Organic carbon, reduced sulfur, and iron in Miocene to Holocene sediments from the Oman and Beguela upwelling systems

We examined sediments from Neogene and Quaternary sections of the Benguela and Oman upwelling systems (DSDP Site 532, ODP Sites 723 and 722) to determine environmental and geochemical factors which control and limit pyrite formation in organic-carbon-rich marine sediments. Those samples from the upwelling sites, which contained low to moderate concentrations of total organic carbon (0.7%-3%), had C/S ratios typical of normal marine sediments, i.e., around 2.8. In these sediments, TOC availability probably limited pyrite formation. Results that do not conform with accepted models were found for the sediments high in TOC (3^0-12.4%). The organic matter was of marine origin and contained considerable pyrolytic hydrocarbons, a fact that we take as a sign of low degradation, yet significant concentrations of dissolved sulfate coexisted with it (> 5 mmol/L in the case of Sites 532 and 723). Detrital iron was probably not limiting in either case, because the degree of pyritization was always less than 0.65. Therefore, controls on sulfate reduction and pyrite formation in the organic matter-rich sediments do not appear to conform simply to generally accepted diagenetic models. The data from these thermally immature, old, and organic-rich marine sediments imply that (1) the total reduced sulfur content of organic-rich marine upwelling sediments rarely exceeds an approximate boundary of 1.5% by weight, (2) the C/S ratio of these sediments is not constant and usually much higher than the empirical values proposed for marine sediments. We conclude that sedimentary pyrite formation in upwelling sediments is limited by an as yet unknown factor, and that caution is advised in using C/S ratios and C vs. S diagrams in paleoenvironmental reconstructions for organic-rich sediments.

Stable isotope ratios, carbon content and primary production of three sediment cores from the subantarctic eastern Atlantic

We present three new benthic foraminiferal delta13C, delta18O, and total organic carbon time series from the eastern Atlantic sector of the Southern Ocean between 41°S and 47°S. The measured glacial delta13C values belong to the lowest hitherto reported. We demonstrate a coincidence between depleted late Holocene (LH) delta13C values and positions of sites relative to ocean surface productivity. A correction of +0.3 to +0.4 [per mil VPDB] for a productivity-induced depletion of Last Glacial Maximum (LGM) benthic delta13C values of these cores is suggested. The new data are compiled with published data from 13 sediment cores from the eastern Atlantic Ocean between 19°S and 47°S, and the regional deep and bottom water circulation is reconstructed for LH (4-0 ka) and LGM (22-16 ka) times. This extends earlier eastern Atlantic-wide synoptic reconstructions which suffered from the lack of data south of 20°S. A conceptual model of LGM deep-water circulation is discussed that, after correction of southernmost cores below the Antarctic Circumpolar Current (ACC) for a productivity-induced artifact, suggests a reduced formation of both North Atlantic Deep Water in the northern Atlantic and bottom water in the southwestern Weddell Sea. This reduction was compensated for by the formation of deep water in the zone of extended winter sea-ice coverage at the northern rim of the Weddell Sea, where air-sea gas exchange was reduced. This shift from LGM deep-water formation in the region south of the ACC to Holocene bottom water formation in the southwestern Weddell Sea, can explain lower preformed d13CDIC values of glacial circumantarctic deep water of approximately 0.3 per mil to 0.4 per mil. Our reconstruction brings Atlantic and Southern Ocean d13C and Cd/Ca data into better agreement, but is in conflict, however, with a scenario of an essentially unchanged thermohaline deep circulation on a global scale. Benthic delta18O-derived LGM bottom water temperatures, by 1.9°C and 0.3°C lower than during the LH at deepest southern and shallowest northern sites, respectively, agree with the here proposed reconstruction of deep-water circulation in the eastern South Atlantic Ocean.

Stable carbon isotope record and geochemistry of late Miocene-early Pliocene sediments from DSDP Site 90-590 in the Tasman Sea

