Paleoceanography

Submillennial-to-millennial variability of diatom production off Mauritania, NW Africa, during the last glacial cycle

Authors


Abstract

[1] The low-latitude upwelling regime off the Mauritanian coast in the subtropical NE Atlantic accounts for a significant part of global export production. Although productivity variations in coastal upwelling areas are usually attributed to changes in wind stress and upwelling intensity, productivity dynamics off Mauritania are less straightforward because of the complex atmospheric and hydrographic setting. Here we integrate micropaleontological (diatoms) and geochemical (bulk biogenic sediment components, X-ray fluorescence, and alkenones) proxies to examine on submillennial-to-millennial changes in diatom production that occurred off Mauritania, NW Africa, for the last 25 ka. During the Last Glacial Maximum (LGM, 19.0–23.0 ka B.P.), moderate silicate content of upwelled waters coupled with weakened NE trade winds determined moderate diatom productivity. No significant cooling is observed during the LGM, suggesting that our alkenone-based SST reconstruction represents a local, upwelling-related signal rather than a global insolation related one. Extraordinary increases in diatom and opal concentrations during Heinrich event 1 (H1, 15.5–18.0 ka B.P.) and the Younger Dryas (YD, 13.5–11.5 ka B.P.) are attributed to enhanced upwelling of silica-rich waters and an enlarged upwelling filament, due to more intense NE trade winds. The synchronous increase of CaCO3 and K intensity and the decreased opal and diatoms values mark the occurrence of the Bølling/Allerød (BA, 13.5–15.5 ka B.P.) due to weakened eolian input and more humid conditions on land. Although the high export of diatoms is inextricably linked to upwelling intensity off Mauritania, variability in the nutrient content of the thermocline also plays a decisive role.

1. Introduction

[2] Coastal upwelling accounts for ∼50% of the global export production in eastern boundary current systems [Longhurst et al., 1995]. One of these systems is the low-latitude regime off the coast of Mauritania, NW Africa, in the tropical NE Atlantic. At present, this high-productivity area is characterized by weak seasonal upwelling, offshore streaming upwelling filaments, and strong eolian input, which is present throughout most of the year [Mittelstaedt, 1991; Barton, 1998]. Although productivity variations in coastal upwelling areas are usually attributed to changes in upwelling intensity due to wind stress [Summerhayes et al., 1995; Nave et al., 2003; Romero et al., 2006], the productivity dynamics off the Mauritanian coast is less straightforward because of its complex atmospheric and hydrographic setting [Mittelstaedt, 1991; Barton, 1998]. Being at the crossroads of water masses of North and South Atlantic origin, the primary productivity off the Mauritanian coast is strongly influenced by the nutrient content of the upwelled waters [Zenk et al., 1991]. Nutrient rich upwelled waters can have surface chlorophyll a concentrations of up to 9.0 mg m−3 [Van Camp et al., 1991] and promotes high primary production year round over the shelf, and especially just beyond the shelf break [Longhurst et al., 1995]. High seasonal and interannual variability of production is expected because of advection of phytoplankton biomass from the costal upwelling regime to the open ocean [Van Camp et al., 1991], fluctuations in the timing of maximal cell growth rate depending on cross-shelf location of the upwelling center [Gabric et al., 1996], and variations in the source waters for coastal upwelling [Zenk et al., 1991].

[3] The occurrence of a large, offshore spreading upwelling filament greatly enhances the area of high surface water productivity between 20° and 24°N off Mauritania [Helmke et al., 2005]. This upwelling filament, caused by the convergence of the southward flowing Canary Current and the poleward flowing North Equatorial Countercurrent south off Cape Blanc, is much more extensive than that of the coastal band of upwelling [Mittelstaedt, 1991], and establishes a plume of high pigment concentration extending offshore from the Mauritanian coastal zone to up to 450 km into the open ocean [Van Camp et al., 1991]. According to satellite-derived images, the filament occurs independently of the season and the local wind system [Van Camp et al., 1991], and persists with varying intensity throughout the year as well as between years [Barton, 1998; Helmke et al., 2005]. The large offshore transport of cool, nutrient rich water is thought to be partly responsible for both enhanced offshore primary production, and the occurrence of coastal phytoplankton found several hundred kilometers offshore [Gabric et al., 1996]. Present hydrographic and nutrient conditions off Mauritania are adequate for diatom occurrence [Romero et al., 2002], where diatom productivity depends on the availability of silicate in the upwelled waters just as the accumulation of opaline debris in sediments depends on the availability of silicate in the waters below the mixed layer [Ragueneau et al., 2000].

[4] In comparison to the Subarctic Atlantic, which is generally assumed to be the active trigger of both abrupt and long-term climatic changes, low-latitude ocean areas are often thought to have been passive during the Last Glacial Maximum (LGM) and the following deglaciation events [Alley and Clark, 1999]. Recent paleoclimatic data and climatic models, however, highlight the importance of the subtropics during the last glacial/interglacial transition [e.g., Peterson et al., 2000; Hendy and Kennett, 2003; Lea et al., 2003; Ivanochko et al., 2005]. A number of studies indicate that meltwater input and subsequent cooling of the North Atlantic should initiate low-latitude hydrologic changes via displacements of the Intertropical Convergence Zone (ITCZ) [e.g., deMenocal et al., 2000a; Lea et al., 2003; Chiang and Bitz, 2005]. Regarding late Quaternary climate variations in subtropical northern Africa, several multiparameter reconstructions illustrate the complex relationship between Saharan ecosystems and climate throughout the periods of aridification [Gasse, 2000; Garcin et al., 2007; Talbot et al., 2007; Kröpelin et al., 2008] as well as abrupt, large-scale changes along the NW African upwelling system [deMenocal et al., 2000a, 2000b; Kuhlmann et al., 2004; Adkins et al., 2006; Kim et al., 2007].

