Pliocene-Pleistocene alkenone, nitrogen and opal record of ODP Site 175-1082

In this study we present combined high-resolution records of sea surface temperature (SST), phytoplankton productivity, and nutrient cycling in the Benguela Upwelling System (BUS) for the past 3.5 Ma. The SST record provided evidence that upwelling activity off Namibia mainly intensified ca. 2.4-2.0 Ma ago in response to the cooling of the Southern Ocean and the resultant strengthening of trade winds. As revealed by productivity-related proxies, BUS intensification led to a major transition in regional biological productivity when considering the termination of the Matuyama Diatom Maximum (a diatom high-production event). Major oceanic reorganization in the Benguela was accompanied by nutrient source changes, as indicated by a new nitrogen isotopic (delta15N) record that revealed a stepwise increase at ca. 2.4 and ca. 1.5 Ma ago. The change in source region likely resulted from significant changes in intermediate water formation tied to the reorganization of oceanic conditions in the Southern Ocean, which may have in turn mainly controlled the global ocean N cycle, and therefore the N isotopic composition of nutrients since 3.5 Ma ago.

Radiolarian faunal turnover across the Oligocene/Miocene boundary in the equatorial Pacific Ocean

The global warming trend of the latest Oligocene was interrupted by several cooling events associated with Antarctic glaciations. These cooling events affected surface water productivity and plankton assemblages. Well-preserved radiolarians were obtained from upper Oligocene to lower Miocene sediments at Ocean Drilling Program (ODP) Leg 199 Sites 1218 and 1219 in the equatorial Pacific, and 110 radiolarian species were identified. Four episodes of significant radiolarian faunal changes were identified: middle late Oligocene (27.5 to 27.3 Ma), latest Oligocene (24.4 Ma), earliest Miocene (23.3 Ma), and middle early Miocene (21.6 Ma). These four episodes approximately coincide with increases and decreases of biogenic silica accumulation rates and increases in delta18O values coded as "Oi" and "Mi" events. These data indicate that Antarctic glaciations were associated with change of siliceous sedimentation patterns and faunal changes in the equatorial Pacific. Radiolarian fauna was divided into three assemblages based on variations in radiolarian productivity, species richness and the composition of dominant species: a late Oligocene assemblage (27.6 to 24.4 Ma), a transitional assemblage (24.4 to 23.3 Ma) and an early Miocene assemblage (23.3 to 21.2 Ma). The late Oligocene assemblage is characterized by relatively high productivity, low species richness and four dominant species of Tholospyris anthophora, Stichocorys subligata, Lophocyrtis nomas and Lithelius spp. The transitional assemblage represents relatively low values of productivity and species richness, and consists of three dominant species of T. anthophora, S. subligata and L. nomas. The characteristics of the early Miocene assemblage are relatively low productivity, but high species richness. The two dominant species present in this assemblage are T. anthophora and Cyrtocapsella tetrapera. The most significant faunal turnover of radiolarians is marked at the boundary between the transitional/early Miocene assemblages.

A Middle Pleistocene nannofossil record of sediment core MD03-2699

Changes in paleoclimate and paleoproductivity patterns have been identified by analysing, in conjunction with other available proxy data, the coccolithophore assemblages from core MD03-2699, located in the Portuguese margin in the time interval from the Marine Isotope Stage (MIS) 13/14 boundary to MIS 9 (535 to 300 ka). During the Mid-Brunhes event, the assemblages associated with the eccentricity minima are characterised by higher nannoplankton accumulation rate (NAR) values and by the blooming of the opportunistic genus Gephyrocapsa. Changes in coccolithophore abundance are also related to glacial-interglacial cycles. Higher NAR and numbers of coccoliths/g mainly occurred during the interglacial periods, while these values decreased during the glacial periods. Superimposed on the glacial/interglacial cycles, climatic and paleoceanographic variability has been observed on precessional timescales. The structure of the assemblages highlights the prevailing long-term influence of the Portugal (PC) and Iberian Poleward (IPC) Currents, following half and full precession harmonics, related to the migration of the Azores High (AH) Pressure System. Small Gephyrocapsa and Coccolithus pelagicus braarudii are regarded as good indicators for periods of prevailing PC influence. Gephyrocapsa caribbeanica, Syracosphaera spp., Rhabdosphaera spp. and Umbilicosphaera sibogae denote periods of IPC influence. Our data also highlights the increased percentages of Coccolithus pelagicus pelagicus during the occurrence of episodes of very cold and low salinity surface water, probably related to abrupt climatic events and millennial-scale oscillations of the AH/Icelandic Low (IL) System.

