Paleoceanography

Testing the impact of seasonal phytodetritus deposition on δ13C of epibenthic foraminifer Cibicidoides wuellerstorfi: A 31,000 year high-resolution record from the northwest African continental slope

Authors


Abstract

[1] Studies of temporal changes of ocean circulation and deepwater ventilation often rely on δ13C records of epibenthic foraminifer Cibicidoides wuellerstorfi. However, primary productivity-related overprints may distort the signal and simulate a chemical age of ambient water mass that is too old and simulates poorly ventilated ambient bottom waters. To further constrain the use of C. wuellerstorfi δ13C records from high-productivity areas, we analyzed a 14CAMS-dated gravity core from the upwelling regime off northwest Africa at 12°N. We compare this new record with 37 radiocarbon dated δ13C records from the eastern Atlantic Ocean between 45°N and 25°S that are bathed by the same water mass. Only during Heinrich events 1 and 2, when the investigated core site off northwest Africa experienced year-round, sustained deposition of organic matter, the δ13C values at this site faithfully record deepwater ventilation states. During times of predominantly seasonal deposition of fresh phytodetritus, however, δ13C values were significantly lower than at the reference sites. This underscores that reconstruction of paleocirculation and deep ocean ventilation using C. wuellerstorfi δ13C from regions that experienced seasonal phytodetritus deposition needs to be validated by additional proxies that are not affected by local productivity.

1. Introduction

[2] The distribution of δ13C of dissolved inorganic carbon (DIC) within the ocean is controlled by a combination of biological carbon uptake at the sea surface, air-sea gas exchange, organic matter decomposition in deeper water masses and the general circulation of the ocean. The epibenthic foraminifer Anomalina wuellerstorfi Schwager 1866, also assigned to Planulina, Cibicidoides, Cibicides, Fontbotia, and lately, based on small subunit rDNA sequences, to Cibicidoides again [Schweizer et al., 2009], are used by paleoceanographers for reconstructing the δ13CDIC of past ocean deep and bottom water masses that has implications for ocean circulation and atmospheric CO2 concentration [e.g., Curry et al., 1988; Duplessy et al., 1988; Sarnthein et al., 1994; Bickert and Mackensen, 2004; Curry and Oppo, 2005]. However, strongly depleted benthic δ13C values are observed on occasion that are inconsistent with paleowater mass architecture, and coincide with areas of increased upwelling and high biological productivity [Sarnthein et al., 1988]. It has been demonstrated that low δ13C values of C. wuellerstorfi tests from glacial core sections underneath highly productive upwelling areas off West Africa and from the eastern South Atlantic polar frontal system were linked with high organic carbon fluxes to the seafloor [Sarnthein et al., 1994; Bickert and Wefer, 1996; Mackensen et al., 2001; Diz et al., 2007]. In the region of the South Atlantic polar front, Mackensen et al. [2001] estimated a 0.4‰ negative deviation of foraminiferal calcite δ13C from ambient bottom water δ13CDIC during times of highly seasonal primary production. Rutberg and Peacock [2006] compared model-derived deep ocean δ13C values with benthic foraminiferal δ13C values from the high-latitude North Atlantic and Southern Ocean, and observed a low-δ13C overprint in high-productivity regions. Covariations between neodymium isotope ratios and C. wuellerstorfi δ13C, further demonstrates the impact of productivity on 13C depletion in C. wuellerstorfi tests [Piotrowski et al., 2004].