Biogenic components of sediment accumulated at high rates beneath frontal zones of the Indian and Pacific oceans during the late Miocene and early Pliocene. The delta13C of bulk and foraminiferal carbonate also decreased during this time interval. Although the two observations may be causally linked, and signify a major perturbation in global biogeochemical cycling, no site beneath a frontal zone has independent records of export production and delta13C on multiple carbonate phases across the critical interval of interest. Deep Sea Drilling Project (DSDP) site 590 lies beneath the Tasman Front (TF), an eddy-generating jetstream in the southwest Pacific Ocean. To complement previous delta13C records of planktic and benthic foraminifera at this location, late Neogene records of CaCO3 mass accumulation rate (MAR), Ca/Ti, Ba/Ti, Al/Ti, and of bulk carbonate and foraminiferal delta13C were constructed at site 590. The delta13C records include bulk sediment, bulk sediment fractions (<63 µm and 5-25 µm), and the planktic foraminifera Globigerina bulloides, Globigerinoides sacculifer (with and without sac), and Orbulina universa. Using current time scales, CaCO3 MARs, Ca/Ti, Al/Ti and Ba/Ti ratios are two to three times higher in upper Miocene and lower Pliocene sediment relative to overlying and underlying units. A significant decrease also occurs in all delta13C records. All evidence indicates that enhanced export production - the 'biogenic bloom' - extended to the southwest Pacific Ocean between ca. 9 and 3.8 Ma, and this phenomenon is coupled with changes in delta13C - the 'Chron C3AR carbon shift'. However, CaCO3 MARs peak ca. 5 Ma whereas elemental ratios are highest ca. 6.5 Ma; foraminiferal delta13C starts to decrease ca. 8 Ma whereas bulk carbonate delta13C begins to drop ca. 5.6 Ma. Temporal discrepancies between the records can be explained by changes in the upwelling regime at the TF, perhaps signifying a link between changes in ocean-atmosphere circulation change and widespread primary productivity.

On the Southern Ocean CO2 uptake and the role of the biological carbon pump in the 21st century, links to supplementary material

We use a suite of eight ocean biogeochemical/ecological general circulation models from the MAREMIP and CMIP5 archives to explore the relative roles of changes in winds (positive trend of Southern Annular Mode, SAM) and in warming- and freshening-driven trends of upper ocean stratification in altering export production and CO2 uptake in the Southern Ocean at the end of the 21st century. The investigated models simulate a broad range of responses to climate change, with no agreement ona dominance of either the SAM or the warming signal south of 44° S. In the southernmost zone, i.e., south of 58° S, they concur on an increase of biological export production, while between 44 and 58° S the models lack consensus on the sign of change in export. Yet, in both regions, the models show an enhanced CO2 uptake during spring and summer. This is due to a larger CO 2 (aq) drawdown by the same amount of summer export production at a higher Revelle factor at the end of the 21st century. This strongly increases the importance of the biological carbon pump in the entire Southern Ocean. In the temperate zone, between 30 and 44° S all models show a predominance of the warming signal and a nutrient-driven reduction of export production. As a consequence, the share of the regions south of 44° S to the total uptake of the Southern Ocean south of 30° S is projected to increase at the end of the 21st century from 47 to 66% with a commensurable decrease to the north. Despite this major reorganization of the meridional distribution of the major regions of uptake, the total uptake increases largely in line with the rising atmospheric CO2. Simulations with the MITgcm-REcoM2 model show that this is mostly driven by the strong increase of atmospheric CO2, with the climate-driven changes of natural CO2 exchange offsetting that trend only to a limited degree (~10%) and with negligible impact of climate effects on anthropogenic CO2 uptake when integrated over a full annual cycle south of 30° S.

Oxygen and carbon isotope ratios for six planktonic foraminifer species from different water depth, Ontong-Java Plateau, Pacific (Table 2)

Stable isotope analyses and scanning electron micrographs have been carried out on six planktonic forminifera species, Pulleniatina obliquiloculata, Globorotalia tumida, Sphaeroidinella dehiscens, Globigerinoides ruber, Globigerinoides sacculifer and Globigerinoides quadrilobatus from eleven box-cores taken at increasing depths in the equatorial Ontong-Java Plateau (Pacific). This allows us to describe the way dissolution affects the microstructures of the tests of the different species and to quantify the changes of isotopic composition. We may conclude that: 1) dissolution effects on test morphology and stable isotope compositions are species dependent, species with a similar habitat showing a similar trend; 2) the shallow water, thin-shelled species are the first to disappear: scanning electron microscope (SEM) work shows alteration of outer layers. Deep water, thick-shelled species are present in all samples: SEM work shows breakdown and disparition of inner layers; 3) for all species there is a similar trend towards increasing delta18O values with increasing water depths and increasing dissolution. This effect may be as high as 0.6 ? per thousand meters for Globorotalia tumida; 4) below the lysocline, around 3500 m, it appears that 13C/12C ratios slightly increase towards equilibrium values for thick shelled species: G. tumida, P. obliquiloculata and S. dehiscens. 14C dates and isotope stratigraphy of two box-cores show that all samples are recent in age, and exclude upward mixing of glacial deposits as an important factor.

