Stable isotopes, core logging data, relative paleointensity, dry bulk density, biogenic opal, CN-data (TC, TOC, CaCO3, TN), element concentrations, and coarse fraction of three sediment cores from the western Bering Sea

We used piston cores recovered in the western Bering Sea to reconstruct millennial-scale changes in marine productivity and terrigenous matter supply over the past ~180 kyr. Based on a geochemical multi-proxy approach, our results indicate closely interacting processes controlling marine productivity and terrigenous matter supply comparable to the situation in the Okhotsk Sea. Overall, terrigenous inputs were high, whereas export production was low. Minor increases in marine productivity occurred during intervals of Marine Isotope Stage 5 and interstadials, but pronounced maxima were recorded during interglacials and Termination I. The terrigenous material is suggested to be derived from continental sources on the eastern Bering Sea shelf and to be subsequently transported via sea ice, which is likely to drive changes in surface productivity, terrigenous inputs, and upper-ocean stratification. From our results we propose glacial, deglacial, and interglacial scenarios for environmental change in the Bering Sea. These changes seem to be primarily controlled by insolation and sea-level forcing which affect the strength of atmospheric pressure systems and sea-ice growth. The opening history of the Bering Strait is considered to have had an additional impact. High-resolution core logging data (color b*, XRF scans) strongly correspond to the Dansgaard-Oeschger climate variability registered in the NGRIP ice core and support an atmospheric coupling mechanism of Northern Hemisphere climates.

Particulate organic matter (POM) of samples from the Lena River delta

Particulate organic matter (POM) derived from permafrost soils and transported by the Lena River represents a quantitatively important terrestrial carbon pool exported to Laptev Sea sediments (next to POM derived from coastal erosion). Its fate in a future warming Arctic, i.e., its remobilization and remineralization after permafrost thawing as well as its transport pathways to and sequestration in marine sediments, is currently under debate. We present one of the first radiocarbon (14C) data sets for surface water POM within the Lena Delta sampled in the summers of 2009 - 2010 and spring 2011 (n = 30 samples). The bulk D14C values varied from -55 to -391 per mil translating into 14C ages of 395 to 3920 years BP. We further estimated the fraction of soil-derived POM to our samples based on (1) particulate organic carbon to particulate nitrogen ratios (POC : PN) and (2) on the stable carbon isotope (d13C) composition of our samples. Assuming that this phytoplankton POM has a modern 14C concentration, we inferred the 14C concentrations of the soil-derived POM fractions. The results ranged from -322 to -884 per mil (i.e., 3060 to 17 250 14C years BP) for the POC : PN-based scenario and from -261 to -944 per mil (i.e., 2370 to 23 100 14C years BP) for the d13C-based scenario. Despite the limitations of our approach, the estimated D14C values of the soil-derived POM fractions seem to reflect the heterogeneous 14C concentrations of the Lena River catchment soils covering a range from Holocene to Pleistocene ages better than the bulk POM D14C values. We further used a dual-carbon-isotope three-end-member mixing model to distinguish between POM contributions from Holocene soils and Pleistocene Ice Complex (IC) deposits to our soil-derived POM fraction. IC contributions are comparatively low (mean of 0.14) compared to Holocene soils (mean of 0.32) and riverine phytoplankton (mean of 0.55), which could be explained with the restricted spatial distribution of IC deposits within the Lena catchment. Based on our newly calculated soil-derived POM D14C values, we propose an isotopic range for the riverine soil-derived POM end member with D14C of -495 ± 153 per mil deduced from our d13C-based binary mixing model and d13C of -26.6 ± 1 per mil deduced from our data of Lena Delta soils and literature values. These estimates can help to improve the dual-carbon-isotope simulations used to quantify contributions from riverine soil POM, Pleistocene IC POM from coastal erosion, and marine POM in Siberian shelf sediments.

(Table 1) Stable carbon isotope record and total nitrogen concentration in ODP Hole 175-1085B sediments

Variations in the strength of coastal upwelling in the South East Atlantic Ocean and summer monsoonal rains over South Africa are controlled by the regional atmospheric circulation regime. Although information about these parameters exists for the last glacial period, little detailed information exists for older time periods. New information from ODP Site 1085 for Marine Isotope Stages (MIS) 12-10 shows that glacial-interglacial productivity trends linked to upwelling variability followed a pattern similar to the last glacial cycle, with maximums shortly before glacial maxima, and minimums shortly before glacial terminations. During the MIS-11/10 transition, several periodic oscillations in productivity and monsoonal proxies are best explained by southwards shifts in the southern sub-tropical high-pressure cells followed by abrupt northwards shifts. Comparison to coeval sea-surface temperature measurements suggests that these monsoonal cycles were tightly coupled to anti-phased hemispheric climate change, with an intensified summer monsoon during periods of Northern (Southern) Hemisphere cooling (warming). The timing of these events suggests a pacing by insolation over precession periods. A lack of similar regional circulation shifts during the MIS-13/12 transition is likely due to the large equatorwards shift in the tropical convection zone that occurred during this extreme glaciation.