[3] Productivity induced 13C depletion of −0.4 to −0.6 ‰ in tests of live C. wuellerstorfi was shown first in Rose Bengal stained specimens from the South Atlantic Polar Frontal Zone. This area experiences highly seasonal productivity with pulsed phytodetritus deposition and the development of organic fluff layers at the seafloor, from which it was suggested that epibenthic C. wuellerstorfi preferentially calcifies during periods of rapid phytodetritus decomposition at the seafloor and thus records a lowered “interstitial” fluff layer δ13CDIC signal [Mackensen et al., 1993; Mackensen and Bickert, 1999]. This observation was contrasted by results from recent Pacific sediments [McCorkle and Keigwin, 1994] that suggested C. wuellerstorfi faithfully records ambient bottom water δ13CDIC values. A study of Holocene benthic foraminifers from core tops in the North Atlantic [Corliss et al., 2006] concluded that neither year-round nor seasonally pulsed primary production affect the δ13C of C. wuellerstorfi. Also Eberwein and Mackensen [2006] did not find productivity related δ13C deviations in C. wuellerstorfi tests from the center of the northwest African upwelling area that produces a sustained high organic matter flux year-round.

[4] To further elaborate on whether and when C. wuellerstorfi δ13C reliably reflects ambient bottom water δ13CDIC, we here compare a new high-resolution C. wuellerstorfi δ13C downcore record from the NW African high-productivity area with 37 14C-dated records from eastern Atlantic core sites at equivalent water depths and in the same water mass. Comparing millennial-scale changes in intensity and seasonality of productivity and organic matter flux [Zarriess and Mackensen, 2010] with benthic δ13C patterns, demonstrates that δ13C of C. wuellerstorfi at our site did not reliably record deepwater ventilation changes during most of the last 31 kyr. Only in intermittent episodes of sustained perennial productivity, that is, during Heinrich events (H) 1 and 2, C. wuellerstorfi δ13C correctly reflects bottom water mass δ13CDIC.

2. Material and Methods

[5] Gravity core GeoB9526-5 and corresponding surface sediment samples were recovered off West Africa at 12°26.1′N, 18°03.4′W in 3231 m water depth (Figure 1). The 10.8 m long core was sampled at 5 cm intervals. For stable isotope measurements, between 2 and 5 tests of Cibicidoides wuellerstorfi were separated from the size fraction >125 μm. Isotope measurements were performed on a Finnigan MAT 251 isotope ratio mass spectrometer coupled to an automatic carbonate preparation device (Kiel II). External reproducibility was better than ±0.08‰ and ±0.06‰ for oxygen and carbon isotopes, respectively. All values are given in δ notation versus Vienna Peedee belemnite standard (VPDB) (Table 1; also see auxiliary material).

Figure 1.

(a) Meridional transect of dissolved phosphate across the East Atlantic Basin. The data are from Garcia et al. [2006] and R. Schlitzer (Ocean Data View, 2008, http://odv.awi.de) and visualize the modern geometry of deepwater distribution. Box outlines latitude versus water depth diagram shown in Figure 1b. (b) Distribution of core sites used in this study. Site GeoB9526 is highlighted by a red triangle, and the other sites are used as reference for comparison. Core positions are given in Table 1. Open circles and italic labels mark core sites that offer centennial time resolution.

