Benthic δ13C Research Articles

Astronomically paced changes in deep-water circulation in the western North Atlantic during the middle Eocene

North Atlantic Deep Water (NADW) currently redistributes heat and salt between Earth's ocean basins, and plays a vital role in the ocean-atmosphere CO2 exchange. Despite its crucial role in today's climate system, vigorous debate remains as to when deep-water formation in the North Atlantic started. Here, we present datasets from carbonate-rich middle Eocene sediments from the Newfoundland Ridge, revealing a unique archive of paleoceanographic change from the progressively cooling climate of the middle Eocene. Well-defined lithologic alternations between calcareous ooze and clay-rich intervals occur at the ∼41-kyr beat of axial obliquity. Hence, we identify obliquity as the driver of middle Eocene (43.5–46 Ma) Northern Component Water (NCW, the predecessor of modern NADW) variability. High-resolution benthic foraminiferal δ18O and δ13C suggest that obliquity minima correspond to cold, nutrient-depleted, western North Atlantic deep waters. We thus link stronger NCW formation with obliquity minima. In contrast, during obliquity maxima, Deep Western Boundary Currents were weaker and warmer, while abyssal nutrients were more abundant. These aspects reflect a more sluggish NCW formation. This obliquity-paced paleoceanographic regime is in excellent agreement with results from an Earth system model, in which obliquity minima configurations enhance NCW formation.

Changes of deep Pacific overturning circulation and carbonate chemistry during middle Miocene East Antarctic ice sheet expansion

East Antarctic ice sheet expansion (EAIE) at ∼13.9 Ma in the middle Miocene represents a major climatic event during the long-term Cenozoic cooling, but ocean circulation and carbon cycle changes during this event remain unclear. Here, we present new fish teeth isotope (εNd) and benthic foraminiferal B/Ca records from the South China Sea (SCS), newly integrated meridional Pacific benthic foraminiferal δ18O and δ13C records and simulated results from a biogeochemical box model to explore the responses of deep Pacific Ocean circulation and carbon cycle across EAIE. The εNd and meridional benthic δ13C records reveal a more isolated Pacific Deep Water (PDW) and a sluggish Pacific meridional overturning circulation during the post-EAIE with respect to the pre-EAIE owing to weakened southern-sourced deep water formation. The deep-water [CO2−3] and calcium carbonate mass accumulation rate in the SCS display markedly similar increases followed by recoveries to the pre-EAIE level during EAIE, which were probably caused by a shelf–basin shift of CaCO3 deposition and strengthened weathering due to a sea level fall within EAIE. The model results show that the ∼1‰ positive δ13C excursion during EAIE could be attributed to increased weathering of high-δ13C shelf carbonates and a terrestrial carbon reservoir expansion. The drawdown of atmospheric CO2 over the middle Miocene were probably caused by combined effects of increased shelf carbonate weathering, expanded land biosphere carbon storage and a sluggish deep Pacific meridional overturning circulation.

Late Holocene intensification of the westerly winds at the subantarctic Auckland Islands (51° S), New Zealand

Abstract. The Southern Hemisphere westerly winds (SHWWs) play a major role in controlling wind-driven upwelling of Circumpolar Deep Water (CDW) and outgassing of CO2 in the Southern Ocean, on interannual to glacial–interglacial timescales. Despite their significance in the global carbon cycle, our understanding of millennial- and centennial-scale changes in the strength and latitudinal position of the westerlies during the Holocene (especially since 5000 yr BP) is limited by a scarcity of palaeoclimate records from comparable latitudes. Here, we reconstruct middle to late Holocene SHWW variability using a fjord sediment core collected from the subantarctic Auckland Islands (51° S, 166° E), located in the modern centre of the westerly wind belt. Changes in drainage basin response to variability in the strength of the SHWW at this latitude are interpreted from downcore variations in magnetic susceptibility (MS) and bulk organic δ13C and atomic C ∕ N, which monitor influxes of lithogenous and terrestrial vs. marine organic matter, respectively. The fjord water column response to SHWW variability is evaluated using benthic foraminifer δ18O and δ13C, both of which are influenced by the isotopic composition of shelf water masses entering the fjord. Using these data, we provide marine and terrestrial-based evidence for increased wind strength from ∼ 1600 to 900 yr BP at subantarctic latitudes that is broadly consistent with previous studies of climate-driven vegetation change at the Auckland Islands. Comparison with a SHWW reconstruction using similar proxies from Fiordland suggests a northward migration of the SHWW over New Zealand during the first half of the last millennium. Comparison with palaeoclimate and palaeoceanographic records from southern South America and West Antarctica indicates a late Holocene strengthening of the SHWW after ∼ 1600 yr BP that appears to be broadly symmetrical across the Pacific Basin. Contemporaneous increases in SHWW at localities on either side of the Pacific in the late Holocene are likely controlled atmospheric teleconnections between the low and high latitudes, and by variability in the Southern Annular Mode and El Niño–Southern Oscillation.