[5] Except for Site ODP658 [Zhao et al., 1995; deMenocal et al., 2000a, 2000b; Adkins et al., 2006], high-resolution climatic records are still lacking to define abrupt climatic and hydrographic variations during the LGM and the following deglacial period on submillennial-to-millennial time scales off Mauritania. To further assess climatic and hydrographic changes in the subtropical NE Atlantic, we studied a well-dated gravity core spanning the entire period from late MIS2 through the last deglaciation into the Holocene (0–25 ka B.P.). Site GeoB7926-2 is located in a transition zone off Mauritania between North and South Atlantic originated water masses, under the influence of the huge upwelling filament and the wind plume blowing from the northern African continent into the subtropical North Atlantic. Here we integrate high-resolution records of diatoms and bulk biogenic sediment components to infer past changes in productivity and upwelling intensity, reconstruct alkenone-derived sea surface temperatures (SSTs), and describe variations in wind intensity and land aridity/humidity based on Ti/Ca and K values. We focused our study on the submillennial-to-millennial variations of diatom paleoproductivity and its relation with the nutrient content of upwelled waters, ITCZ migrations, and SST variations off Mauritania during the last 25 ka.

2. Modern Oceanographic and Climatic Setting

[6] The Mauritanian shelf is generally narrow (about 45–55 km wide) with the exception of the Banc d'Arguin (about 140 km wide). The continental slope is about 45 km wide with an average inclination of 2–3°. Numerous small and several large canyons are located off the shelf edge [Fütterer, 1983, and references therein].

[7] Because of the fairly steady and strong trade winds off Cape Blanc, present-day upwelling occurs throughout the year and is most intense in April, June/July and October/November [Barton, 1998]. The 50–70 km broad coastal band of “primary upwelling” is separated by a front from the Canary Current where “secondary upwelling” occurs [Mittelstaedt, 1991]. Upwelled waters, with SSTs below 17°C [Mittelstaedt, 1991], are responsible for high primary production over the shelf and, especially, beyond the shelf break [Barton, 1998]. Present upwelling source waters are either the salty and relatively nutrient poor North Atlantic Central Water (NACW), present at latitudes down to 22–23°N, or the less saline and nutrient rich South Atlantic Central Water (SACW), present south of 20–21°N. SACW is modified from its original Southern Hemisphere form by mixing en route. Its immediate trajectory is via the North Equatorial Countercurrent and northward along the eastern boundary off of western Africa [Barton, 1998]. Where this water arrives at the confluence with the equatorward flow of the Canary Current, a major water mass front is established with the NACW at about 20–23°N [Zenk et al., 1991]. Additionally, an area of elevated pigment and nutrient concentration off Mauritania, the “upwelling filament,” extends far into the open ocean as the result of the combined effects of offshore export of upwelled nutrients [Van Camp et al., 1991] and in situ growth of phytoplankton in offshore waters [Helmke et al., 2005].

[8] Surface wind and rainfall distribution patterns over tropical Africa are primarily controlled by the migration of the ITCZ, which has a northerly position during boreal summer and a southerly position during boreal winter [Nicholson, 1986] (Figure 1). The ITCZ is a relatively simple, east-west oriented feature which separates the dry, NE trade winds from a moist, southwesterly airflow.

Figure 1.

Location of site GeoB7926-2 (black star) off Mauritania in the NE Atlantic Ocean. The location of Site ODP658 [deMenocal et al., 2000a] is also shown. Black arrows represent present-day surface water circulation. The insert at the lower right depicts the present-day location of the Intertropical Convergence Zone (ITCZ, dotted line) for August and February.

3. Material and Methods

3.1. Gravity Core GeoB7926-2

[9] Site GeoB7926-2 is ideally situated to monitor past variations in the climate and hydrography of northwest Africa. This site is positioned directly below the upwelling filament off Mauritania and the axis of the present-day African dust plume, which transports dust from the Subsaharan and Sahelian regions of NW Africa to the adjacent NE Atlantic (Figure 1). Marine gravity core GeoB7926-2 was recovered during R/V Meteor Cruise 53/1c (20°13′N, 18°27′W, 2500 m water depth) [Meggers and Cruise Participants, 2003]. The total core length is 1328 cm. In this work, we present results for the upper 820 cm (0–25 ka B.P.) The sediment color is predominantly olive in the upper 820 cm with two short olive gray intervals between 50 and 130 cm, and 350 and 390 cm depth. The dominant lithology is foraminifer bearing nanofossil or diatom ooze, with short intervals marked by clayey and quartz-bearing parts (between 543 and 558, 762 and 780, and 809 and 820 cm). Only a few turbidites, between 1 and 3 cm thick, with upward and downward fining features and erosional contacts at the base, are present throughout the upper 820 cm [Meggers and Cruise Participants, 2003].

[10] For this study, two sample series were taken: one of 1.5 cm3 at 1-cm intervals for diatoms and opal studies, and another one of ∼10 cm3 at 5-cm interval for stable isotopes, total organic carbon (TOC), calcium carbonate (CaCO3), and alkenone analyses. Average temporal resolution is <40 years for diatom, opal and XRF generated measurements in the interval from 25 to 10 ka B.P., whereas for the other proxies temporal resolution is ∼200 years.

3.2. Stratigraphy

[11] The age control for gravity core GeoB7926-2 is based on 17 Accelerator Mass Spectrometry (AMS) 14C dates determined on monospecific samples of the planktonic foraminifer Globigerina inflata (>150 μm fraction; Table 1, Leibniz Laboratory for Radiometric Dating and Stable Isotope Research, Kiel University, Germany). The 14C ages were converted into calendar years using Calib Execute Version 5.0.2 (http://calib.qub.ac.uk/) [Stuiver et al., 1998]. All ages were corrected for 13C and for a reservoir age of 400 years. Since no information on regional reservoir age is available for this region, we used the mean ocean reservoir age [Bard, 1988]. Reservoir ages, however, might be larger than the mean ocean reservoir age of 400 years because of the upwelling of older subsurface waters [deMenocal et al., 2000a]. Calendar ages between dated levels were obtained by linear interpolation between the nearest AMS dating points (Figure 2a).

Figure 2.

(a) 14C radiocarbon dates and (b) sedimentation rate (black line, cm ka−1) and total accumulation rate (blue line, g cm−2 ka−1) at site GeoB7926-2 for the past 25 ka (upper 820 cm).

Table 1. Age Control Points for Gravity Core GeoB7926-2
Lab NumberInterval (cm)Age (14C years B.P.)± Error (years)2σ (years B.P.)Agea (ΔR = 0 Years)b (cal years B.P.)Analyzed Material
  • a

    To convert uncorrected 14C ages into calendar ages, the CALIB REV5.0.2 program was used [Stuiver et al., 1998].