Table 1. Mean δ13C and δ18O Values of Cibicidoides wuellerstorfi at Eastern Atlantic Core Sites
CoreLatitude (°N)Longitude (°E)Water Depth (m)0–4 kyr B.P.7–9.8 kyr B.P.11.7–12.7 kyr B.P.15.8–16.8 kyr B.P.18.3–23.5 kyr B.P.24.3–25.4 kyr B.P.28.3–29.5 kyr B.P.Reference
δ13C (‰VPDB)δ18O (‰VPDB)δ13C (‰VPDB)δ18O (‰VPDB)δ13C (‰VPDB)δ18O (‰VPDB)δ13C (‰VPDB)δ18O (‰VPDB)δ13C (‰VPDB)δ18O (‰VPDB)δ13C (‰VPDB)δ18O (‰VPDB)δ13C (‰VPDB)δ18O (‰VPDB)
V29-17944.00−24.533331  1.02.6    0.64.2    Sarnthein et al. [1994]
CHN82502043.50−29.873020  1.22.5    0.74.3    Sarnthein et al. [1994]
KS-1140.69−10.213590  0.92.8    0.74.1    Sarnthein et al. [1994]
CH72-0240.60−21.703485  0.92.9    0.84.3    Sarnthein et al. [1994]
SU81-1837.77−10.1831350.82.80.82.8    0.14.0    Sarnthein et al. [1994]
T86-15P30.43−37.073375  1.02.7    0.54.1    Sarnthein et al. [1994]
GIK12309-226.84−15.1128201.12.60.92.8    0.24.3    Sarnthein et al. [1994]
GIK12310-3/423.50−18.7230801.12.40.72.6    0.14.0    Sarnthein et al. [1994]
GIK1232821.15−18.5727780.92.70.53.00.53.60.23.70.34.50.14.30.34.1Sarnthein et al. [1994]
GIK12329-4/619.37−19.9333200.82.50.92.6    0.24.1    Sarnthein et al. [1994]
V30-4918.43−21.083093  0.72.3    0.24.0    Sarnthein et al. [1994]
ODP65918.08−21.0330691.02.41.02.6    0.24.4    Sarnthein et al. [1994]
ALB-22617.95−21.0531000.92.60.62.6    0.24.0    Sarnthein et al. [1994]
GIK1233715.95−18.1330880.72.60.62.60.23.3−0.13.7      Sarnthein et al. [1994]
V22-19714.17−18.583167  0.72.50.63.2  0.24.30.14.40.04.0Sarnthein et al. [1994]
GIK1323913.88−18.313156  0.72.80.53.5−0.14.20.24.40.04.20.33.9Sarnthein et al. [1994]
ENO66-106.65−21.903527  0.82.3    0.44.0    Sarnthein et al. [1994]
GIK135195.66−19.8528621.12.71.02.7    0.44.5    Sarnthein et al. [1994]
ENO66-165.46−21.143152  1.02.5    0.44.2    Sarnthein et al. [1994]
GIK164575.39−21.7232911.12.70.92.7    0.34.3    Sarnthein et al. [1994]
GIK164585.34−22.0635180.93.00.62.90.83.4  0.24.30.14.00.43.8Sarnthein et al. [1994]
ENO66-445.26−21.713428  1.12.6    0.54.4    Sarnthein et al. [1994]
ENO66-384.92−20.502931  1.12.7    0.54.0    Sarnthein et al. [1994]
CH71-074.38−20.8730831.12.9      0.34.4    Sarnthein et al. [1994]
GIK16771−0.82−15.5127641.32.6      0.34.1    Sarnthein et al. [1994]
GeoB1105-4−1.67−12.4332251.02.40.82.90.63.70.04.5−0.24.40.24.10.43.9Bickert and Mackensen [2004]
GeoB1115-4−3.56−12.5629211.02.41.02.70.23.2−0.24.10.04.2  0.33.9Bickert and Mackensen [2004]
GeoB1112-4−5.78−10.7531250.82.70.92.7    0.44.3    Bickert and Mackensen [2004]
GeoB1903-3−8.68−11.8531611.02.50.83.2    0.34.2    Bickert and Mackensen [2004]
GeoB1417-1−15.54−12.712845  1.12.7    0.54.3    Bickert and Mackensen [2004]
GeoB1905-3−17.14−13.992974  1.03.1    0.74.0    Bickert and Mackensen [2004]
GeoB1031-4−21.887.1031050.82.80.82.6    0.24.2    Bickert and Mackensen [2004]
GeoB5121-3−24.17−12.3634861.02.91.02.9    0.63.8    Bickert and Mackensen [2004]
MD95-204237.80−10.1731461.03.30.93.4  0.14.60.25.00.04.90.24.6Shackleton et al. [2000]
BOFS30-3K19.75−20.7235801.02.50.92.6    0.24.1    Beveridge et al. [1995]
BOFS31-1K19.00−20.1633001.02.60.82.7    0.24.2    Beveridge et al. [1995]
MD95-203940.58−10.3533810.92.70.82.80.83.50.23.80.14.40.14.30.04.1Schönfeld et al. [2003]