Enhanced δ13C and δ18O Differences Between the South Atlantic and South Pacific During the Last Glaciation: The Deep Gateway Hypothesis

AbstractEnhanced vertical gradients in benthic foraminiferal δ13C and δ18O in the Atlantic and Pacific during the last glaciation have revealed that ocean overturning circulation was characterized by shoaling of North Atlantic sourced interior waters; nonetheless, our understanding of the specific mechanisms driving these glacial isotope patterns remains incomplete. Here we compare high‐resolution depth transects of Cibicidoides spp. δ13C and δ18O from the Southwest Pacific and the Southwest Atlantic to examine relative changes in northern and southern sourced deep waters during the Last Glacial Maximum (LGM) and deglaciation. During the LGM, our transects show that water mass properties and boundaries in the South Atlantic and Pacific were different from one another. The Atlantic between ~1.0 and 2.5 km was more than 1‰ enriched in δ13C relative to the Pacific and remained more enriched through the deglaciation. During the LGM, Atlantic δ18O was ~0.5‰ more enriched than the Pacific, particularly below 2.5 km. This compositional difference between the deep portions of the basins implies independent deep water sources during the glaciation. We attribute these changes to a “deep gateway” effect whereby northern sourced waters shallower than the Drake Passage sill were unable to flow southward into the Southern Ocean because a net meridional geostrophic transport cannot be supported in the absence of a net east‐west circumpolar pressure gradient above the sill depth. We surmise that through the LGM and early deglaciation, shoaled northern sourced waters were unable to escape the Atlantic and contribute to deep water formation in the Southern Ocean.

Trace metal evidence for a poorly ventilated glacial Southern Ocean

Glacial benthic δ13C and Δ14C measurements from the Atlantic Ocean have been interpreted to indicate the existence of a poorly ventilated Southern Ocean with greater CO2 and nutrient contents compared to present. Enhanced storage of CO2 in the deep ocean predicts that oxygen concentrations should have declined at the same time—a prediction increasingly supported by evidence for oxygen depletion in the glacial Southern Ocean. Here we take a novel approach by using a suite of redox-sensitive trace metals (Ag, Cd, Re and Mo) to show that Southern Ocean sediments from two cores in the Atlantic sector were suboxic during and prior to deglaciation, implying changes to ocean circulation and/or elevated export production that significantly altered deep water chemistry. In the Cape Basin, enrichments of the authigenically deposited trace metal Re are comparable to those found in oxygen minimum zones, pointing to substantial decreases in oxygenation. Furthermore, trace metal results suggest potential spatial heterogeneity in the glacial Southern Ocean, and a more complicated oceanographic and oxygenation history than has previously been assumed.

Closing an early Miocene astronomical gap with Southern Ocean δ18O and δ13C records: Implications for sea level change

