Benthic Foraminiferal δ13C Research Articles

Continued meltwater influence on North Atlantic Deep Water instabilities during the early Holocene

The transition into the Holocene marks the last large, orbitally derived climatic event and ultimately led to the onset of modern oceanic conditions. The influence of this climatic change on North Atlantic Deep Water (NADW) formation and circulation remains ambiguous. High-resolution records from southern Gardar Drift, south of Iceland, show abrupt decreases in benthic foraminiferal δ13C values at discrete intervals during the early Holocene, suggesting that NADW shoaled episodically. Intervals of lower δ13C values are coincident with higher Δδ18ON. pachyderma (s)–G. bulloides and high abundance of lithic grains/g, indicating that these periods also had enhanced surface water stratification, due to increased meltwater in the circum-North Atlantic region. Our new high-resolution planktonic and benthic foraminiferal stable isotopic data show that increased meltwater delivery led to brief reorganizations of deepwater currents. These southern Gardar surface and deep water records indicate that the early Holocene was a period of multiple abrupt climatic events that were propagated to the North Atlantic during the final break up of ice sheets in the Northern Hemisphere, and suggest that some component of the residual early Holocene sea level rise can be attributed to Northern Hemispheric sources.

Modelling δ13C in benthic foraminifera: Insights from model sensitivity experiments

The δ13C value measured on benthic foraminiferal tests is widely used by palaeoceanographers to reconstruct the distribution of past water masses. The biogeochemical processes involved in forming the benthic foraminiferal δ13C signal (δ13Cforam), however, are not fully understood and a sound mechanistic description is still lacking. We use a reaction–diffusion model for calcification developed by Wolf-Gladrow et al. (1999) and Zeebe et al. (1999) in order to quantify the effects of different physical, chemical, and biological processes on δ13Cforam of an idealised benthic foraminiferal shell. Changes in the δ13C value of dissolved inorganic carbon (δ13CDIC) cause equal changes in δ13Cforam in the model. The results further indicate that temperature, respiration rate, and pH have a significant impact on δ13Cforam. In contrast, salinity, pressure, the δ13C value of particulate organic carbon (δ13CPOC), total alkalinity, and calcification rate show only a limited influence. In sensitivity experiments we assess how combining these effects can influence δ13Cforam. We can potentially explain 33 to 47% of the interglacial-to-glacial decrease in δ13Cforam by changes in temperature and pH, without invoking changes in δ13CDIC. Furthermore, about a quarter of the − 0.4‰ change in δ13Cforam observed in phytodetritus layers can be accounted for by an increase in respiration rate and a reduction in pH.

Deglacial whole-ocean δ13C change estimated from 480 benthic foraminiferal records

Terrestrial carbon storage is dramatically decreased during glacial periods due to cold temperatures, increased aridity, and the presence of large ice sheets on land. Most of the carbon released by the terrestrial biosphere is stored in the ocean, where the light isotopic signature of terrestrial carbon is observed as a 0.32–0.7‰ depletion in benthic foraminiferal δ13C. The wide range in estimated δ13C change results from the use of different subsets of benthic δ13C data and different methods of weighting the mean δ13C by volume. We present a more precise estimate of glacial-interglacial δ13C change of marine dissolved inorganic carbon using benthic Cibicidoides spp. δ13C records from 480 core sites (more than 3 times as many sites as previous studies). We divide the ocean into eight regions to generate linear regressions of regional δ13C versus depth for the Late Holocene (0–6 ka) and Last Glacial Maximum (19–23 ka) and estimate a mean δ13C decrease of 0.38 ± 0.08‰ (2σ) for 0.5–5 km. Estimating large uncertainty ranges for δ13C change in the top 0.5 km, below 5 km, and in the Southern Ocean, we calculate a whole-ocean change of 0.34 ± 0.19‰. This implies a terrestrial carbon change that is consistent with recent vegetation model estimates of 330–694 Gt C. Additionally, we find that a well-constrained surface ocean δ13C change is essential for narrowing the uncertainty range of estimated whole-ocean δ13C change.

