Benthic stable-isotope records of ODP Site 208-1262, South Atlantic

The Late Paleocene and Early Eocene were characterised by warm greenhouse climates, punctuated by a series of rapid warming and ocean acidification events known as "hyperthermals", thought to have been paced or triggered by orbital cycles. While these hyperthermals, such as the Paleocene Eocene Thermal Maximum (PETM), have been studied in great detail, the background low-amplitude cycles seen in carbon and oxygen-isotope records throughout the Paleocene-Eocene have hitherto not been resolved. Here we present a 7.7 million year (myr) long, high-resolution, orbitally-tuned, benthic foraminiferal stable-isotope record spanning the late Paleocene and early Eocene interval (~52.5 - 60.5 Ma) from Ocean Drilling Program (ODP) Site 1262, South Atlantic. This high resolution (~2-4 kyr) record allows the changing character and phasing of orbitally-modulated cycles to be studied in unprecedented detail as it reflects the long-term trend in carbon cycle and climate over this interval. The main pacemaker in the benthic oxygen-isotope (d18O) and carbon-isotope (d13C) records from ODP Site 1262, are the long (405 kyr) and short (100 kyr) eccentricity cycles, and precession (21 kyr). Obliquity (41 kyr) is almost absent throughout the section except for a few brief intervals where it has a relatively weak influence. During the course of the Early Paleogene record, and particularly in the latest Paleocene, eccentricity-paced negative carbon-isotope excursions (d13C, CIEs) and coeval negative oxygen-isotope (d18O) excursions correspond to low carbonate (CaCO3) and coarse fraction (%CF) values due to increased carbonate dissolution, suggesting shoaling of the lysocline and accompanied changes in the global exogenic carbon cycle. These negative CIEs and d18O events coincide with maxima in eccentricity, with changes in d18O leading changes in d13C by ~6 (±5) kyr in the 405-kyr band and by ~3 (±1) kyr in the higher frequency 100-kyr band on average. However, these phase lags are not constant, with the lag in the 405-kyr band extending from ~4 (±5) kyr to ~21 (±2) kyr from the late Paleocene to the early Eocene, suggesting a progressively weaker coupling of climate and the carbon-cycle with time. The higher amplitude 405-kyr cycles in the latest Paleocene are associated with changes in bottom water temperature of 2-4ºC, while the most prominent 100 kyr-paced cycles can be accompanied by changes of up to 1.5ºC. Comparison of the 1262 record with a lower resolution, but orbitally-tuned benthic record for Site 1209 in the Pacific allows for verification of key features of the benthic isotope records which are global in scale including a key warming step at 57.7 Ma.

Organic matter preservation in turbidites on the Madeira Abyssal Plain

Oxidized intervals of five organic-rich Madeira Abyssal Plain (MAP) turbidites deposited during the Miocene, Pliocene, and Pleistocene all displayed comparable major loss of total organic carbon (TOC) (84 ± 3.1%) accompanied by a negative isotopic (d13C) shift ranging from -0.3 to -2.9 per mil. Major but significantly lower loss of total nitrogen (Ntot, 61 ± 7.1%) also occurred, leading to a decrease in TOC relative to Ntot (C/Ntot) and a +1.3 to 2.7 per mil Ntot isotopic (d15N) shift. Compound specific isotopic measurements on plant wax n-alkanes indicate the terrestrial organic component in the unoxidized deposits is 13C-enriched owing to significant C4 contribution. Selective preservation of terrestrial relative to marine organic carbon could account for the d13C behavior of TOC upon oxidation but only if a 13C-depleted component of the bulk terrestrial signal is selectively preserved in the process. Although the C/Ntot decrease and positive d15N shift seems inconsistent with selective terrestrial organic preservation, results from analysis of a Modern eolian dust sample collected in the vicinity indicate these observations are compatible. Regardless of the specific explanation for these isotopic observations, however, our findings provide evidence that paleoreconstruction of properties such as pCO2 using the d13C of TOC is a goal fraught with uncertainty whether or not the marine sedimentary record considered is 'contaminated' with significant terrestrial input. Nonetheless, despite major and selective loss of both marine and terrestrial components as a consequence of postdepositional oxidation, intensive organic geochemical proxies such as the alkenone unsaturation index, UK'37, appear resistant to change and thereby retain their paleoceanographic promise.