[6] The age model of core GeoB9526-5 is based on δ18O records of planktic foraminifer Globigerinoides ruber “pink” (315–400 μm), tied to eight 14C accelerator mass spectrometry (AMS) radiocarbon dates that were determined on multispecific planktic foraminiferal samples from >125 μm at the Leibnitz-Laboratory for Radiometric Dating and Stable Isotope Research, Kiel. The resulting radiocarbon ages were corrected for a reservoir effect of 400 years and calibrated to calendar ages using Fairbanks et al. [2005]. For details see Zarriess and Mackensen [2010]. Seven time intervals were selected covering periods of major environmental changes including Dansgaard/Oeschger (D/O) interstadials 3 to 4 between 29.5 and 28.3 kyr B.P., the Last Glacial Maximum (LGM, 23.5–18.3 kyr B.P.), Heinrich event 2 (H2, 25.4–24.3 kyr B.P.) and Heinrich event 1 (H1, 16.8–15.8 kyr B.P.), the Younger Dryas (YD, 12.7–11.7 kyr B.P.), the middle Holocene (MH, 9.8–7 kyr B.P.), and the late Holocene (LH, 4–0 kyr B.P.).

[7] The GeoB9526-5 δ13C record was compared to radiocarbon-dated δ13C records measured on Cibicidoides wuellerstorfi from the eastern Atlantic between 45°N and 25°S latitude and water depths of 3.2 ± 0.4 km (Table 1 and Figure 1). The position of cores used for this intercomparison were chosen such that all sites are located in areas bathed by the same ambient water mass, both outside and from within upwelling areas that experience increased biologic productivity [Sarnthein et al., 1994; Bickert and Mackensen, 2004]. We computed mean δ13C values of 37 records for the LGM, MH and LH and from nine selected records that offer centennial resolution. Means were computed for D/O interstadials 3–4, H1, H2, and the YD.

[8] To test the significance of differences between δ13C means of GeoB9526-5 and reference cores we used a two-tailed t test (Table 2). Acceptance of H0 requires that δ13C mean values of reference cores and GeoB9526-5 are equal.

Table 2. Offset of δ13C Means From Reference Core Sites (Table 1) and Core GeoB9526-5a
Timeslice (cal kyr B.P.)δ13C, Reference Dataδ13C, GeoB9526-5Calculated t ValueDFProbability (%)Critical t ValueH0 Rejected
MeanSDnMeanSDn
  • a

    SD is standard deviation, n is number of observations, DF is degrees of freedom from Welch-Satterthwaite equation, and calculated and critical t values are from Welch's t characteristics.

0–4.00.960.14230.580.2663.50598.03.36yes
7.0–9.80.860.16350.540.0846.42599.85.89yes
11.7–12.70.510.2380.130.0224.68799.54.03yes
15.8–16.80.010.177−0.060.1260.891095.02.23no
18.3–23.50.330.2136−0.050.11107.612899.93.67yes
24.3–25.40.070.087−0.160.2452.04495.02.78no
28.3–29.50.240.148−0.220.1944.33498.03.75yes

3. Results

[9] Glacial δ13C values of Cibicidoides wuellerstorfi in Core GeoB9526-5 are low, varying between −0.2 and 0.0‰ with minimum values around −0.5‰ during millennial-scale cold events, while interglacial δ13C is high with maximum values of 1.0‰ (Figure 2a). Along most of the 31 kyr long record at Site GeoB9526, mean δ13C values are significantly lower than mean values at reference sites. Mean δ13C offsets between Site GeoB9526 and reference sites vary between ∣0.3∣ and ∣0.4∣‰ (Table 2 and Figures 3 and 4). The offset of means increases by |0.1| to |0.2|‰ if reference sites are excluded that may have been influenced by high productivity [Sarnthein et al., 1994; Bickert and Mackensen, 2004]. Only during Heinrich events, δ13C means of GeoB9526-5 coincide with those obtained from the reference sites. Smaller differences are observed between the core sites but are not significant at the 95% level (Table 2 and Figure 4). Mean δ18O values do not differ significantly between reference sites and GeoB core 9526-5 for all time intervals compared (Figures 3 and 4).

