Specific microbial biomarker signatures from Congo fan sediments of ODP site 175-1075, Congo soils, wetland and estuary sediments

Methane (CH4) is a strong greenhouse gas known to have perturbed global climate in the past, especially when released in large quantities over short time periods from continental or marine sources. It is therefore crucial to understand and, if possible, quantify the individual and combined response of these variable methane sources to natural climate variability. However, past changes in the stability of greenhouse gas reservoirs remain uncertain and poorly constrained by geological evidence. Here, we present a record from the Congo fan of a highly specific bacteriohopanepolyol (BHP) biomarker for aerobic methane oxidation (AMO), 35-aminobacteriohopane-30,31,32,33,34-pentol (aminopentol), that identifies discrete periods of increased AMO as far back as 1.2 Ma. Fluctuations in the concentration of aminopentol, and other 35-aminoBHPs, follow a pattern that correlates with late Quaternary glacial-interglacial climate cycles, with highest concentrations during warm periods. We discuss possible sources of aminopentol, and the methane consumed by the precursor methanotrophs, within the context of the Congo River setting, including supply of methane oxidation markers from terrestrial watersheds and/or marine sources (gas hydrate and/or deep subsurface gas reservoir). Compound-specific carbon isotope values of -30 per mil to -40 per mil for BHPs in ODP 1075 and strong similarities between the BHP signature of the core and surface sediments from the Congo estuary and floodplain wetlands from the interior of the Congo River Basin, support a methanotrophic and likely terrigenous origin of the 35-aminoBHPs found in the fan sediments. This new evidence supports a causal connection between marine sediment BHP records of tropical deep sea fans and wetland settings in the feeding river catchments, and thus tropical continental hydrology. Further research is needed to better constrain the different sources and pathways of methane emission. However, this study identifies the large potential of aminoBHPs, in particular aminopentol, to trace and, once better calibrated and understood, quantify past methane sources and fluxes from terrestrial and potentially also marine sources.

Siliceous phytoplankton in middle Eocene sediments of ODP Hole 207-1260A

The Middle Eocene diatom and silicoflagellate record of ODP Site 1260A (Demerara Rise) is studied quantitatively in order to throw light on the changes that siliceous phytoplankton communities experienced during a Middle Eocene warming event that occurred between 44.0 and 42.0 Ma. Both Pianka's overlap index, calculated per couple of successive samples, and cluster analysis, point to a number of significant turnover events highlighted by changes in the structure of floristic communities. The pre-warming flora, dominated by cosmopolitan species of the diatom genus Triceratium, is replaced during the warming interval by a new and more diverse assemblage, dominated by Paralia sulcata (an indicator of high productivity) and two endemic tropical species of the genus Hemiaulus. The critical warming interval is characterized by a steady increase in biogenic silica and a comparable increase in excess Ba, both reflecting an increase in productivity. In general, it appears that high productivity not only increased the flux of biogenic silica, but also sustained a higher diversity in the siliceous phytoplankton communities. The microflora preserved above the critical interval is once again of low diversity and dominated by various species of the diatom genus Hemiaulus. All assemblages in the studied material are characterized by the total absence of continental and benthic diatoms and the relative abundance of neritic forms, suggesting a transitional depositional environment between the neritic and the oceanic realms.

High resolution color reflectance and hematite and goethite record of ODP Hole 184-1143A in the South China Sea

Precipitation has a larger variability than temperature in tropical monsoon regions, thus it is an important climate variable. However, reconstructions of long-term rainfall histories are scarce because of the lack of reliable proxies. Here we document that iron oxide minerals, specifically the ratio of hematite to goethite (Hm/Gt), is a reasonable precipitation proxy. Using diffuse reflectance spectrophotometry, we measured samples from Ocean Drilling Program (ODP) 1143 drilling site (9°21.72'N, 113°17.11'E, 2777 m water depth) for hematite and goethite, whose formation processes are favored by opposing climate conditions. In order to determine the content of hematite and goethite we produced a set of calibration samples by removing the iron oxides to generate the natural matrix to which hematite and goethite in known percentages were added. From these calibration samples we developed a transfer function for determining hematite and goethite concentration from a sample's spectral reflectance. Applying this method to ODP 1143 sediments (top 34 m of a 510 m core with sampling interval of 10 cm) we were able to reconstruct a continuous precipitation history for SE Asia of the past 600 kyr using the Hm/Gt ratio as a proxy of the precipitation variability of Asian monsoon. The reliability of this Hm/Gt proxy is corroborated by its consistency with the stalagmite delta18O data from South China. Comparing long-term Hm/Gt records with the surface temperature gradient of equatorial Pacific Ocean, we found that monsoon precipitation and El Niño are correlated for the last 600 kyr. The development of El Niño-like conditions decreased SE Asia precipitation, whereas precipitation increases in response to La Niña intensification