Comparison of the stable carbon and nitrogen isotopic values of gill and white muscle tissue of fish

The potential use of stable carbon and nitrogen isotope ratios (d13C, d15N) of fish gills for studies on fish feeding ecology was evaluated by comparing the d13C and d15N of gill tissue with the more commonly used white muscle tissue. To account for the effect of lipid content on the d13C signatures, a study-specific lipid correction model based on C:N ratios was developed and applied to the bulk d13C data. For the majority of species in the study, we found no significant difference in d13C values between gill and muscle tissue after correction, but several species showed a small (0.3-1.4 per mil) depletion in 13C in white muscle compared to gill tissue. The average species difference in d15N between muscle and gill tissue ranged from -0.2 to 1.6 per mil for the different fish species with muscle tissue generally more enriched in 15N. The d13C values of muscle and gill were strongly linearly correlated (R**2 = 0.85) over a large isotopic range (13 per mil), suggesting that both tissues can be used to determine long-term feeding or migratory habits of fish. Muscle and gill tissue bulk d15N values were also strongly positively correlated (R**2= 0.76) but with a small difference between muscle and gill tissue. This difference indicates that the bulk d15N of the two tissue types may be influenced by different isotopic turnover rates or a different composition of amino acids.

Age determination, uranium, carbon and oxygen isotopes ratios of a stalagmite of the Villars cave

The signature of Dansgaard-Oeschger events - millennial-scale abrupt climate oscillations during the last glacial period - is well established in ice cores and marine records (Labeyrie, 2000, doi:10.1126/science.290.5498.1905; Blunier and Brook, 2001, doi:10.1126/science.291.5501.109: Bond et al., 2001, doi:10.1126/science.1065680). But the effects of such events in continental settings are not as clear, and their absolute chronology is uncertain beyond the limit of 14C dating and annual layer counting for marine records and ice cores, respectively. Here we present carbon and oxygen isotope records from a stalagmite collected in southwest France which have been precisely dated using 234U/230Th ratios. We find rapid climate oscillations coincident with the established Dansgaard-Oeschger events between 83,000 and 32,000 years ago in both isotope records. The oxygen isotope signature is similar to a record from Soreq cave, Israel (Bar-Mathews et al., 2000, doi:10.1016/S0009-2541(99)00232-6), and deep-sea records (Bond et al., 1993, doi:10.1038/365143a0; Shackleton and Hall, 2001, doi:10.1029/2000PA000513), indicating the large spatial scale of the climate oscillations. The signal in the carbon isotopes gives evidence of drastic and rapid vegetation changes in western Europe, an important site in human cultural evolution. We also find evidence for a long phase of extremely cold climate in southwest France between 61.2 +/-0.6 and 67.4 0.9 kyr ago.

Isotopic data of atmospheric methane from EDML and Vostok ice cores over a full glacial cycle

The response of natural CH4 sources to climate changes will be an important factor to consider as concentrations of this potent greenhouse gas continue to increase. Polar ice cores provide the means to assess this sensitivity in the past and have shown a close connection between CH4 levels and northern hemisphere temperature variability over the last glacial cycle. However, the contribution of the various CH4 sources and sinks to these changes is still a matter of debate. Contemporaneous stable CH4 isotope records in ice cores provide additional boundary conditions for assessing changes in the CH4 sources and sinks. Here we present new ice core CH4 isotope data covering the last 160,000 years, showing a clear decoupling between CH4 loading and carbon isotopic variations over most of the record. We suggest that d13CH4 variations were not dominated by a change in the source mix but rather by climate- and CO2-related ecosystem control on the isotopic composition of the methane precursor material, especially in seasonally inundated wetlands in the tropics. In contrast, relatively stable d13CH4 intervals occurred during large CH4 loading changes concurrently with past climate changes implying that most CH4 sources (most notably tropical wetlands) responded simultaneously.

Planktic foraminifers, oxygen- and carbon isotopic compositions, and radiocarbon age in bottom sediments from Core AMK4-316GC

