the equatorial pathway. Blue arrows indicate surface to intermediate oceanic currents and dashed black arrows the main Precambrian continental Nd sources. Volcanic Nd sources are in red. (b) The interval of strong activity of the South East Indian Ridge (SEIR) from 60 to 50 Ma is underlined by a red broken line. The red zones (“D” and “K”) correspond to the Deccan Traps province and Kerguelen plateau (see text).

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[35] At 90–80 Ma, the South Indian Ocean was surrounded by old continental blocks derived from Gondwana breakup (Antarctica-Australia, Africa, South America, India) and the seawaterεNd was on the order of −9. As discussed by Robinson et al. [2010], this value is relatively high if we consider the very low values of surrounding old continental crust. Presently, the εNd value of the North Atlantic Ocean, which has a similar geological environment, is on the order of εNd = −13 [Goldstein and Hemming, 2003]. The Kerguelen Plateau and Hot spot in the center of the Southern Indian Ocean are the source of radiogenic Nd that probably explains this isotopic composition.

[36] From 90 to 65 Ma, the end of the supercontinent Gondwana splitting into several continental masses produced an increase of surface exchange between ocean and continent (Figures 1a and 5a), which enhanced erosion inputs to the ocean, mainly through transport by rivers [Donnadieu et al., 2004]. This increase of unradiogenic Nd inputs could explain the 2 to 3 units decrease of εNd values in ancient Indian seawater recorded at ODP sites 1135, 758 and 762 (Figure 2a). The lowest εNd values were reached at about 65 Ma in ODP sites 1135 (−10.5), 758 (−11.5) and 762 (−11.2).

[37] The trend toward more radiogenic values reversed sharply after 65 Ma, implying a strong radiogenic Nd input in the region which persisted until ∼50 Ma. From −60 to −50 Ma, εNd seawater value remained relatively constant at −9 ± 0.5. There is no evidence of abrupt tectonic, or oceanic circulation changes in the South Indian Basin around 65 Ma which could explain this sharp εNd increase. However, two major volcanic events that brought to the surface materials of mantle origin with high Nd radiogenic values occurred then in Indian Ocean (Figure 5b): (1) the gigantic eruption of Deccan Traps [Chenet et al., 2007; Courtillot et al., 1988], (2) the sudden increase of the South East Indian Ridge (SEIR) spreading rate [Cande et al., 2010; Cogné and Humler, 2004, 2006; Müller et al., 2008a; Patriat and Achache, 1984] as shown in Figure 2b. It was shown in the previous section that, with the probable exception of the first million year after the emplacement of the Traps, the Deccan traps might have had a small influence on the Indian Ocean seawater εNd value from ∼66 to 50/55 Ma. Interestingly, from 65 to 50 Ma, the εNd pattern follows the SEIR spreading rate pattern (Figure 2) suggesting that Nd from the ridge could have significantly influenced the seawater εNd value in this region. After 50 Ma, Indian Ridge activity dramatically dropped down from ∼16–18 cm/yr to a value lower than 7 cm/yr (Figure 2b). This was synchronously accompanied by a spectacular decrease of Indian Ocean εNd back to −11.

[38] The ridge crest contribution to the oceanic Nd budget has been the subject of several discussions. In an early paper, the ridge contribution was considered to be significant, as is the case for Sr and Pb [Elderfield and Greaves, 1982]. Later, it was shown that this flux was too small to influence the Nd isotopic composition of the global ocean [Bertram et al., 1992; Halliday et al., 1992; Michard et al., 1983; Piepgras and Wasserburg, 1985]. However, these discussions focused on the ridge influence on the average εNd of an entire ocean and not on the εNd signal at a regional scale. Today, the εNdsignal in the vicinity of the ultra-fast spreading East Pacific Rise (EPR) suggests a source of radiogenic Nd in this region. HighεNdvalues are found in Fe-Mn crusts in the EPR region [O'Nions et al., 1978; Piepgras and Wasserburg, 1985; Piepgras et al., 1979], which contrasts with other water data in the Pacific Ocean [Aplin et al., 1986; Frank et al., 1999; O'Nions et al., 1978; Piepgras and Wasserburg, 1985; Piepgras et al., 1979; van de Flierdt et al., 2004a]. The contribution of the ridge crest to the εNd signal of seawater can be a direct result of submarine volcanism on the ridge, hydrothermal activity but also seawater alteration of ridge crests and MnO2precipitation. Moreover, it has recently been shown that there is a local circulation of seawater toward the surface of the ocean close to ultra-fast spreading ridges. This circulation disperses chemical elements produced above the ridge and inhibits the fast scavenging near the ridge [Adams et al., 2011; Lupton and Craig, 1975; Von Damm et al., 1985]. ODP sites were located at 1500 mbsl, only ∼1000 m above high spreading rate ridge crests [Westphal et al., 2002]. The sites are therefore in the depth range affected by local low-density circulation. We suggest that the Nd isotopic composition of seawater in the region of ultra-high spreading ridge might be influenced by the ridge.