AbstractWe present orbital‐scale resolution (~10 kyr) benthic foraminiferal δ18O and δ13C records from the Kerguelen Plateau (Ocean Drilling Program Sites 751 and 747) from 14.5 to 20.0 Ma spanning the Miocene climate optimum (15–17 Ma). Our records fill a critical gap from ~17 to 18 Ma, a time when many other deep‐sea records are affected by dissolution. We tested the fidelity of published magnetobiostratigraphic age models for these sites by astronomically tuning to the 405 kyr eccentricity cycle. A comparison of spectral estimates between the untuned and tuned records, as well as coherency with Laskar's (2004) eccentricity solution, revealed quasi‐100 kyr cyclicity in δ18O and δ13C. There is only a weak signal associated with the 41 kyr obliquity cycle, likely due to the 10 kyr sampling limiting resolution. The δ18O variations point to persistent 405 and quasi‐100 kyr modulations of temperature and sea level changes through the early to middle Miocene as predicted by astronomical solutions, with changing dominance of the 100 and 41 kyr beat. Comparison of δ18O records with early to middle Miocene sequences from the New Jersey shelf, northeast Australian margin, Bahamas, and Maldives suggests that the dominant sea level period preserved is the 1.2 Myr obliquity cycle, with sequence boundaries associated with δ18O increases or maxima. On the New Jersey margin, higher‐order sequences reflect the quasi‐100 kyr eccentricity cycles as modulated by 405 kyr cycles. We suggest that “nesting” of stratigraphic cycles is a function of the following: (1) pervasive (though changing) Milankovitch forcing of global mean sea level change and (2) preservation that depends on sufficient sediment supply and accommodation.

Mediterranean Outflow Water dynamics during the past ~570 kyr: Regional and global implications

AbstractThe Gulf of Cadiz constitutes a prime area to study teleconnections between the North Atlantic Ocean and climate change in the Mediterranean realm. In particular, the highly saline Mediterranean Outflow Water (MOW) is an important modulator of the North Atlantic salt budget on intermediate water levels. However, our understanding of its paleoceanographic evolution is poorly constrained due to the lack of high‐resolution proxy records that predate the last glacial cycle. Here we present the first continuous and high‐resolution (~ 1 kyr) benthic δ18O and δ13C as well as grain size records from Integrated Ocean Drilling Program Site U1386 representing the last ~570 kyr. We find three distinct phases of MOW variability throughout the Late to Middle Pleistocene at Site U1386 associated with prominent shifts in its composition and flow strength. We attribute this long‐term variability to changes in water mass sourcing of the MOW. Superimposed on the long‐term change in water mass sourcing is the occurrence of distinct and precession paced δ18O enrichment events, which contrast the pattern of global ice volume change as inferred from the global mean δ18O signal (i.e., LR04) but mimics that of the adjacent Mediterranean Sea. We attribute these enrichment events to a profound temperature reduction and salinity increases of the MOW, aligning with similar changes in the Mediterranean source region. These events might further signify ice volume increases as inferred from significant sea level drops recorded in the Red Sea and/or increased influence of North Atlantic intermediate water masses when MOW influence was absent at Site U1386.

Calibration of the carbon isotope composition (δ13C) of benthic foraminifera

AbstractThe carbon isotope composition (δ13C) of seawater provides valuable insight on ocean circulation, air‐sea exchange, the biological pump, and the global carbon cycle and is reflected by the δ13C of foraminifera tests. Here more than 1700 δ13C observations of the benthic foraminifera genus Cibicides from late Holocene sediments (δ13CCibnat) are compiled and compared with newly updated estimates of the natural (preindustrial) water column δ13C of dissolved inorganic carbon (δ13CDICnat) as part of the international Ocean Circulation and Carbon Cycling (OC3) project. Using selection criteria based on the spatial distance between samples, we find high correlation between δ13CCibnat and δ13CDICnat, confirming earlier work. Regression analyses indicate significant carbonate ion (−2.6 ± 0.4) × 10−3‰/(μmol kg−1) [CO32−] and pressure (−4.9 ± 1.7) × 10−5‰ m−1 (depth) effects, which we use to propose a new global calibration for predicting δ13CDICnat from δ13CCibnat. This calibration is shown to remove some systematic regional biases and decrease errors compared with the one‐to‐one relationship (δ13CDICnat = δ13CCibnat). However, these effects and the error reductions are relatively small, which suggests that most conclusions from previous studies using a one‐to‐one relationship remain robust. The remaining standard error of the regression is generally σ ≅ 0.25‰, with larger values found in the southeast Atlantic and Antarctic (σ ≅ 0.4‰) and for species other than Cibicides wuellerstorfi. Discussion of species effects and possible sources of the remaining errors may aid future attempts to improve the use of the benthic δ13C record.