Obliquity and long eccentricity pacing of the Middle Miocene climate transition

AbstractThe Middle Miocene East Antarctic ice sheet expansion (EAIE), which is indicated by an abrupt ~1‰ increase in global benthic foraminiferal δ18O at ~13.8 Ma, marks the Middle Miocene climate transition (MMCT) and has been related to astronomically modulated changes in the global carbon cycle. Here, we present high resolution (3–4 kyr) benthic foraminiferal δ18O and δ13C records from IODP Site U1337 in the central equatorial Pacific, which spans the period 12.2–15.8 Ma. The isotopic records clearly demonstrate significant imprints from periodic variations in the Earth's orbital parameters, particularly the obliquity (40 kyr) and the long eccentricity (400 kyr) cycles. While the benthic δ18O and δ13C exhibit nearly identical amplitudes for glacial‐interglacial cycles from 15.8 to 12.2 Ma, the long‐term trends in the benthic δ18O and δ13C had started to reverse after the beginning of the EAIE. Within the 400‐kyr band, the benthic −δ18O and δ13C displays a constant phase relationship between 15.8 and 12.2 Ma. At the 41‐kyr band, however, a phase reversal reaching >180° between −δ18O and δ13C occurs from 13.8 Ma to 14.0 Ma during the period of the EAIE. A similar phase relationship of benthic foraminiferal −δ18O and δ13C at the 400‐kyr band and the 41‐kyr band is also observed at ODP Site 1146 from the northern South China Sea. This phase jump occurs when the long‐term trends in δ18O and δ13C split, suggesting a decoupling of the global ice volume and ocean carbon reservoir changes during the Middle Miocene.

Paleoproductivity during the middle Miocene carbon isotope events: A data‐model approach

AbstractTo what extent are individual middle Miocene eccentricity‐scale benthic foraminiferal carbon isotope maxima (the so‐called CM events) related to changes in marine export productivity? Here we use benthic foraminiferal accumulation rates from three sites in the Pacific and Southern Oceans and a geochemical box model to assess relationships between benthic foraminiferal δ13C records, export productivity, and the global carbon cycle. Results from Deep Sea Drilling Project Hole 588 and Ocean Drilling Program Site 747 show a distinct productivity maximum during CM 6 at 13.8 Ma, the time of major expansion of ice on Antarctica. Productivity maxima during other CM events are only recorded at high‐latitude Site 747. A set of numerical experiments tests whether changes in foraminiferal δ13C records (CM events) and export productivity can be simulated solely by sea level fluctuations and the associated changes in global weathering‐deposition cycles, by sea level fluctuations plus global climatic cooling, and by sea level fluctuations plus invigorated ocean circulation. Consistent with data, the periodic forcing of sea level and albedo (and associated weathering cycles) produces δ13C variations of the correct temporal spacing, albeit with a reduced amplitude. A productivity response of the correct magnitude is achieved by enhancing ocean circulation during cold periods. We suggest that the pacing of middle Miocene δ13C fluctuations is associated with cyclical sea level variations. The amplitude, however, is muted perhaps due to the competing effects of a time‐lagged response to sea level lowstands but an immediate response to invigorated ocean circulation during cold phases.

An early Lutetian carbon‐cycle perturbation: Insights from the Gorrondatxe section (western Pyrenees, Bay of Biscay)

A distinctive low‐carbonate interval interrupts the continuous limestone‐marl alternation of the deep‐marine Gorrondatxe section at the early Lutetian (middle Eocene) C21r/C21n Chron transition. The interval is characterized by increased abundance of turbidites and kaolinite, a 3‰ decline in the bulk δ13C record, a >1‰ decline in benthic foraminiferal δ13C followed by a gradual recovery, a distinct deterioration in foraminiferal preservation, high proportions of warm‐water planktic foraminifera and opportunistic benthic foraminifera, and reduced trace fossil and benthic foraminiferal diversity, thus recording a significant environmental perturbation. The onset of the perturbation correlates with the C21r‐H6 event recently defined in the Atlantic and Pacific oceans, which caused a 2°C warming of the seafloor and increased carbonate dissolution. The perturbation was likely caused by the input of 13C‐depleted carbon into the ocean‐atmosphere system, thus presenting many of the hallmarks of Paleogene hyperthermal deposits. However, from the available data it is not possible to conclusively state that the event was associated with extreme global warming. Based on our analysis, the perturbation lasted 226 kyr, from 47.44 to 47.214 Ma, and although this duration suggests that the triggering mechanism may have been similar to that of the Paleocene‐Eocene Thermal Maximum (PETM), the magnitude of the carbon input and the subsequent environmental perturbation during the early Lutetian event were not as severe as in the PETM.