[39] Because the shift toward radiogenic values (from ∼−11 to ∼−8.5) is relatively small (2.5 εNd units), this contribution is clearly limited. With a first order mass balance calculation, we can estimate this contribution. Only 13% of radiogenic Nd (respectively 11%) is needed to shift the seawater value from −11 to −8.5, if we consider a value of εNdof +8 (respectively +12) for the ridge contribution. We can use this calculation to estimate the isotopic shift that would induced a similar contribution in the present-day Pacific Ocean seawater near the EPR. Seawater values would change from −4 (resp. −3) to ∼−2.3 (resp. ∼−1.4). This difference is compatible with the small seawaterεNd values in the region [Piepgras and Wasserburg, 1985; Piepgras et al., 1979], but it is difficult to go deeper in the comparison if we consider the analytical precision on seawater Nd isotopic data (±0.5 εNd units).

[40] Assuming that an ultra-fast spreading rate was a source of radiogenic Nd would explain whyεNdsuddenly rose after 65 Ma and strongly decreased to less radiogenic values after the India-Asia collision at ∼50 Ma, when the spreading rate dropped sharply. One could also suggest that the collision triggered theεNd decrease at ∼50 Ma through large inputs of eroded material from the Indian continent (i.e., rocks from the Himalayan belt). However, Métivier et al. [1999]showed that this erosion was associated with the uplift that occurred 20 to 25 Ma after the India-Asia collision [Tapponnier et al., 2001].

[41] The last 10 Myr of our record, from 50 to 40 Ma, are characterized by a large and rapid increase of εNd ancient seawater values, by ∼3 εNd units for ODP site 1135, and ∼2 εNd units for ODP sites 757 and 762 (Figure 2a). This could result from an increase of radiogenic input and/or be due to a decreasing contribution from unradiogenic sources. The radiogenic contribution could result from the erosion of the Deccan traps as they transited through the equatorial humid belt as suggested by Kent and Muttoni [2008] (Figure 5c). The increase of εNd is precisely dated at 50 (±1) Ma and thus can be explain by the Deccan Traps weathering only if it started at 50 Ma. However, Kent and Muttoni [2008] also suggested that the enhancement of the Deccan Traps weathering induced the decrease recorded in the seawater osmium isotopes at 55 Ma [Kent and Muttoni, 2008; Nielsen et al., 2009; Turekian and Pegram, 1997]. Changes in the Os and South Indian seawater εNd toward more volcanic values were not concomitant (5 Ma difference). Therefore, we suggest that only one of these two seawater records can reflect the weathering of Deccan Traps. considering the residence time of these two elements (∼30 Kyr for Os i.e., [Levasseur et al., 1999, 2000; Luck and Turekian, 1983; Oxburgh, 1998; Peucker-Ehrenbrink and Ravizza, 1996; Ravizza and Turekian, 1992] and ∼1 Kyr for Nd [Bertram and Elderfield, 1993; Tachikawa et al., 1999]) and the proximity of the Deccan Traps with our target region, it is difficult to envisage that the Deccan weathering affected the Os signal without disturbing the regional Nd record. Therefore, we suggest that the enhancement of the Deccan Traps weathering, if it ever occurred, started at 50 Ma.

[42] The second scenario, that does not exclude an enhancement of the Deccan Traps weathering at 50 Ma, involves the closure of the connection between the Pacific and North Atlantic basins and consequently the end of the Tethys Circumglobal Current (TCC). Due to the geostrophic oceanic circulation, the Indian-Pacific Equatorial current was diverted along the Eastern coast of India toward the South Indian Ocean (Figure 5c). The strong mixing of radiogenic Pacific seawater with less radiogenic Indian seawater induced an increase of εNd seawater values in our target region. This is also supported by the similarity of εNdvalues over a large geographic distance which suggests a well-mixed ocean.