  • b

    Regional 14C reservoir ages (ΔR) are assumed to be 400 years [Bard, 1988].

KIA 24287811703550630Globigerina inflata
KIA 2581248445535724423G. inflata
KIA 2581098897045309495G. inflata
KIA 24286173100507018012230G. inflata
KIA 22417248108307017012452G. inflata
KIA 24285293105507016012850G. inflata
KIA 24285378122207010013602G. inflata
KIA 2903041813050709014485G. inflata
KIA 27310483142201008016286G. inflata
KIA 29029508142906011017199G. inflata
KIA 22416553146708014017535G. inflata
KIA 273096881710013025019293G. inflata
KIA 290288032045016030023854G. inflata
KIA 2428386823650220–23027027946G. inflata

3.3. Stable Oxygen Isotope Measurements

[12] Stable oxygen isotopes were measured on shells of the planktic foraminifer Globigerina bulloides with a Finnigan MAT 251 mass spectrometer (Isotope Laboratory, MARUM, Bremen). Twenty individual shells were picked for each measurement. The isotopic composition of the carbonate sample was measured on the CO2 gas evolved by treatment with phosphoric acid at a constant temperature of 75°C. For all stable oxygen isotope measurements a working standard (Burgbrohl CO2 gas) was used, which was calibrated against Vienna Pee Dee Belemnite (VPDB) by using the NBS 18, 19 and 20 standards. Consequently, all δ18O data given here are relative to the VPDB standard. Analytical standard deviation is about 0.07‰.

3.4. Diatom Analysis

[13] For the study of diatoms, samples were prepared following the method proposed by Schrader and Gersonde [1978]. Qualitative and quantitative analyses were done at ×1000 magnifications using a Zeiss Axioscope with phase contrast illumination. Counts were carried out on permanent slides of acid cleaned material (Mountex mounting medium). Several traverses across the coverslip were examined, depending on valve abundances (between 400 and 1300 valves per coverslip were counted). At least two coverslips per sample were scanned in this way. Diatom counting of replicate slides indicates that the analytical error of the concentration estimates is equation image15%. The counting procedure and definition of counting units for diatoms followed those proposed by Schrader and Gersonde [1978].

3.5. Bulk Geochemistry

[14] Samples for bulk geochemical analyses were freeze dried and ground in an agate mortar. Total carbon contents (TC) were measured on untreated samples. After decalcification of the samples with 6 N HCl, the total organic carbon (TOC) content was obtained by combustion at 1050°C using a Heraeus CHN-O-Rapid elemental analyzer [Müller et al., 1994]. Carbonate was calculated from the difference between TC and TOC, and expressed as calcite (CaCO3 = (TC – TOC) * 8.33). Opal content was determined by the sequential leaching technique by DeMaster [1981], with modifications by Müller and Schneider [1993].

3.6. X-Ray Fluorescence (XRF) Scanning

[15] The XRF scanner is a nondestructive method for high-resolution and relatively fast analyses of major and minor elements by scanning split sediment cores [Jansen et al., 1998]. Element intensities, given as counts per second (cps), were measured at MARUM (Bremen) on the split cores in 1-cm intervals in the upper 3.5 m of GeoB7926-2 and every 2 cm from 3.5 m down core. From the whole set of elements measured, we show here Ti/Ca and Potassium (K). Calcium (Ca) mainly reflects the biogenic carbonate content whereas titanium (Ti) is related to siliciclastic sediment components and varies directly with the terrigenous fraction of the sediment [Arz et al., 1998]. Potassium and illite, its main mineralogical carrier, have been used as indicators of terrigenous supply into the marine sediments along the NW African margin [Kuhlmann et al., 2004, and references therein].

3.7. Alkenone and SST Estimations

[16] To determine past SST variations, alkenones were extracted from 1 to 2 g portions of freeze-dried and homogenized sediment following the procedure described by Kim et al. [2002]. The extracts were analyzed by capillary gas chromatography using a gas chromatograph (HP 5890A) equipped with a 60 m column (J&W DB1, 0.32 mm x 0.25 μm), a split injector (1:10 split modus), and a flame ionization detector. Quantification of the alkenone content was achieved using squalane as an internal standard. The alkenone unsaturation index U37K′ was calculated from U37K′ = (C37:2)/(C37:2 + C37:3) as defined by Prahl and Wakeham [1987], where C37:2 and C37:3 are the diunsaturated and triunsaturated C37 methyl alkenones. The U37K′ values were converted into temperature values applying the culture calibration of Prahl et al. [1988] (U37K′ = 0.034 * T + 0.039), which has also been validated by core top compilations [Müller et al., 1998]. The precision of the measurements (±1σ) was better than 0.003 U37K′ units (or 0.1°C), on the basis of multiple extractions and analyses of a sediment sample used as a laboratory internal reference from the South Atlantic.

4. Results

4.1. Age Model and Planktonic δ18O

[17] We arbitrarily assume that changes in radiocarbon reservoir ages have been similar throughout the last 25 ka. The marine 14C reservoir correction was nearly twice as large as the modern value during the Younger Dryas (YD) because of changes in deep ocean circulation and related changes in ocean atmosphere radiocarbon partitioning [Bard et al., 1994]. Large increases in the intensity of Mauritanian upwelling (see below subchapters 5.2.) would have sufficed to increase the local apparent reservoir correction age of both Heinrich event 1 (H1) and the YD to a minimum of ∼800–900 years off Mauritania [deMenocal et al., 2000a, 2000b]. We did not force, however, the best possible match for the rapid deglacial events. The transition times for the events during Termination I must be considered maximum estimates because they were derived from radiocarbon dates of a nonlaminated marine sediment sequence, neither of which have the temporal resolution to precisely date decadal- to centennial-scale transitions. The LGM is temporally defined between 23.0 and 19.0 ka B.P.; Termination I between 19.0 and 11.5 ka B.P, and the Holocene started at 11.5 ka B.P. and extends into the present. The rapid events within the deglaciation are temporally constrained as follows: Heinrich event 1 (H1): 18.0–15.5 ka B.P.; Bølling-Allerød (B/A): 15.5–13.5 ka B.P., and Younger Dryas (YD): 13.5–11.5 ka B.P.