In a sediment core AMK4-316 (460 cm long) on the basis of radiocarbon, oxygen isotope, and lithological data climatostratigraphy is established for time interval about 145 ka. The method of factor analysis and spline interpolation applied to data on distribution of planktic foraminifera species has allowed to reconstruct average annual and seasonal temperatures and salinity at the surface and at depth 100 m. The optimum of the Last Interglaciation (5e) is characterized by maximal temperatures, low amplitudes of seasonal fluctuations, and by increased thickness of the upper homogeneous layer. The glacial hydrological mode has arisen here 115 ka ago. Coolings outstripped appropriate events of the global continental glaciation. Minimal average annual temperatures (4-4.5°C) are reconstructed for 47-45, 42, 36, 29-30, and 10 ka. For 50-30 ka interval numerous strong temperature fluctuations that reflect migrations of the polar front are established. Maximal differences of salinity at the surface and depth 100 m showing influence of melting waters were in the beginning of deglaciations (135 and 20 ka) and repeatedly arose in 50-30 ka interval. The Last Glacial Maximum (18 ka) is characterized by the lowest salinity but not by a peak of low temperatures at the surface. Surface temperature was lowered up to 10 ka. Average annual surface temperature of the Holocene optimum was 2°C above the modern one and 2°C below temperature in the Interglaciation optimum (5e), thickness of the upper homogeneous layer exceeded 100 m.

Benthic organic carbon flux and oxygen penetration depth in the Southern Ocean

For the investigation of organic carbon fluxes reaching the seafloor, oxygen microprofiles were measured at 145 sites in different sub-regions of the Southern Ocean. At eleven sites, an in situ oxygen microprofiler was deployed for the measurement of oxygen profiles and the calculation of organic carbon fluxes. At four sites, both in situ and ex situ data were determined for high latitudes. Based on this dataset as well as on previous published data, a relationship was established for the estimation of fluxes derived by ex situ measured O2 profiles. The fluxes of labile organic matter range from 0.5 to 37.1 mgC m**2/day. The high values determined by in situ measurements were observed in the Polar Front region (water depth of more than 4290 m) and are comparable to organic matter fluxes observed for high-productivity, upwelling areas like off West Africa. The oxygen penetration depth, which reflects the long-term organic matter flux to the sediment, was correlated with assemblages of key diatom species. In the Scotia Sea (~3000 m water depth), oxygen penetration depths of less than 15 cm were observed, indicating high benthic organic carbon fluxes. In contrast, the oxic zone extends down to several decimeters in abyssal sediments of the Weddell Sea and the southeastern South Atlantic. The regional pattern of organic carbon fluxes derived from micro-sensor data suggest that episodic and seasonal sedimentation pulses are important for the carbon supply to the seafloor of the deep Southern Ocean.

(Table 1) Organic carbon, hydrogen and oxygen index and hydrocarbons at DSDP Hole 93-603B

C2-C8 hydrocarbon concentrations (about 35 compounds identified, including saturated, aromatic, and olefinic compounds) from 27 shipboard-sealed, deep-frozen core samples of DSDP Hole 603B off the east coast of North America were determined by a gas-stripping/thermovaporization method. Total yields representing the hydrocarbons dissolved in the pore water and adsorbed on the mineral surfaces of the sediments vary from 22 to 2400 ng/g of dryweight sediment. Highest yields are measured in the two black shale samples of Core 603B-34 (hydrogen index of 360 and 320 mg/g Corg, respectively). In organic-carbon-normalized units these samples have hydrocarbon contents of 12,700 and 21,500 ng/g Corg, respectively, indicating the immaturity of their kerogens. Unusually high organic-carbonnormalized yields are associated with samples that are extremely lean in organic carbon. It is most likely that they are enriched by small amounts of migrated light hydrocarbons. This applies even to those samples with high organic-carbon contents (1.3-2.2%) of Sections 603B-28-4, 603B-29-1, 603B-49-2, and 603B-49-3, because they have an extremely low hydrocarbon potential (hydrogen index between 40 and 60 mg/g Corg). Nearly all samples were found to be contaminated by varying amounts of acetone that is used routinely in large quantities on board ship during core-cutting procedures. Therefore, 48 samples from the original set of 75 collected had to be excluded from the present study.

(Appendix) Total organic carbon, pyrolysis characteristics, and organic matter types at DSDP Holes 77-535 and 77-540

Lower to middle Cretaceous sediments in the eastern Gulf of Mexico are richer in organic matter and have a more marine organic facies than their counterparts in the nearby western North Atlantic, suggesting that the Gulf was the more productive of the two areas. As in the western North Atlantic, the rate of supply of terrestrial organic matter was high when the rate of supply of noncarbonate clastic materials was high (at times of low sea level) and diminished as sea level rose. The rate of supply of marine organic matter was lower in the Early Cretaceous than in the Cenomanian, perhaps in response to the global rise in sea level over this period. Where they are thermally mature, the organic matterrich units drilled at Sites 535 and 540 should be excellent sources for liquid hydrocarbons. The Pleistocene sediments of the eastern Gulf are dominated by terrestrial organic matter representing Mississippi River effluent.