Multiple early Eocene benthic foraminiferal assemblage and δ13C fluctuations at DSDP Site 401 (Bay of Biscay — NE Atlantic)

Abstract Within the last decade, several early Eocene hyperthermals have been detected globally. These transient warming events have mainly been characterized geochemically – using stable isotopes, carbonate content measurements or XRF core scanning – yet detailed micropaleontological records are sparse, limiting our understanding of the driving forces behind hyperthermals and of the contemporaneous paleoceanography. Here, detailed geochemical and quantitative benthic foraminiferal records are presented from lower Eocene pelagic sediments of Deep Sea Drilling Project Site 401 (Bay of Biscay, northeast Atlantic). In calcareous nannofossil zone NP11, several clay-enriched levels correspond to negative δ13C and δ18O bulk-rock excursions with amplitudes of up to ~ 0.75‰, suggesting that significant injections of 12C-enriched greenhouse gasses and small temperature rises took place. Coeval with several of these hyperthermal events, the benthic foraminiferal record reveals increased relative abundances of oligotrophic taxa (e.g. Nuttallides umbonifera) and a reduction in the abundance of buliminid species followed by an increase of opportunistic taxa (e.g. Globocassidulina subglobosa and Gyroidinoides spp.). These short-lived faunal perturbations are thought to be caused by reduced seasonality of productivity resulting in a decreased Corg flux to the seafloor. Moreover, the sedimentological record suggests that an enhanced influx of terrigenous material occurred during these events. Additionally, the most intense δ13C decline (here called level δ) gives rise to a small, yet pronounced long-term shift in the benthic foraminiferal composition at this site, possibly due to the reappraisal of upwelling and the intensification of bottom water currents. These observations imply that environmental changes during (smaller) hyperthermal events are also reflected in the composition of deep-sea benthic communities on both short (

Stable-isotope stratigraphy of the Pliocene–Pleistocene climate transition in the northwestern subtropical Pacific

From Ocean Drilling Program (ODP) Site 1208 on Shatsky Rise below the Kuroshio Current Extension, we present the North Pacific's first orbital-scale benthic-foraminiferal δ18O and δ13C time series to span the Pliocene–Pleistocene climate transition. Excellent agreement between the Site 1208 δ18O record and the global δ18O stack of Lisiecki and Raymo (2005) provides orbital-scale age control and confirms continuous stratigraphy from 3.7 to 1.8Ma at the single-hole site. Cross-spectral analysis of the δ18O and δ13C time series reveals that these are coherent to 80% confidence at the 41-kyr obliquity band prior to 3.3Ma (glacial isotope stage M2) and increase to the 95% level thereafter. Throughout, δ18O cycles consistently lead δ13C cycles by ~3kyr. This suggests that global-ocean δ13C variations, as produced by terrestrial–marine 12C transfers, were responsive to obliquity-induced climate changes before the Northern Hemisphere glaciations (NHG) reached mid latitudes at 2.7Ma. In contrast, 41-kyr carbonate sedimentation (as derived from sediment reflectance) cycles, maxima tightly coupled to (>95% confidence) and in phase with minima in the δ18O record, do not emerge until 2.7Ma. Foraminiferal fragmentation counts indicate that carbonate preservation is not the primary process behind enhanced carbonate deposition during interglacials. Thus, we surmise that hydrography-related changes in biogenic opal and carbonate production in surface water best explain glacial–interglacial carbonate cycles beginning with significant NHG. Firm establishment of orbital-scale age control on the stratigraphically complete Site 1208 section now provides a platform for high-resolution paleoceanographic reconstruction of the relatively understudied North Pacific.

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.