[18] The δ18O record of the planktonic foraminifer Globigerina bulloides at site GeoB7926-2 exhibits moderate amplitudes (∼1.6 to −0.3‰) for the last 25 ka B.P. (Figure 3). Isotopic values range from 1.4 to 1.0‰ for the last MIS2 and the entire LGM (19.0–23.0 ka B.P.). A sharp enrichment occurred around 17.5 ka B.P., associated with the beginning of H1 (15–17.5 ka). From 13.6 through 12.5 ka B.P., δ18O varies between 0.9 and −0.1‰. The Holocene (11.5–0 ka B.P.) is marked by relatively depleted values of 0.3 to −0.3‰.

Figure 3.

Down core variation of accumulation rates and δ18O (‰ VPDB) of the planktonic foraminifer Globigerina bulloides at site GeoB7926-2. Diatoms (red line, valves cm−1 ka−2), opal (black line, g cm−1 ka−2), TOC (green line, g cm−1 ka−2), CaCO3 (black line, g cm−1 ka−2), total accumulation rates (blue line, g cm−1 ka−2), and δ18O (purple line, ‰ VPDB) of the planktonic foraminifer Globigerina bulloides during the past 25 ka B.P. at GeoB7926-2. Shadings are as follows: light blue, LGM (Last Glacial Maximum, 23.0–19.0 ka B.P.); light yellow, H1 (Heinrich event 1, 18.0–15.5 ka B.P.) and YD (Younger Dryas, 13.5–11.5 ka B.P.). B/A, Bølling-Allerød (15.5–13.5 ka B.P.). Open triangles at the top represent 14C radiocarbon dating points (see Table 1).

4.2. Variations of Sedimentation and Accumulation Rates

[19] Considering its hemipelagic location, site GeoB7926-2 has an unusually high average sedimentation rate (∼96 cm ka−1) for the time period 0–25 ka. The sedimentation rate ranges between ∼110 and ∼320 cm ka−1 during H1 and the YD (Figure 2b). The variation pattern of the total accumulation rate (AR) at this site is strongly defined by the 14C stratigraphy for the last 25 ka (Figure 2b). The highest total ARs of 24.3–48.1 and 49.7–102.7 g cm−2 ka−1 are noted at 19.3–17.2 ka B.P. and 13.6–12.2 ka B.P., respectively. The lowest AR is recorded between 9.3 ka B.P. and the present (5.4–3.9 g cm−2 ka−1). The AR of biogenic compounds in bulk biogenic sediment (Figure 3) shows a strong similarity in their general trend with that of the total AR. Among the bulk sediment components, CaCO3 records the highest AR (47.3 g cm−2 ka−1 at 12.2 ka B.P.), followed by opal (up to 3.4 g cm−2 ka−1) and TOC (up to 2.4 g cm−2 ka−1 at 12.2 ka B.P.). Diatom AR is highest between 12.2 and 12.4 ka B.P. (6.2 – 5.7 × 107 valves cm−2 ka−1).

4.3. Diatom Variations

[20] Throughout the LGM (19.0–23.0 ka B.P.) and the following deglacial events, diatom concentration exhibits strong variations on submillennial time scales (Figure 4). Diatom concentrations were low before 22.0 ka B.P. and increased steadily toward the end of the LGM. Concentrations were the highest during H1 (18.0–15.5 ka B.P.) and the YD (13.5–11.5 ka B.P.) and decreased again into the Holocene (Figure 5a). We identified ∼200 diatom species. The diatom assemblage is mostly dominated by a highly diversified community of resting spores (RS) of the centric genus Chaetoceros (Figure 5a). Virtually no vegetative cells of Chaetoceros were recorded. The relative contribution of RS of Chaetoceros was highest between the LGM and the late YD, with contributions exceeding 60% of the total diatom assemblage. The dominance of Chaetoceros RS dropped by the end of the YD and at this time, the most important contributor of RS was C. affinis (average = 61.0%) accompanied by C. pseudo-brevis, C. diadema, C. compresus, and Chaetoceros sp. 1 (Figure 5a). The highest contribution of C. affinis spores correlates well with high diatom concentration between the LGM and early H1. Between H1 and the late YD, relative contribution of C. affinis RS becomes anticorrelated with the total diatom concentration. Spores of C. pseudo-brevis, C. diadema, C. compresus and Chaetoceros sp. 1 contributed the most around 17.0 ka B.P., between 15.5 and 14.5 ka B.P. and again during the first half of the YD.

Figure 4.

Down core variation of total diatom concentration (red line, valves g−1), opal (black line, wt %), TOC (green line, wt %), CaCO3 (black line, wt %), alkenone-derived SST (orange line, °C), K intensity (blue line, counts per second (cps)), Ti/Ca ratio and δ18O (‰ VPDB) of the planktonic foraminifer Globigerina bulloides (purple line) during the past 25 ka at GeoB7926-2, and δ18O (‰ SMOW) from Greenland (GRIP). The Last Glacial Maximum (LGM, 23.0–19.0 ka B.P.) is indicated by the blue shaded area while Heinrich event 1 (H1, 18.0–15.5 ka B.P.) and the Younger Dryas (YD, 13.5–11.5 ka B.P.) are indicated by the yellow shaded areas. Open triangles at the top represent 14C radiocarbon dating points (see Table 1).

Figure 5a.

Down core variations of total diatom concentration (red line, valves g−1), the relative concentration of the entire community of Chaetoceros resting spores (black line, %), and the relative contribution (%, five-points running average) of the most abundant Chaetoceros spores: C. affinis (blue line, %), C. pseudo-brevis (black line, %), C. diadema (orange line, %), Chaetoceros sp. 1 (black line, %), and C. compresus (green line, %) at site GeoB7926-2 during the past 25 ka. The gray line in the background for each species represents nonaveraged values. The Last Glacial Maximum (LGM, 23.0–19.0 ka B.P.), Heinrich event 1 (H1, 18.5–15.5 ka B.P.), the Younger Dryas (YD, 13.5–11.5 ka B.P.), and the Bølling-Allerød (B/A, 15.5–13.5 ka B.P.) are indicated.