Figure 2.

GeoB9526-5 stable isotope records and benthic foraminiferal assemblage faunal principal component analysis: (a) δ13C record, (b) δ18O record of C. wuellerstorfi, (c) principle component (PC) loading PC1 record (“Low-Productivity Phytodetritus” fauna, Epistominella exigua), (d) PC2 (“High-Productivity Phytodetritus” fauna, Uvigerina peregrina), and (e) PC3 (“Sustained High-Productivity” fauna: Globobulimina affinis, Cassidulina reniforme, Chilostomella oolina, and Stainforthia concava). Level of significance is given at >0.4. Grey bars indicate time windows of Dansgaard/Oeschger (D/O) interstadials 3–4, Heinrich events H1 and H2, LGM, Younger Dryas (YD), and middle (MH) and late Holocene (LH). PC data are from Zarriess and Mackensen [2010].

Figure 3.

Comparison of δ13C and δ18O means of C. wuellerstorfi from Core GeoB9526-5 (solid symbols) and 37 reference cores (open symbols) for the LGM (blue circles), middle Holocene (MH; green triangles), and late Holocene (LH; red diamonds). For data listings see Tables 1 and 2. Error bars give standard deviation of means.

Figure 4.

Comparison of δ13C and δ18O means of C. wuellerstorfi from Core GeoB9526-5 (solid symbols) and nine reference cores (open symbols) offering centennial resolution. Means are plotted for D/O interstadials 3–4 (red triangles), Heinrich events H1 (green diamonds) and H2 (blue circles), and Younger Dryas (YD; purple triangles). For data listings see Tables 1 and 2. Error bars give standard deviation of means.

4. Discussion

[10] The δ13C recorded by C. wuellerstorfi in core GeoB9526-5 reflects millennial climate and productivity changes in addition to a glacial/interglacial shift (Figure 2) [Curry et al., 1988; Duplessy et al., 1988]. If C. wuellerstorfi specimens calcified close to bottom water δ13CDIC, test calcite δ13C either mirrors changes in bottom water formation and preformed δ13CDIC values or changes in ocean circulation and deepwater mass architecture [Woodruff et al., 1980; Belanger et al., 1981; Curry et al., 1988; Duplessy et al., 1988]. Our data comparison shows that it is only during H1 and H2, that low δ13C means of GeoB9526-5 coincide with likewise low means at the reference sites (Figures 3 and 4), reflecting a consistent pattern of reduced deepwater ventilation in response to meltwater induced perturbations of the thermohaline circulation [Lynch-Stieglitz et al., 2007].

[11] Mean δ13C of GeoB9526-5 up to 0.4‰ below that at the reference sites (Figures 3 and 4) reflects an influence of seasonally pulsed phytodetritus input on the δ13C of C. wuellerstorfi through δ13CDIC depletion of interstitial waters [Mackensen et al., 1993; Mackensen and Bickert, 1999]. Indeed, in a companion study Zarriess and Mackensen [2010] used benthic foraminiferal assemblage compositions and indicator species, as well as a suite of sedimentary geochemical proxies to reconstruct the upwelling history and productivity changes during the last 31 kyr at this site (Figure 2). From this study it is concluded that seasonal input of phytodetritus with subsequent development of a fluff layer, as indicated by specific phytodetritus faunas [Zarriess and Mackensen, 2010], was prevalent at the site for most of the past 31 kyr (Figures 2c and 2d).