Combined carbonate carbon isotopic and cellular ultrastructural studies of individual benthic foraminifera: 2. Toward an understanding of apparent disequilibrium in hydrocarbon seeps

[1] Numerous previous studies show disequilibrium between stable carbon isotope ratios of foraminiferal calcite and pore water dissolved inorganic carbon in hydrocarbon seeps, calling into question the utility of this widely used paleoceanographic tracer as a proxy. We use a recently developed method to compare stable carbon isotope ratios of foraminiferal carbonate with cell ultrastructural observations from individual benthic foraminifera from seep (under chemosynthetic bivalves) and nonseep habitats in Monterey Bay, California, to better understand control(s) of benthic foraminiferal carbon isotope ratios. Two attributes previously proposed to cause the isotopic offsets are diet and symbionts. Ultrastructural analysis shows that positive staining with Rose Bengal indicates presence of foraminiferal cytoplasm, bacterial biomass, or a combination of both and, thus, is not an unequivocal indicator of viability. We also show for the first time that some living seep foraminifera have endobionts. Results from our unique, yet limited, data set are consistent with suggestions that, in our sites, several foraminiferal species collected from seep clam beds may not survive there, diet and symbiont presence do not appear to be major contributors to disequilibrium, little calcification of seep-tolerant foraminiferal species occurs while seep conditions prevail, and microscale variability in habitats could influence δ13C of benthic foraminiferal carbonate. Results further suggest that our knowledge of benthic foraminiferal ecology and biomineralization, especially in extreme habitats such as seeps, must be bolstered before we fully understand the fidelity of paleoenvironmental records derived from benthic foraminiferal test δ13C data.

Ocean overturning since the Late Cretaceous: Inferences from a new benthic foraminiferal isotope compilation

Benthic foraminiferal oxygen isotopic (δ18O) and carbon isotopic (δ13C) trends, constructed from compilations of data series from multiple ocean sites, provide one of the primary means of reconstructing changes in the ocean interior. These records are also widely used as a general climate indicator for comparison with local and more specific marine and terrestrial climate proxy records. We present new benthic foraminiferal δ18O and δ13C compilations for individual ocean basins that provide a robust estimate of benthic foraminiferal stable isotopic variations to ∼80 Ma and tentatively to ∼110 Ma. First‐order variations in interbasinal isotopic gradients delineate transitions from interior ocean heterogeneity during the Late Cretaceous (>∼65 Ma) to early Paleogene (35–65 Ma) homogeneity and a return to heterogeneity in the late Paleogene–early Neogene (35–0 Ma). We propose that these transitions reflect alterations in a first‐order characteristic of ocean circulation: the ability of winds to make water in the deep ocean circulate. We document the initiation of large interbasinal δ18O gradients in the early Oligocene and link the variations in interbasinal δ18O gradients from the middle Eocene to Oligocene with the increasing influence of wind‐driven mixing due to the gradual tectonic opening of Southern Ocean passages and initiation and strengthening of the Antarctic Circumpolar Current. The role of wind‐driven upwelling, possibly associated with a Tethyan Circumequatorial Current, in controlling Late Cretaceous interior ocean heterogeneity should be the subject of further research.

Interpretation of benthic foraminiferal stable isotopes in subtidal estuarine environments

Abstract. Here we present a novel approach for the interpretation of stable isotope signatures recorded in benthic foraminifera from subtidal estuarine environments. The stable isotopic composition (δ18O and δ13C) of living Ammonia tepida and Haynesina germanica is examined at four stations in the Auray River estuary (Gulf of Morbihan, France) sampled in two contrasting seasons, spring 2006 and winter 2007. Comparing benthic foraminiferal δ18O measurements with theoretical oxygen isotopic equilibrium values, calculated on the basis of water temperature and salinity measurements in the upper and lower estuary, i.e., T-S-δ18Oeq. diagrams, strongly suggests that foraminiferal faunas sampled at the four stations calcified during different periods of the year. This interpretation can be refined by using the benthic foraminiferal δ13C, which is mainly determined by the mixing of sea and river water. In the upper estuary foraminifera mainly calcified in early spring and winter, whereas in the lower estuary calcification mainly took place in spring, summer and autumn. This new method provides insight into the complexity of estuarine benthic foraminiferal stable isotope records. In addition, it can also be used to obtain new information on preferred calcification periods of benthic foraminiferal taxa in different parts of the estuary.