Stable isotope composition of early Danian sediments from the Atlantic Ocean

A long-standing question in Paleogene climate concerns the frequency and mechanism of transient greenhouse gas-driven climate shifts (hyperthermals). The discovery of the greenhouse gas-driven Paleocene-Eocene Thermal Maximum (PETM; ~55 Ma) has spawned a search for analogous events in other parts of the Paleogene record. On the basis of high-resolution bulk sediment and foraminiferal stable isotope analyses performed on three lower Danian sections of the Atlantic Ocean, we report the discovery of a possible greenhouse gas-driven climatic event in the earliest Paleogene. This event - that we term the Dan-C2 event - is characterized by a conspicuous double negative excursion in delta13C and delta18O, associated with a double spike in increased clay content and decreased carbonate content. This suggests a double period of transient greenhouse gas-driven warming and dissolution of carbonates on the seafloor analogous to the PETMin the early Paleocene at ~65.2 Ma. However, the shape of the two negative carbon isotope excursions that make up the Dan-C2 event is different from the PETM carbon isotope profile. In the Dan-C2 event, these excursions are fairly symmetrical and each persisted for about ~40 ky and are separated by a short plateau that brings the combined duration to ~100 ky, suggesting a possible orbital control on the event. Because of the absence of a long recovery phase, we interpret the Dan-C2 event to have been associated with a redistribution of carbon that was already in the biosphere. The Dan-C2 event and other early Paleogene hyperthermals such as the short-lived early Eocene ELMO eventmay reflect amplification of a regular cycle in the size and productivity of the marine biosphere and the balance between burial of organic and carbonate carbon.

Stable oxygen and carbon isotope composition of benthic foraminifera from the western Mediterranean Sea

The influence of microhabitat, organic matter flux, and metabolism on the stable oxygen and carbon isotope composition of live (Rose Bengal stained) and dead (empty tests) deep-sea benthic foraminifera from the Gulf of Lions (western Mediterranean Sea) have been studied. The total range of observed foraminiferal isotope values exceeds 1.0 per mil for d18O and 2.2 per mil for d13C demonstrating a wide range of coexisting disequilibria relative to d18O of equilibrium calcite (d18OEQ) and d13C of bottom water dissolved inorganic carbon (d13CDIC). The mean d18O values reveal strongest disequilibria for the studied epifaunal to shallow infaunal species (Cibicidoides pachydermus, Uvigerina mediterranea, Uvigerina peregrina) while values approach equilibrium in deep infaunal species (Globobulimina affinis, Globobulimina pseudospinescens). The mean d13C values decrease with increasing average living depths of the different species, thus reflecting a dominant microhabitat (pore water) signal. At the axis of the Lacaze-Duthier Canyon a minimum d13CDIC pore water gradient of approximately -2.1 per mil is assessed for the upper 6 cm of the surface sediment. Although live individuals of U. mediterranea were found in different depth intervals their mean d13C values are consistent with calcification at an average living depth around 1 cm. The deep infaunal occurrence of U. mediterranea specimens suggests association with macrofaunal burrows creating a microenvironment with geochemical characteristics similar to the topmost centimeter. This also explains the excellent agreement between stable isotope signals of live and dead individuals. The ontogenetic enrichment in both d18O and d13C values of U. mediterranea suggests a slow-down of metabolic rates during test growth similar to that previously observed in planktic foraminifera. Enhanced organic carbon fluxes and higher proportion of resuspended terrestrial organic material at the canyon axis are reflected by d13C values of U. mediterranea on average 0.58 per mil lower than those from the open slope. These results demonstrate the general applicability of the d13C signal of this species for the reconstruction of past organic matter fluxes in the Mediterranean Sea. Further studies on live specimens are needed for a more quantitative paleoceanographic approach.

Stable oxygen isotope ratios of marine barite from Cenozoic sediments

We report new data on oxygen isotopes in marine sulfate (delta18O[SO4]), measured in marine barite (BaSO4), over the Cenozoic. The delta18O[SO4] varies by 6x over the Cenozoic, with major peaks 3, 15, 30 and 55 Ma. The delta18O[SO4] does not co-vary with the delta18O[SO4], emphasizing that different processes control the oxygen and sulfur isotopic composition of sulfate. This indicates that temporal changes in the delta18O[SO4] over the Cenozoic must reflect changes in the isotopic fractionation associated with the sulfide reoxidation pathway. This suggests that variations in the aerial extent of different types of organic-rich sediments may have a significant impact on the biogeochemical sulfur cycle and emphasizes that the sulfur cycle is less sensitive to net organic carbon burial than to changes in the conditions of that organic carbon burial. The delta18O[SO4] also does not co-vary with the d18O measured in benthic foraminifera, emphasizing that oxygen isotopes in water and sulfate remain out of equilibrium over the lifetime of sulfate in the ocean. A simple box model was used to explore dynamics of the marine sulfur cycle with respect to both oxygen and sulfur isotopes over the Cenozoic. We interpret variability in the delta18O[SO4] to reflect changes in the aerial distribution of conditions within organic-rich sediments, from periods with more localized, organic-rich sediments, to periods with more diffuse organic carbon burial. While these changes may not impact the net organic carbon burial, they will greatly affect the way that sulfur is processed within organic-rich sediments, impacting the sulfide reoxidation pathway and thus the delta18O[SO4]. Our qualitative interpretation of the record suggests that sulfate concentrations were probably lower earlier in the Cenozoic.