Microhabitat preferences of live benthic foraminifera and stable carbon isotopes off SW Svalbard in the presence of widespread methane seepage

Abstract To constrain the use of, and better calibrate benthic foraminiferal carbon-isotope based proxies (δ13C) we analyzed live (Rose Bengal stained) benthic foraminifera from fjords, outer shelves, and the upper continental slope of SW-Svalbard. We further investigated whether the presence of widespread methane seepage altered benthic foraminiferal δ13C. The one-to-one relationship between the δ13C of Cibicidoides wuellerstorfi and Cibicides lobatulus tests and the δ13CDIC of bottom water in Nordic Seas was reconfirmed. We also confirmed an almost constant δ13C of live specimens of infaunal species Melonis zaandamae, Nonion labradoricum, and Globobulimina affinis independent of their subbottom sampling depth and pore water δ13CDIC at any given site. This is in contrast to the wide range of intraspecific δ13C values of both, epifaunal as well as infaunal species, observed between sites. We did not find significant correlations between foraminiferal δ13C minus site-specific δ13CDIC (Δδ13CFor − DIC) of the investigated species and the total organic carbon (TOC) contents of surface sediments. On the contrary, a distinct relationship is observed between high marine organic carbon (MOC) contents and low Δδ13CFor − DIC values of N. labradoricum and, to a lesser extent, G. affinis. These findings are important because on the one hand they confirm a systematic influence of the availability of labile MOC on deep infaunal species δ13C, whereas on the other hand they document the independence of epifaunal and infaunal species δ13C of TOC contents in well-ventilated bottom water and organic-rich surface sediment. Generally, species' mean δ13C reflects their average living depths (ALD5) such that epifaunal and shallow infaunal species depict highest, and deep infaunal species, lowest δ13C values. Mean Δδ13CFor − DIC values of neither epifaunal C. lobatulus and C. wuellerstorfi (− 0.05 ± 0.06‰) nor of infaunal M. zaandamae (− 2.40 ± 0.07‰), N. labradoricum (− 2.69 ± 0.13‰), and G. affinis (− 3.48 ± 0.15‰) seem to be significantly influenced by strongly elevated methane concentrations in the water column, widespread gas seepage at the sea floor, and low pore-water δ13CDIC; the latter possibly partly due to methanotrophic oxidation. The δ13C values of deep infaunal N. labradoricum and G. affinis minus epifaunal δ13C values (Δδ13CNlab − Cib and Δδ13CGaff − Cib), seem to decrease with increasing TOC and MOC contents. However, only the Δδ13CNlab − Cib values correlate significantly with MOC contents and thus marine primary productivity. Dissolved oxygen concentrations, estimated on the basis of Δδ13CGaff − Cib values, of > 230 μmol/kg are in accordance with reported well-oxygenated bottom water conditions at all stations.

Prolonged warming of the Brazil Current precedes deglaciations

Paleoceanographic reconstructions from the Brazil Current are scarce and lack the required temporal resolution to appropriately represent its variability during key periods of the last glacial–interglacial cycles. Here, we present the first high-temporal resolution multiproxy reconstruction of the Brazil Current at 24 °S covering the last 185 ka. During the last and penultimate glacial periods, our Mg/Ca-derived sea surface temperature (SST) record shows a strong cooling at ca. 47 and ca. 156 ka, respectively, that is followed by a warming trend from late-Marine Isotope Stage (MIS) 3 to MIS 1 and from late-MIS 6 to MIS5e, respectively. Importantly, the Brazil Current warmed uninterruptedly towards Termination I (II) after the low SST at ca. 47 and ca. 156 ka, with no SST minima during the Last Glacial Maximum or penultimate glacial maximum. The reason for the strong cooling and the warming trend during late-MIS 3 and late-MIS 6 could reside in the favorable obliquity configuration. However, this mechanism is not sufficient to sustain the warming observed for the rest of the last and penultimate glacial periods. We propose that the change in the Atlantic meridional overturning circulation (AMOC), as described in the literature, from a “warm” to a “cold mode” for MIS 2 and MIS 6 is responsible for the accumulation of warm waters in the subtropical western South Atlantic, preventing SST minima during the last and penultimate glacial maxima in the region. Change in benthic δ13C corroborates that a fundamental modification in the AMOC mode might have triggered the heat accumulation. Our data also show a sudden increase in SST and surface salinity during the last glacial descent (MIS 4), indicating that the western portion of the subtropical gyres may have acted as a heat and salt reservoir, while higher latitude climates transited to a glacial background. Our findings imply that the AMOC “cold mode” induces heat storage in the subtropical western South Atlantic and, because of that, the last two regional SST minima occurred out-of-phase with the glacial maxima of higher latitudes.