Sea surface temperature reconstruction for ODP Hole 178-1098B

The disintegration of ice shelves, reduced sea-ice and glacier extent, and shifting ecological zones observed around Antarctica (Cook et al., 2005, doi:10.1126/science.1104235; Stammerjohn et al., 2008, doi:10.1016/j.dsr2.2008.04.026) highlight the impact of recent atmospheric (Steig et al., 2009, doi:10.1038/nature07669) and oceanic warming (Gille, 2002, doi:10.1126/science.1065863) on the cryosphere. Observations (Cook et al., 2005, doi:10.1126/science.1104235; Stammerjohn et al., 2008, doi:10.1016/j.dsr2.2008.04.026) and models (Pollard and DeConto, 2009, doi:10.1038/nature07809) suggest that oceanic and atmospheric temperature variations at Antarctica's margins affect global cryosphere stability, ocean circulation, sea levels and carbon cycling. In particular, recent climate changes on the Antarctic Peninsula have been dramatic, yet the Holocene climate variability of this region is largely unknown, limiting our ability to evaluate ongoing changes within the context of historical variability and underlying forcing mechanisms. Here we show that surface ocean temperatures at the continental margin of the western Antarctic Peninsula cooled by 3-4 °C over the past 12,000?years, tracking the Holocene decline of local (65° S) spring insolation. Our results, based on TEX86 sea surface temperature (SST) proxy evidence from a marine sediment core, indicate the importance of regional summer duration as a driver of Antarctic seasonal sea-ice fluctuations (Huybers and Denton, 2008, doi:10.1038/ngeo311). On millennial timescales, abrupt SST fluctuations of 2-4 °C coincide with globally recognized climate variability (Mayewski et al., 2004, doi:10.1016/j.yqres.2004.07.001). Similarities between our SSTs, Southern Hemisphere westerly wind reconstructions (Moreno et al., 2010, doi:10.1130/G30962.1) and El Niño/Southern Oscillation variability (Conroy et al., 2008, doi:10.1016/j.quascirev.2008.02.015) indicate that present climate teleconnections between the tropical Pacific Ocean and the western Antarctic Peninsula (Yuan et al., 2004, doi:10.1017/S0954102004002238) strengthened late in the Holocene epoch. We conclude that during the Holocene, Southern Ocean temperatures at the western Antarctic Peninsula margin were tied to changes in the position of the westerlies, which have a critical role in global carbon cycling (Moreno et al., 2010, doi:10.1130/G30962.1; Anderson et al., 2009, doi:10.1126/science.1167441).

Calcareous nannofossils of ODP sites 177-1088 and 177-1090

During ODP Leg 177, a Miocene to Pliocene calcareous nannofossil record was recovered at Sites 1088 and 1090. Site 1088, located at 41°8'S, shows a continuous middle-upper Miocene to Pliocene carbonate sequence that was deposited at relatively high sedimentation rates (Shipboard Scientific Party, 1999a, doi:10.2973/odp.proc.ir.177.103.1999). Moreover, Site 1088 proved suitable for calcareous nannofossil analysis as a means to improve the biostratigraphy at this southern latitude. Site 1090 was drilled at 42°54'S; a tephra layer marks a significant disconformity at the Miocene/Pliocene boundary of this sequence (Shipboard Scientific Party, 1999b, doi:10.2973/odp.proc.ir.177.105.1999). Although nannofossil assemblages are poorly preserved at this site (Shipboard Scientific Party, 1999b), they may help in determining the age of the disconformity and its paleoceanographic significance.