[12] Generally, productivity-induced epibenthic δ13C depletion occurs along the coast of northwest Africa in areas with seasonally pulsed surface productivity (Figures 5 and 6). But the depletion is more significant at sites south of the Intertropical Convergence Zone (ITCZ) influenced by fluvial nutrient input during rainy seasons, than at sites north of the ITCZ affected by coastal upwelling (Figures 5 and 6). In contrast to the monsoon related fluvial nutrient supply, intensive upwelling of nutrient-rich subsurface waters forced by NE trade winds is accompanied by enhanced lateral advection that transports resuspended material, including altered organic matter, from the shelf and upper slope to the open ocean [Karakaş et al., 2006]. Thus, only during H1 and H2, perennial high productivity and sustained input of refractory organic matter is observed as indicated by a specific high-productivity fauna (Figure 2e). It is suggested that high-latitude cold events and variations in low-latitude summer insolation influenced humidity, wind systems, and the position of the tropical rain belt and so lead to changes in intensity and seasonality of primary productivity off NW Africa [Zarriess and Mackensen, 2010].

Figure 5.

(a) Annual chlorophyll concentration (mg/m3) based on the SeaWIFS collection of 2008 (G. C. Feldmann and C. R. McClain, SeaWIFS reprocessing, 2010, http://oceancolor.gsfc.nasa.gov/). (b) Location of the investigated samples, the Intertropical Convergence Zone (ITCZ), productivity areas, and surface and subsurface hydrography in the eastern Atlantic. Areas of permanent and seasonal upwelling and fluvial nutrient input are after Schemainda et al. [1975], Voituriez and Herbland [1982], Jansen et al. [1984], Van Camp et al. [1991], Mittelstaedt [1991], and Shannon and Nelson [1996]. The Northeast trade winds (NE; white arrow) and positions of the ITCZ (yellow dashed line) during February and July are after Stramma and Schott [1999] and Nicholson [2000]. Ocean currents (blue arrows) are after Voituriez and Herbland [1982] and Stramma and Schott [1999]: AC, Angola Current; AD, Angola Dome; AG, Angola Gyre; BCC, Benguela Costal Current; BOC, Benguela Oceanic Current; CC, Canary Current; EUC, Equatorial Under Current; GC, Guinea Current; GD. Guinea Dome; MC, Mauritanian Current; NAC, North Atlantic Current; NASTG, North Atlantic Subtropical Gyre; NEC, North Equatorial Current; NECC, North Equatorial Counter Current; SEC, South Equatorial Current; and SECC, South Equatorial Counter Current.

Figure 6.

Regional distribution of δ13C of C. wuellerstorfi compiled in this study. Black dots indicate locations of reference core sites listed in Table 1, and red stars mark GeoB9526-5. Data distributions are mapped for D/O interstadials 3–4 (29.5–28.3 kyr B.P.), LGM (23.5–18.3 kyr B.P.), Heinrich events H1 (16.8–15.8 kyr B.P.) and H2 (25.4–24.3 kyr B.P.), Younger Dryas (YD; 12.7–11.7 kyr B.P.), middle Holocene (MH; 9.8–7 kyr B.P.), and late Holocene (LH; 4–0 kyr B.P.). Data were gridded with DIVA (Data Interpolating Variational Analysis, http://modb.oce.ulg.ac.be/projects/1/diva) and mapped with Ocean Data View (Schlitzer, Ocean Data View, 2008, http://odv.awi.de).

5. Conclusions

[13] By correlation of δ13C values of C. wuellerstorfi tests with a paleoproductivity record at site GeoB9526 and comparison with published δ13C records we demonstrate that during the last 31 kyr, during times of highly seasonal productivity, δ13C values at GeoB9526 were by about 0.4‰ lower than those from 37 reference sites from the same water mass, except during Heinrich stadials 1 and 2, times of sustained productivity. We conclude that applying δ13C of epibenthic C. wuellerstorfi as a proxy for bottom water ventilation in regions of seasonal deposition of phytodetritus warrants validation from complementary productivity-independent proxies.

Acknowledgments

[14] We thank G. Meyer, L. Schönborn, C. Saukel, and S. Wiebe for technical support and S. Mulitza for discussion. The critical and helpful comments of R. Rutberg, two anonymous reviewers, and the editor R. Zahn helped to focus this note. The DFG Research Centre “The Ocean in the Earth System” supported this work.

Ancillary