Stable carbon and oxygen isotope record of Cretaceous planktonic foraminifera from DSDP (Table 1)

The Cretaceous Heterohelix moremani (Cushman) was the only biserial planktonic foraminiferal species from its first appearance in the late Albian up to the Cenomanian/Turonian boundary. Within that time, it increased gradually in abundance relative to other planktonic foraminifera in five Circum-North Atlantic sections. It is generally rare in upper Albian sediments, common in most of the Cenomanian and very abundant in sediments representing the latest Cenomanian Oceanic Anoxic Event. Short-term variations on the overall abundance trend correlate with positive excursions in the bulk carbonate delta13C record. Maximum rain rates of H. moremani during OAE2 show that this species was an opportunist that did well in extreme conditions, but its overall distribution indicates that it is not necessarily a marker for very high palaeoproductivity environments. Stable oxygen and carbon isotope measurements on foraminiferal species indicate that H. moremani was a surface water dweller at least in part of its geographic range, but incorporated 13C out of equilibrium with ambient seawater. It is depleted in delta13C relative to other planktonic foraminifera, which is attributed to vital effects related to its opportunistic character.

(Table 1) Geochemistry of ODP Hole 130-807C carbonates across the K/T boundary

Paleontological, stable isotopic, trace elemental abundance, and magnetostratigraphic studies have been performed on limestones spanning the Cretaceous/Tertiary boundary transition at Ocean Drilling Program (ODP) Hole 807C. Paleontological evidence exists for considerable resedimentation, which we attribute to the fact that Hole 807C is located in a basement graben. Age estimates based on planktonic foraminiferal biostratigraphy, as well as magnetostratigraphy, indicate that sedimentation rates could have been on the order of 12-14 m/m.y. This is significantly higher than those documented in other important Deep Sea Drilling Project (DSDP) and ODP Cretaceous/Tertiary boundary sections using the same age control points (e.g., DSDP Hole 577 and ODP Hole 690B), although not as high as those documented from DSDP Hole 524. The expanded nature of this succession has resulted in the Cretaceous/Tertiary boundary d13C decrease occurring over approximately a 9-m interval. Ir analysis of these sediments do not show a single large anomaly, as has been found in other Cretaceous/Tertiary boundary sections, but trivial background levels instead. Ce data support the hypothesis that this section has been expanded by secondary sedimentological processes.

Stable carbon and oxygen isotope ratios for different test sizes of live benthic foraminifera from the Arabian Sea

We determined the stable oxygen and carbon isotopic composition of live (Rose Bengal stained) tests belonging to different size classes of two benthic foraminiferal species from the Pakistan continental margin. Samples were taken at 2 sites, with water depth of about 135 and 275 m, corresponding to the upper boundary and upper part of the core region of the oxygen minimum zone (OMZ). For Uvigerina ex gr. U. semiornata and Bolivina aff. B. dilatata, delta13C and delta18O values increased significantly with increasing test size. In the case of U. ex gr. U. semiornata, delta13C increased linearly by about 0.105 per mil for each 100-µm increment in test size, whereas delta18O increased by 0.02 to 0.06 per mil per 100 µm increment. For B. aff. B. dilatata the relationship between test size and stable isotopic composition is better described by logarithmic equations. A strong positive linear correlation is observed between delta18O and delta13C values of both taxa, with a constant ratio of delta18O and delta13C values close to 2:1. This suggests that the strong ontogenetic effect is mainly caused by kinetic isotope fractionation during CO2 uptake. Our data underline the necessity to base longer delta18O and delta13C isotope records derived from benthic foraminifera on size windows of 100 µm or less. This is already common practice in down-core isotopic studies of planktonic foraminifera.