Compilation of organic carbon distribution and sedimentology in the surface sediments on the continental margin offshore southwestern Africa

In this study we demonstrate the relevance of lateral particle transport in nepheloid layers for organic carbon (OC) accumulation and burial across high-productive continental margins. We present geochemical data from surface sediments and suspended particles in the bottom nepheloid layer (BNL) from the most productive coastal upwelling area of the modern ocean, the Benguela upwelling system offshore southwest Africa. Interpretation of depositional patterns and comparison of downslope trends in OC content, organic matter composition, and 14C age between suspended particles and surface sediments indicate that lateral particle transport is the primary mechanism controlling supply and burial of OC. We propose that effective seaward particle transport primarily along the BNL is a key process that promotes and maintains local high sedimentation rates, ultimately causing high preservation of OC in a depocenter on the upper slope offshore Namibia. As lateral transport efficiently displaces areas of enhanced OC burial from maximum production at highly productive continental margins, vertical particle flux models do not sufficiently explain the relationship between primary production and shallow-marine OC burial. On geologic time scales, the widest distribution and strongest intensity of lateral particle transport is expected during periods of rapid sea-level change. At times in the geologic past, widespread downslope lateral transport of OC thus may have been a primary driver of enhanced OC burial at deeper continental slopes and abyssal basins.

Size-partitioned phytoplankton carbon concentrations retrieved from ocean color data, links to data in netCDF format

Owing to their important roles in biogeochemical cycles, phytoplankton functional types (PFTs) have been the aim of an increasing number of ocean color algorithms. Yet, none of the existing methods are based on phytoplankton carbon (C) biomass, which is a fundamental biogeochemical and ecological variable and the "unit of accounting" in Earth system models. We present a novel bio-optical algorithm to retrieve size-partitioned phytoplankton carbon from ocean color satellite data. The algorithm is based on existing methods to estimate particle volume from a power-law particle size distribution (PSD). Volume is converted to carbon concentrations using a compilation of allometric relationships. We quantify absolute and fractional biomass in three PFTs based on size - picophytoplankton (0.5-2 µm in diameter), nanophytoplankton (2-20 µm) and microphytoplankton (20-50 µm). The mean spatial distributions of total phytoplankton C biomass and individual PFTs, derived from global SeaWiFS monthly ocean color data, are consistent with current understanding of oceanic ecosystems, i.e., oligotrophic regions are characterized by low biomass and dominance of picoplankton, whereas eutrophic regions have high biomass to which nanoplankton and microplankton contribute relatively larger fractions. Global climatological, spatially integrated phytoplankton carbon biomass standing stock estimates using our PSD-based approach yield - 0.25 Gt of C, consistent with analogous estimates from two other ocean color algorithms and several state-of-the-art Earth system models. Satisfactory in situ closure observed between PSD and POC measurements lends support to the theoretical basis of the PSD-based algorithm. Uncertainty budget analyses indicate that absolute carbon concentration uncertainties are driven by the PSD parameter No which determines particle number concentration to first order, while uncertainties in PFTs' fractional contributions to total C biomass are mostly due to the allometric coefficients. The C algorithm presented here, which is not empirically constrained a priori, partitions biomass in size classes and introduces improvement over the assumptions of the other approaches. However, the range of phytoplankton C biomass spatial variability globally is larger than estimated by any other models considered here, which suggests an empirical correction to the No parameter is needed, based on PSD validation statistics. These corrected absolute carbon biomass concentrations validate well against in situ POC observations.

(Table 1) Stable oxygen and carbon isotopic ratios of tree rings, and tree ring width of white spruce (Picea glauca), Ennadai Lake

Stable isotope ratios from tree rings and peatland mosses have become important proxies of past climate variations. We here compare recent stable carbon and oxygen isotope ratios in cellulose of tree rings from white spruce (Picea glauca), growing near the arctic tree line; and cellulose of Sphagnum fuscum stems, growing in a hummock of a subarctic peatland, in west-central Canada. Results show that carbon isotopes in S. fuscum correlate significantly with July temperatures over the past ~20 yr. The oxygen isotopes correlate with both summer temperature and precipitation. Analyses of the tree-ring isotopes revealed summer temperatures to be the main controlling factor for carbon isotope variations, whereas tree-ring oxygen isotope ratios are controlled by a combination of spring temperatures and precipitation totals. We also explore the potential of combining high-frequency (annual) climate signals derived from long tree-ring series with low-frequency (decadal to centennial) climate signals derived from the moss remains in peat deposits. This cross-archive comparison revealed no association between the oxygen isotopes, which likely results from the varying sensitivity of the archives to different seasons. For the carbon isotopes, common variance could be achieved through adjustments of the Sphagnum age model within dating error.