Hydrological variations of the intermediate water masses of the western Mediterranean Sea during the past 20 ka inferred from neodymium isotopic composition in foraminifera and cold-water corals

Abstract. We present the neodymium isotopic composition (εNd) of mixed planktonic foraminifera species from a sediment core collected at 622 m water depth in the Balearic Sea, as well as εNd of scleractinian cold-water corals (CWC; Madrepora oculata, Lophelia pertusa) retrieved between 280 and 442 m water depth in the Alboran Sea and at 414 m depth in the southern Sardinian continental margin. The aim is to constrain hydrological variations at intermediate depths in the western Mediterranean Sea during the last 20 kyr. Planktonic (Globigerina bulloides) and benthic (Cibicidoides pachyderma) foraminifera from the Balearic Sea were also analyzed for stable oxygen (δ18O) and carbon (δ13C) isotopes. The foraminiferal and coral εNd values from the Balearic and Alboran seas are comparable over the last ∼ 13 kyr, with mean values of −8.94 ± 0.26 (1σ; n = 24) and −8.91 ± 0.18 (1σ; n = 25), respectively. Before 13 ka BP, the foraminiferal εNd values are slightly lower (−9.28 ± 0.15) and tend to reflect higher mixing between intermediate and deep waters, which are characterized by more unradiogenic εNd values. The slight εNd increase after 13 ka BP is associated with a decoupling in the benthic foraminiferal δ13C composition between intermediate and deeper depths, which started at ∼ 16 ka BP. This suggests an earlier stratification of the water masses and a subsequent reduced contribution of unradiogenic εNd from deep waters. The CWC from the Sardinia Channel show a much larger scatter of εNd values, from −8.66 ± 0.30 to −5.99 ± 0.50, and a lower average (−7.31 ± 0.73; n = 19) compared to the CWC and foraminifera from the Alboran and Balearic seas, indicative of intermediate waters sourced from the Levantine basin. At the time of sapropel S1 deposition (10.2 to 6.4 ka), the εNd values of the Sardinian CWC become more unradiogenic (−8.38 ± 0.47; n = 3 at ∼ 8.7 ka BP), suggesting a significant contribution of intermediate waters originated from the western basin. We propose that western Mediterranean intermediate waters replaced the Levantine Intermediate Water (LIW), and thus there was a strong reduction of the LIW during the mid-sapropel ( ∼ 8.7 ka BP). This observation supports a notable change of Mediterranean circulation pattern centered on sapropel S1 that needs further investigation to be confirmed.

Subsidence of the northern South China Sea and formation of the Bashi Strait in the latest Miocene: Paleoceanographic evidences from 9-Myr high resolution benthic foraminiferal δ18O and δ13C records

We present astronomically tuned high time resolution (3–4kyr) benthic foraminiferal δ18O and δ13C records between 9Ma and 5Ma from Ocean Drilling Program (ODP) Site 1143 in the southern South China Sea (SCS). We found that the late Miocene ocean carbon shift (LMOCS) which was linked with the late Miocene/early Pliocene “Biogenic Bloom” also occurred in the northern and southern SCS. A marked deep-water δ13C gradient as much as ~1.0‰ of the northern and southern SCS was observed between 6.5Ma and 5.9Ma, which was probably caused by the significant difference in paleoproductivity between the northern and southern SCS. The deep δ13C gradient was likely produced by rapid tectonic subsidence in the northern SCS at ~6.0Ma and the final formation of the Bashi Strait at ~6.5Ma. A reversed benthic δ13C gradient of the northern and southern SCS was also observed between 1.2Ma and 0.35Ma, which was probably caused by the consecutive closures of the interarc water gates within the Bashi strait at ~3.0Ma and ~1.2Ma. Closures of the interarc water gates within the Bashi strait at ~3.0Ma and ~1.2Ma further semi-enclosed the SCS basin and made the paleoproductivity the major factor influencing the deep water δ13C, leading to the reversed deep water δ13C gradient of the northern and southern SCS between 1.2Ma and 0.35Ma.

