Epibenthic δ13C Research Articles

Carbon isotope offsets between benthic foraminifer species of the genus Cibicides (Cibicidoides) in the glacial sub‐Antarctic Atlantic

AbstractEpibenthic foraminifer δ13C measurements are valuable for reconstructing past bottom water dissolved inorganic carbon δ13C (δ13CDIC), which are used to infer global ocean circulation patterns. Epibenthic δ13C, however, may also reflect the influence of 13C‐depleted phytodetritus, microhabitat changes, and/or variations in carbonate ion concentrations. Here we compare the δ13C of two benthic foraminifer species, Cibicides kullenbergi and Cibicides wuellerstorfi, and their morphotypes, in three sub‐Antarctic Atlantic sediment cores over several glacial‐interglacial transitions. These species are commonly assumed to be epibenthic, living above or directly below the sediment‐water interface. While this might be consistent with the small δ13C offset that we observe between these species during late Pleistocene interglacial periods (Δδ13C = −0.19 ± 0.31‰, N = 63), it is more difficult to reconcile with the significant δ13C offset that is found between these species during glacial periods (Δδ13C = −0.76 ± 0.44‰, N = 44). We test possible scenarios by analyzing Uvigerina spp. δ13C and benthic foraminifer abundances: (1) C. kullenbergi δ13C is biased to light values either due to microhabitat shifts or phytodetritus effects and (2) C. wuellerstorfi δ13C is biased to heavy values, relative to long‐term average conditions, for instance by recording the sporadic occurrence of less depleted deepwater δ13CDIC. Neither of these scenarios can be ruled out unequivocally. However, our findings emphasize that supposedly epibenthic foraminifer δ13C in the sub‐Antarctic Atlantic may reflect several factors rather than being solely a function of bottom water δ13CDIC. This could have a direct bearing on the interpretation of extremely light South Atlantic δ13C values at the Last Glacial Maximum.

Strong thermodynamic imprint on Recent bottom-water and epibenthic δ13C in the Weddell Sea revealed: Implications for glacial Southern Ocean ventilation

Paleonutrient proxies are widely used to reconstruct the geometry of deep-water masses during the Last Glacial Maximum (LGM). Epibenthic δ13C provides best spatial coverage, and artifacts are well investigated. Discrepancies between reconstructed LGM-circulation patterns derived from models or different benthic nutrient proxies can partly be resolved by varying air–sea signatures of δ13C, i.e. δ13Cas. However, there are very few data available to calculate a δ13Cas of modern bottom water dissolved inorganic carbon (DIC) δ13C, and to document how this signal is recorded in benthic foraminiferal δ13C. Here I show that today bottom water in the Atlantic sector of the Southern Ocean is 13C enriched with δ13CDIC values between 0.4 and 1.0‰ and δ13Cas values >0.4‰, and that this signal is recorded in live and dead epibenthic δ13C. This is in contrast to a uniform modern Antarctic circumpolar δ13CDIC of rather 0.4‰, which hitherto is used as modern framework to compare to low LGM δ13CDIC of southern sourced bottom-water and glacial inter basin differences. I conclude that a potential reduction of the strong Recent thermodynamic imprint during bottom-water generation in glacial times could explain depleted circum Antarctic 13CDIC without associated CO2 enrichment and anoxia in Antarctic bottom waters. The present synoptic compilation of δ13CDIC and live benthic foraminifera δ13C is in support of hypotheses that explain low LGM δ13C by a depletion of southern end-member 13CDIC due to extensive sea-ice formation with low δ13Cas-brine rejection and diminished air–sea gas exchange.

Ventilation changes in intermediate water on millennial time scales in the SE Nordic seas, 65‐14 kyr BP

A glacial epibenthic δ13C record from 773 m water depth in the SE Nordic seas reveals moderate, but distinct changes in the ventilation ∼65,000−14,000 years ago. The Last Glacial Maximum, the warm interstadials, and the shorter stadials are characterized by high δ13C values indicating well‐ventilated intermediate water masses in the Nordic seas. Decreasing δ13C values during the cold Heinrich events signify a reduction in intermediate water ventilation. We attribute the reduction to the development of a halocline causing a stop in convection and outflow from the Nordic seas. The well‐ventilated outflow water is replaced by warmer Atlantic water, which due to the stratification is isolated from the atmosphere. Its initial high δ13C values are reduced due to 'ageing'. We ascribe the lack of response in the subsurface Atlantic Water during the stadials to the smaller geographical extent of these events as compared to the Heinrich events.