Geochemistry and physics on sediment cores from the Peru Basin

We studied preservation/dissolution cycles and paleoproductivity in eight sediment cores from the Peru Basin south of the highly productive surface waters of the eastern equatorial Pacific. Stratigraphy is based on stable oxygen isotopes and on combined magnetostratigraphy and biostratigraphy. Sediment cores which span the last 8 m.y., were retrieved during cruise 79 with RV SONNE close to the carbonate compensation depth (CCD). In general, sediments show Pacific-type carbonate cycles. We interpret a pronounced carbonate peak between 6 and 7 Ma as the result of a western and northern extension of the highly productive Peru Current. Decreased carbonate contents from the late Miocene to the late Pliocene might be associated with a slow contraction of the latitudinal extent of the high-productivity belt north of the study areas. During the Pliocene, carbonate variations showed 400 kyr cycles indicating the growth and decay of ice sheets, which should have been associated with pulsations of the Antarctic ice cap. An abrupt collapse of the carbonate system occurred at 2.4 Ma. Higher frequency variations of the carbonate record indicate the major increase of the northern hemisphere glaciation. During the Quaternary, carbonate fluxes are high during glacials and low during interglacials. Large amplitude variations with long broad minima and maxima, associated with small migrations of the lysocline and the CCD (< 200 m), are indicative of the preservation/dissolution history in the Peru Basin. During the early Pleistocene, climatic forcing by the 41 kyr obliquity cycle is not observed in the carbonate record. During the last 800 kyr, variability in the carbonate record was dominated by the 100 kyr eccentricity cycle. Fluxes of biogenic material (calcium carbonate, organic carbon, opal, and barium) were greatest during glacials, which imply higher productivity and export production of the Peru Current during cold climatic periods. Dissolution was greatest during interglacials as inferred from the relatively poor preservation of planktonic foraminifera and from the low accumulation rate of carbonate. After the Mid-Brunhes Event (400 ka), we observe a plateaulike shift to enhanced dissolution and to intensified productivity.

Organic geochemistry of two sediment cores from the Kara Sea

Within the Russian-German research project on "Siberian River Run-off (SIRRO)" devoted to the freshwater discharge and its influence on biological, geochemical, and geological processes in the Kara Sea, sedimentological and organic-geochemical investigations were carried-out on two well-dated sediment cores from the Yenisei Estuary area. The main goal of this study was to quantify the terrigenous organic carbon accumulation based on biomarker and bulk accumulation rate data, and its relationship to Yenisei river discharge and climate change through Holocene times. The biomarker data at both cores clearly indicate the predominance of terrigenous organic matter, reaching 70 to 100% and 50 to 80% of the total organic carbon within and directly north of the estuary, respectively. During the last about 9 Cal. kyrs. BP represented in the studied sediment section, siliciclastic sediment and (terrigenous) organic carbon input was strongly influenced by postglacial sea-level rise and climate-related changes in river discharge. The mid-Holocene Climatic Optimum is documented by maximum river discharge between 8.2 and 7.3 Cal. kyrs. BP. During the last 2000 years river discharge probably became reduced, and accumulation of both terrigenous and marine organic carbon increased due to increased coagulation of fine-grained material.

Late Neogene carbonate sedimentation and stable isotope record of ODP Site 121-758

The evolution of oceanic and climatic conditions the northeast Indian Ocean during the last 7 m.y. is revealed in the sediments from Site 758. We present detailed and continuous records of d18O and d13C from planktonic foraminifers, weight percent calcium carbonate, weight percent coarse fraction, magnetic susceptibility, and geomagnetic reversals. Sample spacing of the records ranges from 3 to 10 cm and is equivalent to an average time interval of 2000 to 6000 yr. Despite the fact that core recovery ranged between 100% and 105%, recovery gaps as large as 2.7 m occurred at nearly every break between advanced hydraulic piston cores. Approximately 12% of the late Neogene sequence was not recovered in each of the two holes drilled at Site 758. To circumvent the discontinuity introduced by the gaps, a composite depth section was constructed from multiple cores taken from offset holes at Site 758. The resulting composite depth section extends continuously from 0 to 116 mbsf, from the Holocene to the upper Miocene. A detailed chronostratigraphy is based on geomagnetic reversals which extend from the Brunhes Chron to Chron 6, and on d18O stages 1 through 105, which span from 0 to 2.5 Ma. The d18O record is dominated by a ~40-k.y. cycle in the late Pliocene and early Pleistocene, and is followed by a change to a ~100-k.y. cycle in the late Pleistocene. The mid-Pleistocene transition between these two modes of variability occurs between d18O stages 25 and 22 (between 860 and 800 Ka). Thirteen major volcanic ash horizons from the Indonesian arc are observed throughout the sedimentary section and are dated by their relative position within the geomagnetic reversals and the d18O chronostratigraphy. Since 5 Ma, there has been a long-term decline in weight percent CaCO3 and CaCO3 mass accumulation rates, and an associated rise in non-CaCO3 mass accumulation rates. We attribute these changes to a decrease in CaCO3 productivity and an increase in terrigenous sedimentation through enhanced riverine input. Such input may be linked to rapid tectonic uplift of the Himalayas and the Tibetan Plateau via mechanisms such as the intensification of the monsoonal rains, increased fluvial erosion, and regional glaciation. The long-term increase in percent coarse fraction since 5 Ma suggests a gradual increase in CaCO3 preservation. Higher frequency fluctuations in CaCO3 preservation are superimposed on the long-term trend and are related to climate fluctuations. The abrupt drop (-50%) in CaCO3 accumulation at 3.4 Ma signals a dramatic decrease in CaCO3 production that occurred over much of the Indian Ocean.