[21] The Chaetoceros spores are accompanied by a highly diversified community of diatoms, representing a variety of different hydrographic conditions and productivity regimes. Most of these secondary components had only minor contributions before the YD (Figure 5b). Thalassionema nitzschioides var. capitulata peaked between 15.5 and 14.0 ka B.P., followed by a maximum of Actinocyclus octonarius. The relative contribution of Chaetoceros RS decreased by the beginning of the YD and a more diverse diatom community, composed by open ocean, neritic and tycoplanktonic representatives, appeared. Thalassionema nitzschioides var. nitzschioides, Planktoniella sol and Biddulphia alternans contributed the most during the early YD, followed by enhanced values of Thalassiosira oestrupii var. oestrupii during the second half of YD and the early Holocene (Figure 5b). The tycoplanktonic diatom Paralia sulcata increased steadily during the Holocene.

Figure 5b.

Down core variation of total diatom concentration (red line, valves g−1) and the relative concentration (%, five-points running average) of the selected diatom species at GeoB7926-2 during the past 25 ka: Thalassionema nitzschioides var. capitulata (black line, %), Actinocyclus octonarius (blue line, %), Thalassionema nitzschioides var. nitzschioides (black line, %), Planktoniella sol (orange line, %), Fragilariopsis doliolus (black line, %), Thalassiosira oestrupii var. oestrupii (green line, %), Paralia sulcata (black line, %), and Biddulphia alternans (dark brown line, %). The gray line in the background for each species represents nonaveraged values. Shadings are as follows: light blue, LGM (Last Glacial Maximum, 23.0–19.0 ka B.P.); light yellow, H1 (Heinrich event 1, 18.0–15.5 ka B.P.); and YD (Younger Dryas, 13.5–11.5 ka B.P.). B/A, Bølling-Allerød (15.5–13.5 ka B.P.).

4.4. Bulk Components

[22] Site GeoB7926-2 documents rapid and large amplitude changes in sediment composition between the latest Pleistocene and the Holocene. CaCO3 dominated the bulk biogenic sedimentation, followed by opal and TOC. CaCO3 values fluctuated between 30% and 42% between late MIS2 and the early Holocene, and abruptly increased at the transition of Termination I into the Holocene (Figure 4). Opal contribution was moderate before H1 but reached high values during H1 and the YD (up to 27%; Figure 4) and decreased at the transition of Termination I into the Holocene. TOC closely follows the variation pattern of opal, and is mostly anticorrelated to CaCO3. TOC values range from 1.5 to 2.0% during the LGM but decrease into Termination I and then increase at the beginning of H1. TOC decreased below 2% by the early Holocene.

4.5. X-Ray Fluorescence

[23] Here we present variations in Ti/Ca as an indicator of wind strength, and K as a proxy for land aridity/humidity. Ti/Ca ranged from 0.0045 to 0.046 and K intensity varied between ∼30 and 250 cps. Both proxies are highly variable on submillennial-to-millennial time scales (Figure 4). Ti/Ca oscillated mostly between 0.02 and 0.03 throughout the LGM and peaked at ∼21 ka B.P. Ti/Ca variations accompanied rapid deglaciation events: it increases into H1, decreases into the BA, and reached its highest values of the highest record during the YD. Ti/Ca decreases again in the early Holocene and then steadily increases toward the late Holocene. The K intensity was highest before and after rapid deglaciation events and values fluctuate between ∼130 and 210 cps before H1. K intensity decreases into H1, increases during the BA, and decreases again during the YD. Shortly after the YD, an increasing trend initiated and persists well into the late Holocene.

4.6. Alkenone-Derived SST

[24] Alkenone-derived SST reveals a narrow range of variation between 17.2 and 21.5°C over the last 25 ka (Figure 4). During the LGM, the SST varied narrowly between 20.5 and 21.3°C. Toward the transition into Termination I, SST decreased down to ∼19°C with variations of ∼1°C on the submillennial time scale (i.e., around 18.7–18.5 ka B.P.). This decreasing trend continued well into H1. Submillennial time scale SST variations intensified before H1, mainly between 17.7 and 16.6 ka B.P., and continued throughout the deglaciation events (the fastest between ∼16.0 and 15.0 ka B.P., ∼2.3°C difference). The lowest SST of the last 25 ka is recorded during H1. Following H1, SST increased again after 15.8 ka B.P. while the YD marks a return to lower SST. For most of the Holocene, SST varied between 19.5 and 21.3°C.

5. Discussion

[25] Late Quaternary variations of productivity and upwelling intensity in eastern boundary current systems are thought to be uniquely linked to the variability in wind stress [e.g., Summerhayes et al., 1995; Bertrand et al., 1996; deMenocal et al., 2000a, 2000b; Nave et al., 2003]. However, our high-resolution records from the highly dynamic Mauritanian upwelling area suggest that the interplay of several oceanographic processes, including variations in nutrient content of the upwelled waters and the offshore extension of the upwelling filament overlying site GeoB7926-2, coupled with atmospheric processes, such as the latitudinal migration of the ITCZ and changes in wind intensity, markedly influenced diatom productivity during the last 25 ka.

5.1. Last Glacial Maximum (23.0–19.0 ka B.P.)

[26] During the LGM, Meridional Overturning Circulation (MOC) experienced a moderate slowdown in the North Atlantic [Duplessy et al., 1988; McManus et al., 2004], but this signal still propagated into the subtropical NE Atlantic via the Canary Current [Zhao et al., 1995] and the NACW. At present, different water masses upwell in the northern and southern regions of the Mauritanian coast and the transition between the two regions is located approximately off Cape Blanc (∼21°N) [Zenk et al., 1991]. The saltier, oligotrophic NACW dominates north of 21°N, while the less saline, cooler, nutrient richer SACW prevails south of it [Barton, 1998]. The boundary between NACW and SACW is convoluted, variable in position, and characterized by intense mixing and interleaving processes [Mittelstaedt, 1991, and references therein]. We interpret the moderate diatom productivity off Mauritania during the LGM to be mainly due to upwelling of NACW at site GeoB7926-2, which supplied silica poor water. With a moderate content of silica in surface waters, diatom production remained relatively low, in comparison with values reached later during H1 and the YD (see below 5.2.). This regime of moderate siliceous productivity under full glacial conditions resembles the late Quaternary glacial scenario in pelagic areas from the western equatorial Pacific [Berger and Lange, 1998], and from other eastern boundary current systems such as the Santa Barbara Basin [Berger and Lange, 1998] and the central Benguela System [Romero et al., 2003a]. Berger and Lange [1998] suggested that reduced diatom accumulation during glacials in sediments underlying the California Current may reflect a silicate depleted thermocline due to the shutdown or slowdown of NADW production. Preferential dissolution during the LGM, another possible explanation for moderate opal values, is ruled out during glacial periods off Mauritania. Light microscopy observations of the diatom community at site GeoB7926-2 revealed that the main components, Chaetoceros RS, are well preserved and show only minor dissolution effects throughout the LGM.