Variability of the Denmark Strait Overflow during the Last Glacial Maximum

The Denmark Strait Overflow (DSO) today compensates for the northward flowing Norwegian and Irminger branches of the North Atlantic Current that drive the Nordic heat pump. During the Last Glacial Maximum (LGM), ice sheets constricted the Denmark Strait aperture in addition to ice eustatic/isostatic effects which reduced its depth (today ∼630 m) by ∼130 m. These factors, combined with a reduced north-south density gradient of the water-masses, are expected to have restricted or even reversed the LGM DSO intensity. To better constrain these boundary conditions, we present a first reconstruction of the glacial DSO, using four new and four published epibenthic and planktic stable-isotope records from sites to the north and south of the Denmark Strait. The spatial and temporal distribution of epibenthic δ18O and δ13C maxima reveals a north-south density gradient at intermediate water depths from σ0∼28.7 to 28.4/28.1 and suggests that dense and highly ventilated water was convected in the Nordic Seas during the LGM. However, extremely high epibenthic δ13C values on top of the Mid-Atlantic Ridge document a further convection cell of Glacial North Atlantic Intermediate Water to the south of Iceland, which, however, was marked by much lower density (σ0∼28.1) The north-south gradient of water density possibly implied that the glacial DSO was directed to the south like today and fed Glacial North Atlantic Deep Water that has underthrusted the Glacial North Atlantic Intermediate Water in the Irminger Basin.

Late Neogene benthic stable isotope record of Ocean Drilling Program Site 999: Implications for Caribbean paleoceanography, organic carbon burial, and the Messinian Salinity Crisis

We report on epibenthic foraminiferal δ18O and δ13C and percentage coarse fraction records from Caribbean Ocean Drilling Program (ODP) Site 999 (12°44′N, 78°44′W, water depth 2828 m) spanning the interval from 8.5 to 5.3 Ma. Low epibenthic δ13C values and low amounts of sand‐sized particles (mostly foraminifer shells) indicate a poorly ventilated deep Caribbean throughout the late Miocene. At this time the deep Caribbean was dominated by a nutrient‐rich and corrosive water mass. A generally constant δ13C gradient between the Caribbean and deep Atlantic records during the late Miocene suggests that the fluctuations in δ13C reflect global changes in δ13C of the dissolved inorganic carbon due to varying erosion of organic carbon from terrigenous soils and shelf sediments. The observed 100‐kyr cyclicity of epibenthic δ13C is in good accordance with the variability of terrigenous input to the equatorial Atlantic as recorded by magnetic susceptibility records of the Ceara Rise. However, some short‐term gradient changes between 7.0 and 4.5 Ma indicate a poorer ventilation of the deep Atlantic related to a reduced production of deep water in the Atlantic. The Messinian Salinity Crisis between 6.0 and 5.3 Ma did not affect the intermediate to deep water gradient between the Caribbean and the Atlantic. Comparison to the Bahama platform record of ODP Site 1006, however, indicates a poorer ventilation of the shallower northern Caribbean basins coincident with the isolation of the Mediterranean Sea.

Late Quaternary paleoproductivity and deep water circulation in the eastern South Atlantic Ocean: Evidence from benthic foraminifera

Two late Quaternary sediment cores from the northern Cape Basin in the eastern South Atlantic Ocean were analyzed for their benthic foraminiferal content and benthic stable carbon isotope composition. The locations of the cores were selected such that both of them presently are bathed by North Atlantic Deep Water (NADW) and past changes in deep water circulation should be recorded simultaneously at both locations. However, the areas are different in terms of primary production. One core was recovered from the nutrient-depleted Walvis Ridge area, whereas the other one is from the continental slope just below the coastal upwelling mixing area where present day organic matter fluxes are shown to be moderately high. Recent data served as the basis for the interpretation of the late Quaternary faunal fluctuations and the paleoceanographic reconstruction. During the last 450,000 years, NADW flux into the eastern South Atlantic Ocean has been restricted to interglacial periods, with the strongest dominance of a NADW-driven deep water circulation during interglacial stages 1, 9 and 11. At the continental margin, high productivity faunas and very low epibenthic δ13C values indicate enhanced fluxes of organic matter during glacial periods. This can be attributed to a glacial increase and lateral extension of coastal upwelling. The long term glacial-interglacial paleoproductivity cycles are superimposed by high-frequency variations with a period of about 23,000 yr. Enhanced productivity in surface waters above the Walvis Ridge, far from the coast, is indicated during glacial stages 8, 10 and 12. During these periods, cold, nutrient-rich filaments from the mixing area were probably driven as far as to the southeastern flank of the Walvis Ridge.