Global deepwater circulation between 2.4 and 1.7 Ma and its connection to the onset of Northern Hemisphere Glaciation

We have generated an early Pleistocene benthic isotopic record for the Ocean Drilling Program (ODP) Site 807 (2804 m) from the western equatorial Pacific. Between 2.4 and 1.7 Ma, the benthic δ13C of this site and a few other deep Pacific sites was consistently higher than the Southern Ocean Site MV0502-4JC (4286 m), pointing to a reversal relative to the current gradient and hence implying a different circulation regime. We reconstructed the deep water mass distribution of this interval using a collection of benthic isotope records from 15 Pacific and 10 Atlantic sites and a δ13C-δ18O mixing model. A two-end-member mixing regime between the North Atlantic Deep Water (NADW) and the Antarctic Bottom Water (AABW), with properties very different from today, was identified. The Southern Ocean showed strong signs of stratification and AABW with low benthic δ13C but high δ18O values reached out to other basins only below ~4000 m. In contrast, NADW ventilated most of the ocean interior, contributing ~70% to the Pacific Deep Water (PDW) volumetrically. Our model results also reveal a strong remineralization effect at the bottom sites of the Pacific and the Atlantic, suggesting significant accumulation of respired carbon in the bottom water between 2.4 and 1.7 Ma. We propose that such a circulation pattern was initiated by the reversal of salinity gradient between AABW and NADW from 3.0 to 2.4 Ma, possibly linked to Antarctic sea ice expansion and reduced southward heat transport during the onset of Northern Hemisphere Glaciation (NHG).

Glacial water mass structure and rapid δ18O and δ13C changes during the last glacial termination in the Southwest Pacific

Changes in ocean circulation are thought to have contributed to lowering glacial atmospheric CO2 levels by enhancing deep ocean sequestration of carbon that was returned to the atmosphere during glacial terminations. High-resolution benthic foraminiferal δ13C and δ18O records from a depth transect of cores in the Southwest Pacific Ocean presented here provide evidence that both wind- and thermohaline-driven circulation drove CO2 from the ocean during the last deglaciation. Shallow geochemical stratification in the glacial Southern Ocean was followed by a short pulse of rapid δ13C enrichment to intermediate water depths during Heinrich Stadial 1, indicative of better-ventilated intermediate waters co-occurring with documented wind-driven upwelling in the Southern Ocean. Intermediate depth δ13C enrichment paused at the start of the Antarctic Cold Reversal (∼14.7 ka), implying a brief shallow restratification, while deeper layers were progressively flushed of δ13C-depleted and δ18O-enriched waters, likely caused by the increasing influence of deep waters sourced from the North Atlantic. The coincidence of atmospheric CO2 increases with these geochemical shifts in both shallow and deep cores suggests that shifts in both atmospheric and oceanic circulation contributed to the deglacial rise of CO2.

The demise of the early Eocene greenhouse – Decoupled deep and surface water cooling in the eastern North Atlantic

Early Paleogene greenhouse climate culminated during the early Eocene Climatic Optimum (EECO, 50 to 53Ma). This episode of global warmth is subsequently followed by an almost 20 million year-long cooling trend leading to the Eocene-Oligocene glaciation of Antarctica. Here we present the first detailed planktic and benthic foraminiferal isotope single site record (δ13C, δ18O) of late Paleocene to middle Eocene age from the North Atlantic (Deep Sea Drilling Project Site 401, Bay of Biscay). Good core recovery in combination with well preserved foraminifera makes this site suitable for correlations and comparison with previously published long-term records from the Pacific Ocean (e.g. Allison Guyot, Shatsky Rise), the Southern Ocean (Maud Rise) and the equatorial Atlantic (Demerara Rise). Whereas our North Atlantic benthic foraminiferal δ18O and δ13C data agree with the global trend showing the long-term shift toward heavier δ18O values, we only observe minor surface water δ18O changes during the middle Eocene (if at all) in planktic foraminiferal data. Apparently, the surface North Atlantic did not cool substantially during the middle Eocene. Thus, the North Atlantic appears to have had a different surface ocean cooling history during the middle Eocene than the southern hemisphere, whereas cooler deep-water masses were comparatively well mixed. Our results are in agreement with previously published findings from Tanzania, which also support the idea of a muted post-EECO surface-water cooling outside the southern high-latitudes.