Stable carbon and oxygen isotope ratios of Planulina wuellerstorfi from sediments of the Sierra Leone Rise

Causes of change in deep water delta13C can be either global or local in extent. Global causes include (1) climatically-induced changes in the amount of terrestrial biomass which alter the average carbon isotopic composition of the oceanic reservoir (Shackleton, 1977), and (2) erosion and deposition of organic-rich, continental shelf sediments during sea level fluctuations which change the mean oceanic carbon: phosphorus ratio (Broecker, 1982 doi:10.1016/0079-6611(82)90007-6). Regional gradients of delta13C are created by remineralization of organic detritus within the deep ocean itself thus reflecting the distribution of water masses and modern thermohaline flow. Changes in a single geological record of benthic foraminiferal delta13C can result from any combination of these global and abyssal circulation effects. By sampling a large number of cores collected over a wide bathymetric range yet confined to a small geographical region we have minimized the ambiguity. We can assume that each delta13C record was equally affected by global causes of delta13C variation. The differences seen between the delta13C records must, therefore, reflect changes in the distribution of delta13C in the deep ocean. We interpret these differences in distribution in terms of changes in the ocean's abyssal circulation. Benthic foraminiferal carbon isotopic evidence from a suite of Sierra Leone Rise cores indicates that the deeper parts of the eastern Atlantic basins underwent a reduction in [O2] during the maximum of the last glaciation. Reduced advection of O2-rich deep water through low-latitude fracture zones, associated with increased delivery of organic matter to the deep ocean, lowered the delta13C of deep water SumCO2 at all depths below the sill separating the eastern and western Atlantic basins (Metcalf et al., 1964 doi:10.1016/0011-7471(64)91078-2). This decreased advection into the eastern Atlantic Ocean coincides with the overall decrease in deep water production in the North Atlantic during the last glacial maximum (Curry and Lohmann, 1982 doi:10.1016/0033-5894(82)90071-0; Boyle and Keigwin, 1982 doi:10.1126/science.218.4574.784; Schnitker, 1979 doi:10.1016/0377-8398(79)90020-3; Streeter and Shackleton, 1979 doi:10.1126/science.203.4376.168).

Geochemistry and stable carbon isotope composition of kerogen of DSDP Site 124-767

Celebes Basin sediments from Ocean Drilling Program Site 767 (Leg 124) containing both marine and terrestrial organic matter have been investigated through palynofacies and geochemical analyses. The main degradation processes affecting or having affected organic matter are recorded in the sedimentary column as shown by ammonium, phosphate and sulfate pore-water profiles, and by petrographic and geochemical analyses of sediments. In the upper part of the sedimentary section (down to 200 mbsf), the decrease of the ratio of total organic carbon to sulfur (TOC/S) with depth, generally related to the sulfate reduction process, is accompanied by an increase of framboidal pyrite content in the marine organic matter, and by an increasing amount of amorphous marine organic matter relative to the total organic matter. However, as the terrestrial organic input also varies with depth, dilution effects are superimposed on diagenesis. This continental supply affects the TOC/S ratio by increasing total organic carbon and decreasing the ability of the bulk organic matter to be metabolized through sulfate reduction. A positive relationship between the TOC/P ratio and the amount of degraded organic matter of marine origin clearly displays the effect of an organic source on the composition of the sediment. Each lithostratigraphic unit possesses its own characteristics in terms of composition and preservation of organic matter. The effects of diagenesis can only be appreciated within a single lithostratigraphic unit and mainly affect the less-resistant marine organic matter.