[43] The establishment of this Southwest current is supported by the development of warm foraminifera faunas, interpreted as an increase of Pacific warm water input in the southern Indian Ocean [Berggren and Holister, 1977; Hocutt, 1987]. This current would have some similarity with the present South East current that prevails along the coast of Africa (M. Tomczak and S. J. Godfrey, Regional oceanography: An introduction, version 1.0, 2001, e-book available athttp://www.es.flinders.edu.au/∼mattom/regoc/pdfversion.html). Such a rapid rise of εNd was found in the Tasman Sea region on the East flank of the Australian Continent, after the narrowing of the Indonesian Gateway at ∼10 Ma [van de Flierdt et al., 2004b]. The Equatorial Pacific seawater with high εNd was there also diverted to the South and mixed with less radiogenic southern seawater.

5. Conclusion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

[44] The εNd record preserved in ODP core 1135 and validated by ODP cores 757, 758 and 762 revealed large variations, ranging from −11 to −8. A consistent pattern of seawater εNd is present in an area of ∼3000 km in diameter in the Southern Indian Ocean.

[45] Between 90 and 65 Ma, the plate geometry and the Tethys Circum Current isolated the southern Indian Ocean. Precambrian continents resulting from Gondwana breakup surrounded this basin. The εNd decrease recorded during this time interval is interpreted as a result of enhanced erosion due to this continental breakup.

[46] At 65 Ma, the Reunion hot spot formed very rapidly the LIP of the Deccan Traps. Most of the volume was erupted in (possibly much) less than 1 Ma. The emplacement of the Deccan had presumably an impact on the seawater Nd isotopic composition in the region as it has been found to be for Os [Ravizza and Peucker-Ehrenbrink, 2003], but the low sedimentation rate around the K/T boundary did not permit to document such effect.

[47] From 60 to 50 Ma, the South Indian εNdradiogenic values (∼−8.5) and a sudden decrease down to −11 at 50 Ma. No large changes occurred in the positions of our sites or in surface and/or intermediate oceanic currents that could explain these variations over this period. However, the South East Indian Ridge (SEIR) spreading rate drastically changed. At 60 Ma, SEIR became an ultra-fast ridge and at 50 Ma the SEIR activity returned to a more moderate spreading rate [Cande et al., 2010; Cogné and Humler, 2006; Patriat and Achache, 1984]. Simultaneous changes from high spreading rates and Nd radiogenic values to moderate spreading rate and low Nd radiogenic values lead us to suggest a causal relation. Presently, the relatively radiogenic εNdseawater values in the vicinity of the modern ultra-fast East Pacific Rise seem to support this hypothesis.

[48] An enhancement of Deccan Traps erosion when Greater India entered the equatorial humid belt [Kent and Muttoni, 2008] could explain the increase of εNd from 50 to 40 Ma. But, 5 Ma long uncertainty on the age of the onset of the Deccan weathering and the difficulty to correlate Os and Nd records do not allow us to validate this hypothesis.

[49] The large equatorial oceanic seaway between the Pacific and Indian Oceans shrunk with the India-Asia collision at 50 Ma and the Tethys Circum Equatorial Current (TCC) shut down. We interpret the 50–40 MaεNd increase as a result of oceanic change with a larger mixing of Pacific radiogenic waters with South Indian seawater, after the diversion to the South of the Equatorial Pacific Current. This is supported by changes in assemblages of planktonic foraminifera [Berggren and Holister, 1977; Hocutt, 1987].

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

[50] We thank Pascale Louvat and Julien Moureau for their help in the MC-ICP-MS measurements, Jean Louis Birck for his invaluable help on lab geochemistry and Antoine Cogez, Virginia Rojas, and Mickaël Tharaud for REE measurements. We thank also Philippe Patriat and Jerome Dyment for scientific discussions on Indian Ocean marine geophysics. We thank Vincent Courtillot for stimulating discussions and comments. This research used samples provided by Integrated Ocean Drilling Program (IODP). The Kochi ODP Core Center staff is warmly thanked for the quality and the time devoted to sampling. This work was supported by the CNRS- INSU Program SYSTER and the French Ministry of Research. This is IPGP contribution 3291.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

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