Middle‐late Pleistocene deep water circulation in the southwest subtropical Pacific

The modern δ13CDIC distribution in southwest subtropical Pacific deep waters is consistent with a regional mixing regime between water masses of open Pacific Ocean and Tasman Sea origin. This mixing regime is reconstructed across the middle‐late Pleistocene using a record of benthic foraminiferal δ13C in a sediment core from the New Caledonia Trough. The relative influence on the mixing regime from open Pacific Ocean deep waters is seen to be significantly reduced during glacial in comparison to interglacial stages over the past 1.1 Ma. The spatial δ13C gradient in the Southern Ocean between deep waters entering the Tasman Sea and the open Pacific Ocean is shown to be consequently greater during glacial than interglacial stages but was generally reduced across the period of the Middle Pleistocene Transition. The existence of strong spatial chemical gradients in the glacial Southern Ocean limits its capacity to act as an enhanced sink for atmospheric carbon.

Mid‐Miocene paleoproductivity in the Atlantic Ocean and implications for the global carbon cycle

A prominent, middle Miocene (17.5–13.5 Ma) carbon isotope excursion ubiquitously recorded in carbonate sediments has been attributed to enhanced marine productivity and sequestration of 13C depleted organic carbon in marine sediments or enhanced carbon burial in peat/lignite deposits on land. Here we test the hypothesis that the marine δ13C record reflects a change in productivity with proxy records from three Atlantic Ocean sites (Deep Sea Drilling Program Site 608 and Ocean Drilling Program Sites 925 and 1265). Our multiproxy approach is based on benthic foraminiferal accumulation rates, elemental ratios (Ba/Al and P/Al), the δ13C of bulk sedimentary organic matter, and dissolution indices. We compare these proxies to benthic foraminiferal δ13C values measured on the same samples. Our results indicate that marine paleoproductivity in the Atlantic Ocean is not related to the benthic foraminiferal δ13C excursion. A numerical box model confirms that marine productivity cannot account for the δ13C maximum. The model shows that sequestration of 1.5 × 1018 mol C in the terrestrial realm over a period of 3 Ma leads to a 0.9‰ δ13C increase in the deep ocean, which is near the observed records. Therefore, an increase in continental organic carbon sequestration is the most plausible way to enrich the ocean's carbon pool with 13C, which is consistent with coeval lignite deposits worldwide. The δ13C values of bulk sedimentary organic matter parallel the δ13C of dissolved inorganic carbon as reflected by benthic foraminiferal δ13C values suggesting no significant change in atmospheric pCO2 levels over the investigated period.

Evidence for northward expansion of Antarctic Bottom Water mass in the Southern Ocean during the last glacial inception

We investigated deep water changes in the Southern Ocean during the last glacial inception, in relationship to surface hydrology and global climatology, to better understand the mechanisms of the establishment of a glacial ocean circulation. Changes in benthic foraminiferal δ13C from three high‐resolution cores are compared and indicate decoupled intermediate and deep water changes in the Southern Ocean. From the comparison with records from the North Atlantic, South Atlantic, and the Southern Ocean, we show that the early southern deep water δ13C drop observed at the MIS 5.5–5.4 transition occurred before any significant reduction of North Atlantic Deep Water ventilation. We propose that this drop is linked to the northward expansion of poorly ventilated Antarctic Bottom Water (AABW) mass in the Southern Ocean. Associated with an early cooling in the high southern latitudes, the westerly winds and surface oceanic fronts would migrate equatorward, thus weakening the upwelling of Circumpolar Deep Waters. Reduced heat brought to Antarctic surface waters would enhance sea ice formation during winters and the deep convection of cold and poorly ventilated AABW.