Geochemical investigations at Tsanfleuron Glacier, Switzerland

Subg1acially precipitated calcites, formed since 1860, are exposed by the retreating of the Tsanfleuron glacier in the Swiss Alps. They have been sampled together with different types of ice and water, for their isotopic composition. The isotopic study suggests that the initial water, from which calcium carbonate is precipitated by partial freezing, is produced by melting of the basal ice layer, not of glacier ice. Because of the wide range of isotopic composition of basal ice, some doubts are expressed on the possibility given by such subglacially precipitated calcites to determine the isotopic composition of Pleistocene ice sheets and to correct the paleotemperature scale.

Authigenic carbonate of ODP SIte 104-643

Different generations of complex authigenic carbonates formed in siliceous muds (lithologic Unit IV) and hemipelagic clays (lithologic Unit V) of ODP Site 643, Leg 104 Norwegian Sea. The dominant phase in Unit IV is an early diagenetic Mn, Fe-calcite with a strong negative d13C ( -14 to -16 per mil) signature, and slightly negative d180 values. The strong negative d13C results from extensive incorporation of 12C-enriched CO2 derived from bacterial degradation of marine organic matter into early Mn, Fe - calcite cements. Concomitant framboidal pyrite precipitation and abundant SEM microtextures showing excellent preservation of delicate structures of fragile diatom valves by outpourings with early Mn-calcites strongly support their shallow burial formation before the onset of compaction. Later generations of authigenic mineralizations in lithologic Unit IV include minor amounts of a second generation of calcite with platy crystals, possibly precipitated along with opal-A dissolution, and finally opal-CT crystallization in deeper seated environments overgrowing earlier precipitates with films and lepispheres. The last mineralization is collophane (fluor apatite) forming amorphous aggregates and tiny hexagonal crystals. Authigenic mineral assemblages in lithologic Unit V consist of rhodochrosites, transitional rhodochrosite/manganosiderites, and apatite. A negative d13C ( -7.1 to -15.6 per mil) and a fluctuating d18O signal indicates that the micritic to sparitic rhodochrosites, transitional rhodochrosites/manganosiderites were formed at various burial depths. CO2 resulted from organic matter degradation in the lowermost sulfate reduction zone and from biogenic methane generation in the lowermost sediments, resulting in variable and negative d13C signals. The change in carbonate mineralogy reflects major compositional differences compared to sediments in Unit IV. Most prominent is an increase in altered ash as a primary sediment component and a sudden decrease of siliceous microfossils. Upward diffusion of cations, lowered salinities in pore waters, and elevated temperatures provide diagenetic environments favoring increased remobilization processes.

Dinoflagellate cyst assemblages, Mg/Ca based reconstructed temperatures, stable carbon isotopes, and geochemical parameters of sediment core GeoB9508-5

In order to investigate a possible connection between tropical northeast (NE) Atlantic primary productivity, Atlantic meridional overturning circulation (AMOC), and drought in the Sahel region during Heinrich Stadial 1 (HS1), we used dinoflagellate cyst (dinocyst) assemblages, Mg/Ca based reconstructed temperatures, stable carbon isotopes (d13C) and geochemical parameters of a marine sediment core (GeoB 9508-5) from the continental slope offshore Senegal. Our results show a two-phase productivity pattern within HS1 that progressed from an interval of low marine productivity between ~ 19 and 16 kyr BP to a phase with an abrupt and large productivity increase from ~ 16 to 15 kyr BP. The second phase is characterized by distinct heavy planktonic d13C values and high concentrations of heterotrophic dinocysts in addition to a significant cooling signal based on reconstructions of past sea surface temperatures (SST). We conclude that productivity variations within HS1 can be attributed to a substantial shift of West African atmospheric processes. Taken together our results indicate a significant intensification of the North East (NE) trade winds over West Africa leading to more intense upwelling during the last millennium of HS1 between ~ 16 and 15 kyr BP, thus leaving a strong imprint on the dinocyst assemblages and sea surface conditions. Therefore, the two-phase productivity pattern indicates a complex hydrographic setting suggesting that HS1 cannot be regarded as uniform as previously thought.