Changes in East Atlantic Deepwater Circulation over the last 30,000 years: Eight time slice reconstructions

Using 95 epibenthic δ13C records, eight time slices were reconstructed to trace the distribution of east Atlantic deepwater and intermediate water masses over the last 30,000 years. Our results show that there have been three distinct modes of deepwater circulation: Near the stage 3‐2 boundary, the origin of North Atlantic Deep Water (NADW) was similar to today (mode 1). However, after late stage 3 the source region of the NADW end‐member shifted from the Norwegian‐Greenland Sea to areas south of Iceland (mode 2). A reduced NADW flow persisted during the last glacial maximum, with constant preformed δ13C values. The nutrient content of NADW increased markedly near the Azores fracture zone from north to south, probably because of the mixing of upwelled Antarctic Bottom Water (AABW) from below, which then advected with much higher flux rates into the northeast Atlantic. Later, the spread of glacial meltwater over the North Atlantic led to a marked short‐term ventilation minimum below 1800 m about 13,500 14C years ago (mode 3). The formation of NADW recommenced abruptly north of Iceland 12,800–12,500 years ago and reached a volume approaching that of the Holocene, in the Younger Dryas (10,800–10,350 years B.P.). Another short‐term shutdown of deepwater formation followed between 10,200 and 9,600 years B.P., linked to a further major meltwater pulse into the Atlantic. Each renewal of deepwater formation led to a marked release of fossil CO2 from the ocean, the likely cause of the contemporaneous 14C plateaus. Over the last 9000 years, deepwater circulation varied little from today, apart from a slight increase in AABW about 7000 14C years ago. It is also shown that the oxygenated Mediterranean outflow varied largely independent of the variations in deepwater circulation over the last 30,000 years.

Water Mass Conversion in the Glacial Subarctic Pacific (54°N, 148°W): Physical Constraints and the Benthic‐Planktonic Stable Isotope Record

Benthic (Uvigerina spp., Cibicidoides spp., Gyroidinoides spp.) and planktonic (N. pachyderma sinistral, G. bulloides) stable isotope records from three core sites in the central Gulf of Alaska are used to infer mixed‐layer and deepwater properties of the late glacial Subarctic Pacific. Glacial‐interglacial amplitudes of the planktonic δ18O records are 1.1–1.3‰, less than half the amplitude observed at core sites at similar latitudes in the North Atlantic; these data imply that a strong, negative δw anomaly existed in the glacial Subarctic mixed layer during the summer, which points to a much stronger low‐salinity anomaly than exists today. If true, the upper water column in the North Pacific would have been statically more stable than today, thus suppressing convection even more efficiently. This scenario is further supported by vertical (i.e., planktic versus benthic) δ18O and δ13C gradients of >1‰, which suggest that a thermohaline link between Pacific deep waters and the Subarctic Pacific mixed layer did not exist during the late glacial. Epibenthic δ13C in the Subarctic Pacific is more negative than at tropical‐subtropical Pacific sites but similar to that recorded at Southern Ocean sites, suggesting ventilation of the deep central Pacific from mid‐latitude sources, e.g., from the Sea of Japan and Sea of Okhotsk. Still, convection to intermediate depths could have occurred in the Subarctic during the winter months when heat loss to the atmosphere, sea ice formation, and wind‐driven upwelling of saline deep waters would have been most intense. This would be beyond the grasp of our planktonic records which only document mixed‐layer temperature‐salinity fields extant during the warmer seasons. Also we do not have benthic isotope records from true intermediate water depths of the Subarctic Pacific.