Stable isotope composition of Holocene benthic foraminifers from the Eastern Mediterranean Sea

Temporal and regional changes in paleoproductivity and paleoceanography in the eastern Mediterranean Sea during the past 12 kyr were reconstructed on the basis of the stable oxygen and carbon isotope composition of the epibenthic Planulina ariminensis and the shallow endobenthic Uvigerina mediterranea from three sediment cores of the Aegean Sea and Levantine Basin. The Younger Dryas is characterized by high d18O values, indicating enhanced salinities and low temperatures of deep water masses at all investigated sites. With the onset of the Holocene, d18O records show a continuous decrease towards the onset of sapropel S1 formation, mainly caused by a freshening and warming of surface waters at deep water formation sites. In the middle and late Holocene, the similarity of d18O values from the southern Aegean Sea and Levantine Basin suggests the influence of isotopically identical deep water masses. By contrast, slightly higher d18O values are observed the northern Aegean Sea, which probably point to lower temperatures of North Aegean deep waters. The epifaunal d13C records reveal clear changes in sources and residence times of eastern Mediterranean deep waters associated with period of S1 formation. Available data for the early and late phase of sapropel S1 formation and for the interruption around 8.2 kyr display drops by 0.5 and 1.5 per mil, indicating the slow-down of deep water circulation and enhanced riverine input of isotopically light dissolved inorganic carbon from terrestrial sources into the eastern Mediterranean Sea. The decrease in epifaunal d13C signals is particularly expressed in the southern Aegean Sea and Levantine Basin, while it is less pronounced in the northern Aegean Sea. This points to a strong reduction in deep water exchange rates in the southern areas, but the persistence of local deep water formation in the northern Aegean Sea. The d13C values of U. mediterranea records reveal temporal and regional differences in paleoproductivity during the past 12 kyr, with rather eutrophic and mesotrophic conditions in the North Aegean Sea and southeast Levantine Basin, respectively, while the South Aegean Sea is characterized by rather oligotrophic conditions. After S1 formation, increasing d13C values reflect a progressive decrease in surface water productivity in the eastern Mediterranean Sea during the middle and late Holocene. In the northern Aegean Sea, this time interval is marked by repetitive changes in organic matter fluxes documented by significant fluctuations in the d13C signal of U. mediterranea on millennial- to multi-centennial time scales. These fluctuations can be linked to short-term changes in river runoff driven by northern hemisphere climatic variability.

Terrestrial organic matter in marine sediments of the Central Equatorial Atlantic

Terrestrial organic matter (OM) in pelagic sediments is discussed with regard to depositional processes and land-sea interactions in the modern and past glacial/interglacial Equatorial Atlantic. Special emphasis is placed on a critical evaluation of different analytical approaches (C/N, Rock-Eval Pyrolysis, stable carbon isotopes, palynology, organic petrology, and selected biomarkers) which are currently used for the qualitative and quantitative assessment of terrigenous organic carbon. If binary mixing equations are used to calculate terrestrial and marine proportions of organic carbon, we consider the definition of endmember values to be most critical since these values may be biased by a great number of independent controls. A combination of geochemical methods including optical studies (organic petrology and palynology) is therefore suggested to evaluate each individual proxy. Organic geochemical analyses performed on sediments from the modern and Late Quaternary Equatorial Atlantic evidence fluctuations in eolian supply of terrigenous OM related to changes in intensity of the trade winds. Quantification of this organic fraction leads to differing proportions depending on the approach applied, i.e. the organic carbon isotopic composition or maceral analyses. Modern distribution of terrigenous OM reveals a decrease in supply towards the basin contributing less than a fifth of the total OM in pelagic areas. Organic geochemical data indicate that sedimentation in the modern northeastern Brasil Basin is affected by lateral advection of reworked OM probably from southern source areas. Glacial/interglacial deposits from the pelagic Equatorial Atlantic (ODP Site 663), covering isotopic stages 12 and 11, reveal that deposition of terrigenous OM was higher under past glacial conditions, in correspondence to generally enhanced dust fluxes. Proportions of terrigenous OM, however, never exceed 50% of the total OM according to maceral analyses. Other estimates, recently proposed by Verardo and Ruddiman (1996), are considered to be too high probably for analytical reasons. Palynological records in the Equatorial Atlantic parallel dust records. Increased portions of grass pollen suggest the admixture of C4-plant material under modern and past glacial conditions. It is therefore assumed, as one possible interpetation, that C4-plant debris has an effect on sedimentary d13Corg and might explain differences between isotopic and microscopic quantitative estimates. Using the difference between these two records, we calculate that maximum supply of C4-material remains below 20% of the total OM for the deep modern and past glacial/interglacial Equatorial Atlantic.