[27] The relatively high contribution Chaetoceros spores in eastern boundary current systems is usually interpreted as straightforward indication of intense upwelling and high surface water productivity [e.g., Nave et al., 2003]. Our results, however, suggest that different species of Chaetoceros spores are not equally associated with the same intensity of coastal upwelling off Mauritania. Instead, different populations of Chaetoceros might have responded differently to varying oceanic and atmospheric conditions throughout the last 25 ka. Although the ecology of particular resting spores of Chaetoceros from low-latitude eastern boundary current systems is still poorly known [Romero et al., 2003a], we speculate that RS of C. affinis, the most abundant spore, is ecologically ubiquitous, dominating under different levels of silica content and upwelling intensity. Similar observations from other eastern boundary current systems (central Benguela system [Romero et al., 2003a] and the southern Peru-Chile Current [Romero et al., 2006; O. E. Romero, unpublished observations, 2008]) suggest a comparable behavior of C. affinis. Therefore, caution is recommended with the simple interpretation of high relative concentrations of the entire Chaetoceros RS community as indicator of highly productive surface waters in low-latitude coastal areas during the late Quaternary. We propose that the identification and counting of RS on the species level provides significant ecological information on productivity dynamics of low-latitude eastern boundary current systems.

[28] Moderate Ti/Ca values during the LGM suggest weakened eolian input through NE trade winds (Figures 6a and 6b). The weakening of NE trades implies, in turn, that the intensity of the Saharan Air Layer (SAL) might have been strong at the time. Blowing at altitudes between 5 and 6 km in modern times, the westward SAL reaches its maximum strength and highest dust load during the summer months, roughly focusing between 17 and 21°N [Chiapello et al., 1995]. Air mass trajectories show that source regions of eolian transported material deposited off Mauritania are widespread across northern Africa [Romero et al., 2003b], in good agreement with model representations [Marticorena et al., 1997]. We propose that SAL winds dominated over trade winds during the LGM, therefore impeding the efficient offshore Ekman transport and weakening upwelling intensity and the length of the upwelling filament (Figures 6a and 6b). Evidence from north of site GeoB7926-2 suggests that the season of westward SAL winds was longer during the LGM [Martinez et al., 1999, and references therein] while land records demonstrate that active desert dunes migrated seaward during the LGM in response to a strengthened SAL [Lancaster et al., 2002]. No significant cooling is observed during the LGM, compared to the Holocene, suggesting that our alkenone SST record represents a local upwelling-related signal rather than a global insolation related one. This supports our interpretation that NE trade winds over the core site were weakened, and thus the intensity of upwelling and the primary productivity decreased during the LGM.

Figure 6.

Schematic representation of changes in atmospheric and hydrographic features off Mauritania for (a and b) the Last Glacial Maximum (23.0–19.0 ka B.P.), (c and d) Heinrich event 1 (18.0–15.5 ka B.P.), (e and f) the Bølling-Allerød (15.5–13.5 ka B.P.), (g and h) the Younger Dryas (13.5–11.5 ka B.P.), and (i and j) the Holocene/Present (11.5–0 ka B.P.). Figures 6b, 6d, 6f, 6h, and 6j show the Saharan Air Layer (SAL, light orange arrow), trade winds (TW, yellow arrow), contribution of water masses (dark blue and purple arrows), and offshore extension of the upwelling filament (green dashed line). The orange star represents the location of site GeoB7926-2. In Figures 6a, 6c, 6e, 6g, and 6i, arrows represent surface circulation (gray, Canary Current; green, North Equatorial Countercurrent) while the dotted purple line represents the northernmost and southernmost present-day location of the Intertropical Convergence Zone (ITCZ) for August and February, respectively. Differences in letter size of NACW (dark blue) and SACW (purple) and arrow thickness and length represents the strength of their contribution (larger means stronger, and smaller means weaker) to upwelling water off Mauritania.

[29] Given the present-day validity of the Ekman transport mechanism and the extended scale of the predominant trade winds that drive it [Barton, 1998], one might expect that coastal upwelling along the northwestern African coast would be quite uniform over long stretches. The productivity response in the subtropical NE Atlantic, however, differed latitudinally between 15 and 30°N during the LGM. Productivity increases noted in the northernmost latitudes along the NW African continent between 25 and 32°N have been mainly attributed to variations in wind stress [Bertrand et al., 1996; Nave et al., 2003]. Our results for the LGM resemble those of Martinez et al. [1999], who show that productivity diminished in the overlying waters in the midslope off Mauritania. This latitudinal heterogeneity along the NW African coast suggests regional differences with (1) a stronger seasonality north of 21°N due to westerlies during one season and along shore trade winds, favorable for upwelling, during the other [Rognon and Coudé-Gaussen, 1996] and/or with (2) a contrasted effect of the wind stress and direction depending on the orientation of the coastline. Accordingly, these regional differences also involved differences in upwelling intensity and the strength of the offshore extension of upwelling filaments along NW Africa between Mauritania and Morocco.

5.2. Termination I (19.0–11.5 ka B.P.)

5.2.1. Heinrich Event 1 (18.0–15.5 ka B.P.)

[30] The North Atlantic largely experienced dramatic cooling during H1 [Clark et al., 2001; Bond et al., 1992]. The catastrophic iceberg discharge and freshening at high latitudes during H1 caused MOC, sensitive to buoyancy forcing through surface salinity perturbations, to resemble a “drop-dead circulation” with negligible overturning [McManus et al., 2004.] The massive freshwater influx slowed down the MOC and the associated climate signal was propagated into the subtropical NE Atlantic via the Canary Current [Zhao et al., 1995]. This effect probably caused much cooler SST during H1 than earlier during the LGM at 21°N off NW Africa. Records from the western tropical Atlantic [Lea et al., 2003] and modeling studies [Chiang and Bitz, 2005] suggest that meltwater input into the North Atlantic and the associated MOC collapse might have been accompanied by a southward shift of the ITCZ and consequent cooling off the Mauritanian coast [Vellinga and Wood, 2002; Lohmann, 2003] (Figures 6c and 6d). Further insight into the atmospheric conditions is provided by our XRF data. Decreased SST and K values after 17.3 ka B.P. suggest that the Mauritanian upwelling area returned to cold and dry conditions due to intensified trade winds. Moreover, the high sedimentation rates during H1 might be related to a longer trade wind season before 17.3 ka B.P.