Petrologic data from Site U1308 over the last three glaciations

A composite North Atlantic record from DSDP Site 609 and IODP Site U1308 spans the past 300,000 years and shows that variability within the penultimate glaciation differed substantially from that of the surrounding two glaciations. Hematite stained grains exhibit similar repetitive down-core variations within the Marine Isotope Stage (MIS) 8 and 4-2 intervals, but little cyclic variability within the MIS 6 section. There is also no petrologic evidence, in terms of detrital carbonate-rich (Heinrich) layers, for surging of the Laurentide Ice Sheet through the Hudson Strait during MIS 6. Rather, very high background concentration of ice-rafted debris (IRD) indicates near continuous glacial meltwater input that likely increased thermohaline disruption sensitivity to relatively weak forcing events, such as expanded sea ice over deepwater formation sites. Altered (sub)tropical precipitation patterns and Antarctic warming during high orbital precession and low 65° N summer insolation appears related to high abundance of Icelandic glass shards and southward sea ice expansion. Differing European and North American ice sheet configurations, perhaps aided by larger variations in eccentricity leading to cooler summers, may have contributed to the relative stability of the Laurentide Ice Sheet in the Hudson Strait region during MIS 6.

Chronology and petrology in MIS4-2 of DSDP Site 94-609

Ice-rafting evidence for a '1500-year cycle' sparked considerable debate on millennial-scale climate change and the role of solar variability. Here, we reinterpret the last 70,000 years of the subpolar North Atlantic record, focusing on classic DSDP Site 609, in the context of newly available raw data, the latest radiocarbon calibration (Marine09) and ice core chronology (GICC05), and a wider range of statistical methodologies. A ~1500-year oscillation is primarily limited to the short glacial Stage 4, the age of which is derived solely from an ice flow model (ss09sea), subject to uncertainty, and offset most from the original chronology. Results from the most well-dated, younger interval suggest that the original 1500 ± 500 year cycle may actually be an admixture of the ~1000 and ~2000 cycles that are observed within the Holocene at multiple locations. In Holocene sections these variations are coherent with 14C and 10Be estimates of solar variability. Our new results suggest that the '1500-year cycle' may be a transient phenomenon whose origin could be due, for example, to ice sheet boundary conditions for the interval in which it is observed. We therefore question whether it is necessary to invoke such exotic explanations as heterodyne frequencies or combination tones to explain a phenomenon of such fleeting occurrence that is potentially an artifact of arithmetic averaging.

The Paleocene-Eocene boundary in ODP Leg 208 sites

The Paleocene-Eocene thermal maximum (PETM) has been attributed to the rapid release of ~2000 * 10**9 metric tons of carbon in the form of methane. In theory, oxidation and ocean absorption of this carbon should have lowerd deep-sea pH, thereby triggering a rapid (<10,000-year) shoaling of the calcite compensation depth (CCD), followed by gradual recovery. Here we present geochemical data from five new South Atlantic deep-sea sections that constrain the timing and extent of massive sea-floor carbonate dissolution coincident with the PETM. The sections, from between 2.7 and 4.8 kilometers water depth, are marked by a prominent clay layer, the character of which indicates that the CCD shoaled rapidly (<10,000 years) by more than 2 kilometers and recovered gradually (>100,000 years). These findings indicate that a large mass of carbon (>>2000 * 10**9 metric tons of carbon) dissolved in the ocean at the Paleocene-Eocene boundary and that permanent sequestration of this carbon occurred through silicate weathering feedback.

Carbon and oxygen isotopic composition of benthic foraminifers from ODP Site 167-1014 and Hole 167-1020C

Benthic foraminiferal stable isotope data are presented for Sites 1014 (Tanner Basin, 1176 m) and 1020 (Gorda Ridge, 3040 m) to constrain past changes in Pacific deep- and intermediate-water nutrient chemistry associated with the onset of large-amplitude 100-k.y. climate cycles after ~900 ka. The Site 1014 data were based on analyses of separate species of Cibicidoides, whereas only Cibicidoides wuellerstorfi was used to generate the Site 1020 record. The present data span 380-920 and 620-950 ka at Sites 1014 and 1020, respectively.