Deep water circulation within the eastern Mediterranean Sea over the last 95 kyr: New insights from stable isotopes and benthic foraminiferal assemblages

The response of eastern Mediterranean Sea (EMS) circulation to climate forcings over the last 95kyr BP was studied using core MD04-2722, collected at 1780m water depth within the Levantine Sea. Foraminiferal stable isotopes and benthic assemblages were combined in order to reconstruct deep water ventilation and oxygenation in relation to surface water freshening. Over the last deglaciation, benthic foraminiferal δ13C values and oxygen index decreased, while the δ18O gradient between benthic and planktonic foraminifera increased. The results, respectively, indicate slower ventilation, bottom water oxygen depletion, and stronger stratification prior to S1 sapropel deposition. A combination of deglacial sea level rise and fresher north Atlantic surface water contributions were determined to be a precondition for S1 formation within the Levantine Sea. Local Nile River freshwater supplied during the African Humid Period further strengthened water column stratification. As for S3 deposit, the central role of monsoonal precipitation was estimated. For the last glacial period, three events at around 53, 46 and 37ka BP were marked by a deep water circulation reduction at the core location. Considering the influence of north Atlantic surface water salinity on Mediterranean Sea circulation, we propose that the 46 and 37ka BP events are responses to Heinrich Events 4 and 5 that supplied fresher surface water to the Mediterranean Sea. Since the ‘53ka event’ was also characterized by the appearance of low oxygen benthic indicators as observed within the S1 and S3 layers, we tentatively attributed it to ‘missing’ sapropel S2. Our results indicate that intense stagnation within the EMS could occur when both the local freshwater supply and fresher north Atlantic surface water are contributors. The influence of north Atlantic conditions was significant to EMS circulation under warm and cold climate conditions.

Pacific‐Atlantic Circumpolar Deep Water coupling during the last 500 ka

AbstractInvestigating the interbasin deepwater exchange between the Pacific and Atlantic Oceans over glacial‐interglacial climate cycles is important for understanding circum‐Antarctic Southern Ocean circulation changes and their impact on the global Meridional Overturning Circulation. We use benthic foraminiferal δ13C records from the southern East Pacific Rise to characterize the δ13C composition of Circumpolar Deep Water in the South Pacific, prior to its transit through the Drake Passage into the South Atlantic. A comparison with published South Atlantic deepwater records from the northern Cape Basin suggests a continuous water mass exchange throughout the past 500 ka. Almost identical glacial‐interglacial δ13C variations imply a common deepwater evolution in both basins suggesting persistent Circumpolar Deep Water exchange and homogenization. By contrast, deeper abyssal waters occupying the more southern Cape Basin and the southernmost South Atlantic have lower δ13C values during most, but not all, stadial periods. We conclude that these values represent the influence of a more southern water mass, perhaps Antarctic Bottom Water (AABW). During many interglacials and some glacial periods, the gradient between Circumpolar Deep Water and the deeper southern Cape Basin bottom water disappears suggesting either no presence of AABW or indistinguishable δ13C values of both water masses.