[31] The high diatom and opal concentrations during H1 off Mauritania points to increased silica content in upwelled waters coupled with intensified NE trade winds (Figures 6c and 6d). The slowdown of the MOC during H1 [McManus et al., 2004] might have caused the detachment of the Canary Current farther north of 21°N off NW Africa and, hence, caused the predominance of the SACW as upwelling water, supplying the thermocline with silica rich waters. Additionally, intensified NE trade winds might have contributed to more intense offshore Ekman transport building, in turn, a larger upwelling filament than during the LGM, which streamed farther offshore into the open ocean off Mauritania. The large field of the upwelling filament is much more extensive than that of the coastal band of upwelling [Helmke et al., 2005], and allows for a substantial or even major portion of the total primary production to be attributed to upwelling that directly takes place [Barton, 1998]. The seaward transport of nutrient rich waters and its constituent biota from the shelf and upper slope region favored upwelling over GeoB7926-2 and provided favorable conditions for diatom productivity during H1.

[32] The increased supply of upwelled nutrients also might have greatly enlarged the zone where spores of Chaetoceros and accompanying species thrive, displacing farther offshore the boundary between eutrophic and mesotrophic water masses. The complex valve ornamentation of Chaetoceros spores favors the formation of aggregates, which facilitates the rapid sinking of valves produced in surface waters and largely reduces the time spent by the valves in silica undersaturated deep waters [Ragueneau et al., 2000, and references therein]. This is clearly reflected by the very good valve preservation at GeoB7926-2 during H1 and the YD. Present-day sinking of particulates off Mauritania reach speeds of up to 800 m d–1 [Bory et al., 2001]. Although recent studies show that calcareous phytoplankton acts as ballast mineral and facilitates rapid sedimentation of organic matter within high-density aggregates [Klaas and Archer, 2002], our observations at site GeoB7926-2 suggest that diatoms can also play an active role in transporting organic components down the water column and might act as ballast in eastern boundary current systems. Our suggestion is additionally supported by observations gained at the sediment trap deployed at 18°W over the Mauritanian slope where measured opal and diatom fluxes are two to three times higher than (O. E. Romero, unpublished data, 2008) those previously measured at the outer, pelagic site CB (21°N, 20°W [Romero et al., 2002]).

5.2.2. Bølling-Allerød (15.5–13.5 ka B.P.)

[33] The synchronous increase in CaCO3 concentration, SST and K intensity, coupled with decreased values of opal and diatom concentrations, marks the occurrence of the B/A off Mauritania (Figure 5). A decrease in diatom values around 15.2–15.4 ka B.P. reflects the rapid response to less favorable temperature and nutrient conditions in surface waters. Slightly decreased Ti/Ca values compared to H1, and increased K intensity, indicates weakened eolian input and more humid conditions on land than earlier during H1. Although this increase must have been partially due to increasing vegetation cover on the adjacent continent [Gasse and Roberts, 2005], it may also reflect an abrupt decline in the transport capacity of the trade winds blowing from the North African continent, coupled with the greater northward penetration of the ITCZ (Figures 6e and 6f). Such a scenario is compatible with the general decline in eolian activity across northwestern Africa at the end of the abrupt cooling during the deglaciation [Swezey, 2001; Lancaster et al., 2002].

[34] Although RS of Chaetoceros still dominated, quantitative and qualitative variations within the Chaetoceros community, coupled with the occurrence of the pelagic warm water diatom Thalassionema nitzschioides var. capitulata, suggest decreased silica content in upwelled waters during H1. At the end of H1, the heat and salt release from the tropical western Atlantic to the northern high latitudes may have accelerated and amplified warming, and pushed MOC into its interglacial mode [Lohmann, 2003; Weaver et al., 2003]. The rapidity and magnitude of the B/A warming resulted in large part from the reinvigoration of the Atlantic MOC [McManus et al., 2004]. This might, in turn, have strengthened the southward intrusion of silica depleted NACW, and thus caused an abrupt decrease in diatom production off Mauritania.

5.2.3. Younger Dryas (13.5–11.5 ka B.P.)

[35] Together with H1, the YD stands out as the major climatic reorganization during the last deglaciation off Mauritania. A major effect on the surface waters was cooling during the YD and the rapid response of the biogenic components to changing hydrographic and atmospheric conditions. The high sedimentation and accumulation rates during the YD were a consequence of the high surface water productivity and the enhanced lithogenic input due to strengthened NE trade winds and the increased land aridity over the northern African continent at that time [Lancaster et al., 2002; Talbot et al., 2007]. The rapid shift to Heinrich-like conditions, mainly as response to the slowdown of MOC [McManus et al., 2004] and upwelling water predominantly supplied by SACW (Figures 6g and 6h), caused increased concentrations of opal and diatoms. Although atmospheric conditions off Mauritania during the YD resembled those previously discussed for H1, less intense cooling coupled with enhanced wind intensity and the highest sedimentation rates suggests that wind played a crucial role in determining upwelling intensity and seaward Ekman transport. Collapse or slowdown of MOC in the northern North Atlantic [McManus et al., 2004] and southward displacement of the ITCZ during the YD in the Northern Hemisphere [Hughen et al., 1996; Lea et al., 2003; Jennerjahn et al., 2004; Talbot et al., 2007] may have led to stagnation and accumulation of heat and salt in the tropical western Atlantic [Schmidt et al., 2004; Kim and Schneider, 2003]. The occurrence of a more diverse diatom community at GeoB7926-2 reflects changes in the hydrographic and nutrient conditions of the overlying waters. Favorable conditions for diatom productivity in surface waters still prevailed during the YD, although weak incursions of open ocean waters into the coastal area of Cape Blanc are reflected by the occurrence of diatoms typical of pelagic, oligotrophic waters such as Planktoniella sol, Fragilariopsis doliolus and both varieties of Thalassiosira oestrupii [Romero et al., 2002].