Abrupt changes of intermediate-water oxygen in the northwestern Pacific during the last 27 kyr

An oxygen minimum zone (OMZ) currently exists at intermediate water depths on the northern Japanese margin in the northwestern Pacific. The OMZ results largely from a combination of high surface–water productivity and poor ventilation of intermediate waters. We investigated the late Quaternary history (last 27 kyr) of the intensity of this OMZ using changes in benthic foraminiferal carbon isotopes and assemblages in a sediment core taken on the continental slope off Shimokita Peninsula, northern Japan, at a water depth of 975 m. The core was located well within the region of the present-day OMZ and high surface–water productivity. The benthic foraminiferal δ13C values, which indicate millennial-scale fluctuations of nutrient contents at the sediment–water interface, were 0.48‰ lower during the last glacial maximum (LGM) than during the late Holocene. These results do not indicate the formation of glacial intermediate waters of subarctic Pacific origin, but rather the large contribution of high-nutrient water masses such as the Antarctic Intermediate Water, implying that the regional circulation pattern during the LGM was similar to that of modern times. Benthic foraminiferal assemblages underwent major changes in response to changes in dissolved oxygen concentrations in ocean floor sediments. The lowest oxygen and highest nutrient conditions, marked by dysoxic taxa and negative values of benthic foraminiferal δ13C, occurred during the Bolling/Allerod (B/A) and Pre-Boreal warming events. Dysoxic conditions in this region during these intervals were possibly caused by high surface–water productivity at times of reduced intermediate–water ventilation in the northwestern Pacific. The benthic assemblages show dysoxic events on approx. 100- to 200-year cycles during the B/A, reflecting centennial-scale productivity changes related to freshwater cycles and surface–water circulation in the North Pacific.

Astronomically modulated Neogene sediment records from the South China Sea

General global cooling over the Neogene has been modulated by changes in Earth's orbital parameters. Investigations of deep‐sea sediment sequences show that various orbital cycles can dominate climate records for different latitudes or for different time intervals. However, a comprehensive understanding of astronomical imprints over the entire Neogene has been elusive because of the general absence of long, continuous records extending beyond the Pliocene. We present benthic foraminiferal δ18O and δ13C records over the past 23 Ma at Ocean Drilling Program Site 1148 in the northern South China Sea and construct an astronomically tuned timescale (TJ08) for these records based on natural gamma radiation and color reflectance data at this site. Our results show that a 41 ka cycle has dominated sediment records at this location over the Neogene, displaying a linear response to orbital forcing. A 100 ka cycle has also been significant. However, it is correlated nonlinearly with Earth's orbital variations at the 100 ka band. The sediment records also display a prominent 405 ka cycle. Although this cycle was coherent with orbital forcing during the Oligocene and the early Miocene, it was not coherent with Earth's orbital variations at the 405 ka band over the whole Neogene. Amplification of Northern Hemisphere and Southern Hemisphere glaciation since the middle Miocene may be responsible for this change in sedimentary response. Our benthic foraminifera δ18O and δ13C records further exhibit amplitude variations with longer periods of 600, 1000, 1200, and 2400 ka. Apparently, these cycles are nonlinear responses to insolation forcing.

345,000-year-long multi-proxy records off South Africa document variable contributions of Northern versus Southern Component Water to the Deep South Atlantic

Millennial to multi-centennial benthic foraminiferal stable isotope, sortable silt mean grain size (SS―) and benthic foraminiferal Cd/Ca records are presented for core MD96-2080 from 2488 m water depth at the western Agulhas Bank Slope off South Africa. The data demonstrate the interplay between Northern and Southern Component Waters as the Atlantic Meridional Overturning Circulation (MOC) shifted between glacial and interglacial modes. During early phases of Marine Isotope Stages (MIS) 6 and 8, the Atlantic MOC was little different from its interglacial mode. Benthic foraminiferal δ13C modulation during these stages can be explained by mean-ocean δ13C changes, while, only subtle benthic foraminiferal Cd/Ca increases indicate a continued influence of Northern Component Waters (NCW). Later glacial stages are characterized by a progressive incursion of nutrient-enriched waters presumably a mixture of Upper and Lower Southern Component Waters (SCW). Maximum SS― values during late glacial ventilation minima suggest increased near-bottom flow speeds. The combined ventilation and flow-speed pattern indicates an enhanced influence of SCW, most likely linked with a northward migration of the Antarctic Circumpolar Current (ACC) which progressively decreased the influence of NCW at site MD96-2080. Peak-maximum seawater Cd concentrations (Cdsw, derived from benthic foraminiferal Cd/Ca) overlap in time with increased deposition of ice-rafted debris (IRD) in the North Atlantic and plot outside the δ13C/Cdsw field in the South Atlantic. The anomalies conceivably are the South Atlantic equivalents of peak Cdsw maxima associated with Heinrich-type events in the North Atlantic and reflect a substantially reduced Atlantic MOC.