Reduced admixture of North Atlantic Deep Water to the deep central South Pacific during the last two glacial periods

AbstractThe South Pacific is a sensitive location for the variability of the global oceanic thermohaline circulation given that deep waters from the Atlantic Ocean, the Southern Ocean, and the Pacific Basin are exchanged. Here we reconstruct the deep water circulation of the central South Pacific for the last two glacial cycles (from 240,000 years ago to the Holocene) based on radiogenic neodymium (Nd) and lead (Pb) isotope records complemented by benthic stable carbon data obtained from two sediment cores located on the flanks of the East Pacific Rise. The records show small but consistent glacial/interglacial changes in all three isotopic systems with interglacial average values of −5.8 and 18.757 for εNd and 206Pb/204Pb, respectively, whereas glacial averages are −5.3 and 18.744. Comparison of this variability of Circumpolar Deep Water (CDW) to previously published records along the pathway of the global thermohaline circulation is consistent with reduced admixture of North Atlantic Deep Water to CDW during cold stages. The absolute values and amplitudes of the benthic δ13C variations are essentially indistinguishable from other records of the Southern Hemisphere and confirm that the low central South Pacific sedimentation rates did not result in a significant reduction of the amplitude of any of the measured proxies. In addition, the combined detrital Nd and strontium (87Sr/86Sr) isotope signatures imply that Australian and New Zealand dust has remained the principal contributor of lithogenic material to the central South Pacific.

Southeastern Atlantic deep-water evolution during the late-middle Eocene to earliest Oligocene (Ocean Drilling Program Site 1263 and Deep Sea Drilling Project Site 366)

Comparison of new benthic foraminiferal δ18O and δ13C records from Ocean Drilling Program (ODP) Site 1263 (Walvis Ridge, southeast Atlantic, 2100 m paleodepth) and Deep Sea Drilling Project (DSDP) Site 366 (Sierra Leone Rise, eastern equatorial Atlantic, 2200–2800 m paleodepth) with published data from Atlantic and Southern Ocean sites provides the means to reconstruct the development of deep-water circulation in the southeastern Atlantic from the late-middle Eocene to the earliest Oligocene. Our comparison shows that in the late-middle Eocene (ca. 40 Ma), the South Atlantic was characterized by a homogeneous thermal structure. Thermal differentiation began ca. 38 Ma. By 37.6 Ma, Site 1263 was dominated by Southern Component Water; at the same time, warm saline deep water filled the deeper South Atlantic (recorded at southwest Atlantic ODP Site 699, paleodepth 3400 m, and southeast Atlantic ODP Site 1090, paleodepth 3200 m). The deep-water source to eastern equatorial Site 366 transitioned to Northern Component Water ca. 35.6–35 Ma. Progressive cooling at Site 1263 during the middle to late Eocene and deep-water thermal stratification in the South Atlantic may be attributed at least in part to the gradual deepening and strengthening of the proto–Antarctic Circumpolar Current from the late-middle Eocene to the earliest Oligocene, as the Drake and Tasman gateways opened. Our isotopic comparisons across depth and latitude provide evidence of the development of deep-water circulation similar to modern-day Atlantic Meridional Overturning Circulation.

Intensification of North Pacific intermediate water ventilation during the Younger Dryas

Modern North Pacific intermediate water (NPIW) is formed in the mixed water region where the Oyashio and Kuroshio currents meet. The source for cooling and freshening of NPIW is intermediate water in the Okhotsk Sea. The Okhotsk intermediate water outflows to the open Pacific, forming the Oyashio intermediate water by mixing with the subarctic gyre water. In the Oyashio region, the intermediate water originating from the Okhotsk Sea flows mainly at depths shallower than 500 m. On the other hand, ventilation of intermediate water in the subarctic Pacific during the deglaciation remains a topic of debate. In this study, foraminiferal δ18O and δ13C signatures were determined in a dated sediment core collected at 777 m water depth to evaluate the intensity and depth distribution of the source of NPIW since the last deglaciation in the Oyashio region. Benthic foraminiferal δ18O increased by 0.3–0.4‰ from the end of the Bolling/Allerod warm episode to the Younger Dryas cold episode, suggesting intermediate water cooling. Consistent with this trend, benthic δ13C values point to decreased nutrient contents during the cold event. Conversely, benthic δ18O signatures from a nearby core site at a water depth of 1,366 m did not show such cooling. These results suggest that cold intermediate water originating from the north was actively ventilated at depths of at least 700–800 m, and possibly even 1,200 m during the Younger Dryas, implying that NPIW ventilation was thicker and deeper than under modern conditions.