Variability of the Brazil Current during the late Holocene analysed on two marine sediment cores

Our understanding of the centennial-scale variability of the Brazil Current (BC) during the late Holocene is elusive because of the lack of appropriate records. Here we used the Mg/Ca and oxygen isotopic composition of planktonic foraminifera from two marine sediment cores collected at 27° S and 33° S off southeastern South America to assess the late Holocene variability in the upper water column of the BC. Our results show in phase fluctuations of up to 3 °C in sea surface temperatures (SST), and 0.8 per mil in oxygen isotopic composition of surface sea water, a proxy for relative sea surface salinity (SSS). Time-series analyses of our records indicate a cyclicity with a period of ca. 730 yr. We suggest that the observed cyclicity reflects variability in the strength of the BC associated to changes in the Atlantic meridional overturning circulation (AMOC). Positive (negative) SST and SSS anomalies are related to a strong (weak) BC and a weak (strong) AMOC. Moreover, periods of peak strength in the BC occur synchronously to a weak North Brazil Current, negative SST anomalies in the high latitudes of the North Atlantic, and positive (negative) precipitation anomalies over southeastern South America (equatorial Africa), further corroborating our hypothesis. This study shows a tight coupling between the variability of the BC and the high latitudes of the North Atlantic mediated by the AMOC even under late Holocene boundary conditions.

Tie points and goechemical analysis from different Holes of ODP Site 165-1002

The fine-grained sediments of the Cariaco Basin, Venezuela, of the last 130 ky, whose deposition history is well characterized, were analyzed geochemically in order to test the validity of sediment bulk geochemistry as an indicator of detrital provenance. Several binary and ternary diagrams as well as the chemical index of alteration (CIA) were tested for their capacity to discriminate the poorly contrasted detrital sources to the Cariaco Basin, and to describe the temporal evolution of the contributions of these different sources. Most of the diagrams tested did not allow a good discrimination of sources or, when sources were well discriminated, did not allow an interpretation of the temporal variations consistent with the known history. A relatively good discrimination of sources and a consistent interpretation of temporal variations were however obtained using Hf vs. Th and La/Yb vs. Gd/Yb binary diagrams, as well as Ti-Zr-Th, Ti-Zr-La, and Lu-Hf-Th ternary diagrams. Compared to the previous studies of the detrital content of the Cariaco Basin sediments, the geochemical approach permitted the recognition of a sediment contribution eroded from the Unare platform and Gulf of Cariaco during rapid sea level oscillations, and the contribution of Saharan eolian particles during the Younger Dryas-Preboreal and MIS6-5 transition. The choice of plotted elements was determined after considering carrier minerals, so that different elements may be informative in different sedimentary contexts. Overall, mineral sorting during transport appears as a major limit to quantitative estimation of the different contributions. In particular mineral sorting leads to the selective enrichment of elements associated with clays (Al, Rb, Th and LREE) in sediments deposited in the basin. Unless the geochemical effect of mineral sorting can be measured, it appears that quantitative provenance analysis should be performed on fractions of similar grain size instead of bulk sediment.

A 31,000-year high-resolution record from two sediment cores off northwest Africa

Benthic foraminiferal assemblage compositions and sedimentary geochemical parameters were analyzed in two radiocarbon dated sediment cores from the upwelling area off NW Africa at 12°N, to reconstruct productivity changes during the last 31 kyr. High-latitude cold events and variations in low-latitude summer insolation influenced humidity, wind systems, and the position of the tropical rain belt over this time period. This in turn caused changes in intensity and seasonality of primary productivity off the southern Northwest African continental margin. High accumulation rates of benthic foraminifera, carbonate, and organic carbon during times of north Atlantic melt water events Heinrich 2 (25.4 to 24.3 kyr BP) and 1 (16.8 to 15.8 kyr BP) indicate high productivity. Dominance of infaunal benthic foraminiferal species and high numbers of deep infaunal specimens during that time indicate a strong and sustained supply of refractory organic matter reworked from the upper slope and shelf. A more southerly position of the tropical rainbelt and the Northeast trade wind belt during Heinrich 2 and 1 may have enhanced wind intensity and almost permanent upwelling, driving this scenario. A phytodetritus-related benthic fauna indicates seasonally pulsed input of labile organic matter but generally low year-round productivity during the Last Glacial Maximum (23 to 18 kyr BP). The tropical rainbelt is more expanded to the North than during Heinrich Events, and relatively weak NE trade winds resulted in seasonal and weak upwelling, thus lower productivity. High productivity characterized by a seasonally high input of labile organic matter, is indicated for times of orbital forced warming, such as the African Humid Period (9.8 to 7 kyr BP). An intensified African monsoon during boreal summer and the northernmost position of the tropical rainbelt within the last 31 kyr resulted in enhanced river discharge from the northward-extended drainage area (or river basin) initiating intense phytoplankton blooms. In the late Holocene (4 to 0 kyr BP) strong carbonate dissolution may have been caused by even more enhanced organic matter fluxes to the sea floor. Increasing aridity on the continent and stronger NE trade winds induced intensive, seasonal coastal upwelling.