Organic matter rain rates, oxygen availability, and vital effects from benthic foraminiferal δ13C in the historic Skagerrak, North Sea

The sediment cores 225514 and 225510 were recovered from 420 and 285 m water depth, respectively. They were investigated for their benthic foraminiferal δ 13C during the last 500 years. Both cores were recovered from the southern flank of the Skagerrak. The δ 13C values of Uvigerina mediterranea and other shallow infaunal species in both cores indicate that organic matter rain rates to the seafloor varied around a mean value until approximately AD 1950 after which they increased. This increase might result from changes in the North Atlantic Current System and a co-occurring persistently high North Atlantic Oscillation index state in the 1980s to 1990s, rather than from anthropogenic eutrophication. Using δ 13C mean values of multiple species, we reconstruct δ 13C gradients of dissolved inorganic carbon (DIC) within pore waters for the time periods AD 1500 to 1950 and AD 1950 to 2000. The calculated δ 13C DIC ranges, interpreted as indicating total organic matter remineralization due to respiration, are generally bigger in Core 225514 than in Core 225510. Since mean δ 13C values of U. mediterranea suggest that organic matter rain rates were similar at both locations, differences in total organic matter remineralization are attributed to differing oxygen availability. However, oxygen concentrations in the overlying bottom water masses are not likely to have differed significantly. Thus, we suggest that organic matter remineralization was controlled by oxygen availability within the sediments, reflecting strong differences in sedimentation rates at the two investigated core sites. Based on the assumptions that tests of benthic foraminiferal species inhabiting the same microhabitat depth should show equal δ 13C values unless they are affected by vital effects and that Globobulimina turgida records pore water δ 13C DIC, we estimate microhabitat-corrected vital effects for several species with respect to G. turgida: > 0.7‰ for Cassidulina laevigata, > 1.3‰ for Hyalinea balthica, and > 0.7‰ for Melonis barleeanus. Melonis zaandami seems to closely record pore water δ 13C DIC.

Late Miocene carbon isotope records and marine biological productivity: Was there a (dusty) link?

We examine whether or not a relationship exists between the late Miocene carbon isotope shift (∼7.6–6.6 Ma) and marine productivity at four sites from the Indian and Pacific Oceans (Ocean Drilling Program Sites 721, 1146, 1172, and 846). We use a multiproxy approach based on benthic foraminiferal accumulation rates, elemental ratios, and dissolution indices, and we compare these data to benthic foraminiferal δ13C values measured on the same samples. Although some of these sites have been targeted previously in studies of either the late Miocene/early Pliocene “biogenic bloom” (Sites 721 and 846) or the late Miocene carbon isotope shift (Site 1172), our records are the first to establish paired proxy records of carbon isotopes and paleoproductivity allowing a direct assessment of a potential link. Our results indicate that at all sites, productivity increased sometime during the δ13C shift; at three sites (721, 1146, and 846), productivity increased at the beginning of the shift. The correlation coefficients derived from linear regression between micropaleontologically derived productivity and foraminiferal δ13C values are relatively high during the time interval containing the late Miocene δ13C shift (and statistically significant at three of the sites). Carbon flux and isotope mass balance considerations illustrate that transfer of organic matter between the terrestrial and marine reservoirs together with enhanced oceanic upwelling best approximates observed changes in carbon isotope records and paleoproductivity. We note that long‐term trend in the Site 846 paleoproductivity record can be correlated to the long‐term trend in the Site 848 eolian flux reconstructions of Hovan (1995) hinting at a link between strengthened wind regime and productivity during the late Miocene.