[36] Strong regional similarities are recognized in association with rapid deglacial changes and the climatic evolution of the African continent and the subtropical NE Atlantic. In northern Africa, the YD has been associated with a sharp increase in regional aridity [e.g., Gasse, 2000]. Widespread eolian activity prior to 11.5 ka B.P., as revealed by our Ti/Ca record, is reported throughout the Sahara, whereas humid conditions and dune stabilization started by the end of the YD [Lancaster et al., 2002]. In good agreement, our marine record GeoB7926-2 documents a brief interval of increased eolian transport and lower humidity, closely followed in time by the formation and migration of dunes in western Mauritania [Lancaster et al., 2002]. Paleoceanographic records from the tropical and subtropical western Atlantic also provide evidence for stronger NE trade winds during the YD [Hughen et al., 1996; Lea et al., 2003; Jennerjahn et al., 2004]. The decrease in opal and diatom concentrations points to less favorable conditions for siliceous productivity, probably due to the predominance of silica depleted waters of the NACW. The strong diminution in sedimentation and accumulation rates by the end of the last deglaciation also points to lessened productivity off Mauritania. At this time, land records document the resumption of monsoonal activity and increased latitudinal migrations of the ITCZ over the African continent [Garcin et al., 2007; Talbot et al., 2007]. At a global scale, synchronous reorganizations of atmospheric circulation are observed in tropical regions (i.e., in South America, east and west Asia, and Africa) and support the role of strengthened monsoon activity in the tropics as a climatic amplifier during this major climatic boundary [Ivanochko et al., 2005, and references therein], generally associated with northern high latitudes temperature and precipitation changes [Alley and Clark, 1999, and references therein].

5.3. Holocene (11.5–0 ka B.P.)

[37] According to the present stratigraphic framework, core GeoB7926-2 experienced an abrupt decrease in sedimentation and accumulation rates around 9.4 ka B.P. Rapid Holocene changes off Mauritania have been previously described and thoroughly discussed [deMenocal et al., 2000a, 2000b; Adkins et al., 2006]. As already described for ODP658C, CaCO3 dominated over opal and surface waters became warmer off Mauritania. This rapid decrease in sedimentation rate points to the weakened influence of NE trade winds, resembling the scenario described above for the B/A (Figures 6i and 6j).

[38] Throughout the Holocene, decreased diatom productivity and weakened offshore spreading of the upwelling filament is mirrored by the lowest diatom concentrations and the lowest relative contribution of RS of Chaetoceros during the last 25 ka. This scenario of moderate to low diatom productivity during the Holocene has already been described for surface sediments of late Holocene age off Mauritania [Romero et al., 2002]. The diatom assemblage preserved in the upper part of GeoB7926-2 is dominated by robust to moderately silicified neritic species (Paralia sulcata, Biddulphia alternans), mostly associated with periods of very low diatom flux in sediment traps off Cape Blanc [Romero et al., 2002]. For other pelagic species with moderately silicified valves, such as Fragilariopsis doliolus and Planktoniella sol, relative abundances in sediment traps are similar to those found in the surface sediments, and their occurrence in the preserved record indicates inshore incursions of open ocean waters into the coastal area off Cape Blanc. The very low diatom productivity throughout the Holocene reflects a reduction in coastal upwelling intensity compared to H1 and the YD.

6. Concluding Remarks

[39] Site GeoB7926-2 offers an exceptionally high resolution record of environmental changes off Mauritania, NW Africa, during the LGM and the last deglaciation. Our multiproxy approach demonstrates that changes in productivity follow a clear submillennial-to-millennial pattern. The silica content of upwelled waters, the seaward extension of the upwelling filament and fluctuations in wind strength are most likely the main processes driving submillennial-to-millennial variations in diatom productivity off Mauritania.

[40] The occurrence of silica depleted NACW, as upwelling water, along with weakened NW trade winds, resulted in moderate diatom productivity off Mauritania during the LGM (23.0–19.0 ka B.P.). Rather than a global insolation related SST signal, our alkenone SST reconstruction suggests a local, upwelling-related signal for the LGM and supports the occurrence of weakened NE trade winds. The physical forcing for the Mauritanian upwelling system changed during the last deglaciation. The thermocline silica content increased during H1 (18.0–15.5 ka B.P.) and the YD (13.5–11.5 ka B.P.), resulting in exceptionally high diatom production. Rapid offshore advection within the upwelling filament transported cold, nutrient rich waters, and its associated biota, from the Mauritanian coastal area into the hemipelagic ocean, greatly extending the area of high productivity associated with upwelling. Drier land conditions, stronger trade winds, and hence greater terrigenous input from northern Africa, were probably present, because of a more southerly position of the ITCZ.

[41] The production of huge amounts of Chaetoceros spores played a decisive role in the sinking of autochthonous, near surface grown diatoms and the consequent delivering of huge amounts of opal toward the sea bottom. Caution, however, is recommended in the interpretation of high relative contribution of the entire community of Chaetoceros spores as a simple indicator of (high) upwelling intensity in low-latitude eastern boundary current systems during the late Quaternary, since particular Chaetoceros species seem to respond differently to different nutrient and temperature conditions. Under favorable nutrient conditions, diatoms act as important carriers of organic matter to the seafloor in eastern boundary current systems. Moreover, this high correlation reflects the ability of diatoms to quickly respond to physically forced mixing events, and other temporally short-lived conditions, and to out-compete nonsiliceous organisms for new nutrients.

Acknowledgments

[42] We thank H. Buschof, M. Klann, B. Meyer, and M. Segl for their lab work (MARUM, Bremen). Ursula Roehl and her team are acknowledged for their help with the X-ray fluorescence scanner (MARUM, Bremen). Radiocarbon analyses were performed at the Kiel Leibniz Laboratory for Radiometric Dating and Stable Isotope Research. The data are available from the PANGAEA database (http://www.pangaea.de). Isla Castañeda improved the English language. Three anonymous reviewers and Editor G. Dickens are greatly acknowledged for their contributions in correcting and improving this work.

Ancillary