Barium intensities and tie points between holes of Leg 208

The Paleocene - Eocene thermal maximum (PETM) is one of the best known examples of a transient climate perturbation, associated with a brief, but intense, interval of global warming and a massive perturbation of the global carbon cycle from injection of isotopically light carbon into the ocean-atmosphere system. One key to quantifying the mass of carbon released, identifying the source(s), and understanding the ultimate fate of this carbon is to develop high-resolution age models. Two independent strategies have been employed, cycle stratigraphy and analysis of extraterrestrial Helium (HeET), both of which were first tested on Ocean Drilling Program (ODP) Site 690. Both methods are in agreement for the onset of the PETM and initial recovery, or the clay layer ("main body"), but seem to differ in the final recovery phase of the event above the clay layer, where the carbonate contents rise and carbon isotope values return toward background values. Here we present a state-of-the-art age model for the PETM derived from a new orbital chronology developed with cycle stratigraphic records from sites drilled during ODP Leg 208 (Walvis Ridge, Southeastern Atlantic) integrated with published records from Site 690 (Weddell Sea, Southern Ocean, ODP Leg 113). During Leg 208, five Paleocene - Eocene (P-E) boundary sections (Sites 1262 to 1267) were recovered in multiple holes over a depth transect of more than 2200 m at the Walvis Ridge yielding the first stratigraphically complete P-E deep-sea sequence with moderate to relatively high sedimentation rates (1 to 3 cm/kyr). A detailed chronology was developed with non-destructive X-ray fluorescence (XRF) core scanning records on the scale of precession cycles, with a total duration of the PETM now estimated to be ~ 170 kyr. The revised cycle stratigraphic record confirms original estimates for the duration of the onset and initial recovery, but suggests a new duration for the final recovery that is intermediate to the previous estimates by cycle stratigraphy and HeET.

Stable isotopes and Mg/Ca measurements of two sediment cores in the northeastern tropical Atlantic

Two SST records based on Mg/Ca of G. ruber (pink) from the continental slope off West Africa at 15°N and 12°N shed new light on the thermal bipolar seesaw pattern in the northeastern tropical Atlantic during periods of reduced Atlantic Meridional Overturning Circulation (AMOC) associated with Heinrich stadials H1 to H6. The two records indicate that the latitudinal position of the bipolar seesaw's zero-anomaly line, between cooling in the North and warming in the South, gradually shifted southward from H6 to H1. A conceptual model is presented that aims to provide a physically consistent mechanism for the southward migration of the seesaw's fulcrum. The conceptual model suggests latitudinal movements of the Intertropical Convergence Zone, driven by a combination of orbital-forced changes in the meridional temperature gradient within the realm of the Hadley cell and the expansion of the Northern Hemisphere cryosphere, as a major factor.

Geochemistry of selected minerals at DSDP Leg 65 Holes

The majority of the basalts drilled on Leg 65 in the Gulf of California are aphyric to sparsely phyric massive flows ranging in average thickness between 5 meters in the upper part of the sections in Holes 483 and 483B, where they are interlayered with sediment, and 14 meters in Hole 485A, where interlayered sediments constitute more than half of the section. Massive flows interlayered with pillows are generally less than 4 meters thick. The pillow lavas recovered are more phyric (up to 15 modal%) and contain two to three generations of plagioclase and olivine ± clinopyroxene. Plagioclase generally exceeds 60% of any given phenocryst assemblage. Resorbed olivine, clinopyroxene, and plagioclase megacrysts may reflect a high-pressure stage, the phenocrysts crystallizing in the main magma chamber and the skeletal microphenocrysts in dikes. Precise measurements of length/width ratios of different phenocryst types and compositions show low aspect ratios and large crystal volumes for early crystals and high ratios and low volumes for late crystals grown under strong undercooling conditions. The minerals examined show wide ranges in composition: in particular, plagioclase ranges from An92 to An36; clinopyroxene ranges from Ca41Mg51Fe8 in the cores of phenocrysts to Ca40**36 Mg45**49Fe15**20 in the groundmass; and olivine ranges from Fo86 to Fo81. The wide range in mineral compositions, together with evidence of disequilibrium based on textures and comparisons of glass and mineral compositions, indicate complex crystallization histories involving both polybaric crystal fractionation and magma mixing.