Contusotruncana contusa abundances and morphotypes in the cretaceous mid-latitude South Atlantic

Spatial and temporal patterns in test size and shape (test conicity and spiral roundness) and absolute abundance (accumulation rate) of the planktonic foraminifer Contusotruncana contusa were studied in the South Atlantic Ocean (DSDP sites 356, 516, 525 and 527) during an interval corresponding to the last 800 kyr of the Cretaceous. The variation in absolute abundance of C. contusa was characterised by alternating periods of high and low abundance; some of these periods were traceable across the entire mid-latitude South Atlantic Ocean. While the mean spiral roundness did not show any interpretable patterns, a sudden increase of the mean test size and mean test conicity occurred between 65.3 and 65.2 Ma (based on linear interpolation within the Cretaceous part of Subchron C29R) at all sites studied, indicating a poleward migration followed by rapid withdrawal of the low-latitude C. contusa morphotypes from the mid-latitude South Atlantic Ocean. We suggest that this event was caused by a short period of surface-water warming in the southern mid-latitudes corresponding to the brief high-latitude warming event and associated faunal migrations in the Boreal and Austral realms.

Auxiliary material and basic geochemical and benthic foraminiferal stable isotope data for the interval bracketing Eocene Thermal Maximum 2 (ETM2) at DSDP Sites 401 and 550 in the northeastern Atlantic Ocean

Ever since its discovery, Eocene Thermal Maximum 2 (ETM2; ~53.7 Ma) has been considered as one of the "little brothers" of the Paleocene-Eocene Thermal Maximum (PETM; ~56 Ma) as it displays similar characteristics including abrupt warming, ocean acidification, and biotic shifts. One of the remaining key questions is what effect these lesser climate perturbations had on ocean circulation and ventilation and, ultimately, biotic disruptions. Here we characterize ETM2 sections of the NE Atlantic (Deep Sea Drilling Project Sites 401 and 550) using multispecies benthic foraminiferal stable isotopes, grain size analysis, XRF core scanning, and carbonate content. The magnitude of the carbon isotope excursion (0.85-1.10 per mil) and bottom water warming (2-2.5°C) during ETM2 seems slightly smaller than in South Atlantic records. The comparison of the lateral d13C gradient between the North and South Atlantic reveals that a transient circulation switch took place during ETM2, a similar pattern as observed for the PETM. New grain size and published faunal data support this hypothesis by indicating a reduction in deepwater current velocity. Following ETM2, we record a distinct intensification of bottom water currents influencing Atlantic carbonate accumulation and biotic communities, while a dramatic and persistent clay reduction hints at a weakening of the regional hydrological cycle. Our findings highlight the similarities and differences between the PETM and ETM2. Moreover, the heterogeneity of hyperthermal expression emphasizes the need to specifically characterize each hyperthermal event and its background conditions to minimalize artifacts in global climate and carbonate burial models for the early Paleogene.

Chemistry of biogenic sediments from the equatorial Pacific

To increase our understanding of the mechanisms that control the distribution of Al and Ti within marine sediment, we performed sequential extractions targeting the chemical signatures of the loosely bound, exchangeable, carbonate, oxide, organic, opal, and residual fraction of sediment from a carbonate-dominated regime (equatorial Pacific) and from a mixed opal-terrigenous regime (West Antarctic Peninsula). We observe a systematic partitioning of Al and Ti between sediment phases that is related to bulk Al/Ti. We show that, where we can quantify an Al(excess) component, the dissolved Al is preferentially affiliated with the oxide fraction, resulting in Al/Ti molar ratios of 500-3000. This is interpreted as the result of surface complexation in the water column of dissolved Al onto oxyhydroxides. We also observe a previously undetected Ti(excess) with as much as 80% of the total Ti in the organic fraction, which is most likely a function of metal-organic colloidal removal from the water column. In samples where the excess metals are obscured by the detrital load, the Al and Ti are almost exclusively found in the residual phase. This argues for the paired removal of Al (preferentially by the oxide component) and Ti (preferentially by the organic component) from the water column by settling particulate matter. This research builds upon earlier work that shows changes in the bulk ratio of Al to Ti in carbonate sediment from the central-equatorial Pacific that coincide with changes in the sedimentary bulk accumulation rate (BAR). The ratios that are observed are as much as three times higher than typical shale values, and were interpreted as the result of scavenging of dissolved Al onto particles settling in the water column. Because this non-terrigenous Al(excess) accounts for up to 50% of the total sedimentary Al inventory and correlates best with BAR, the bulk Al/Ti may be a sensitive tracer of particle flux and, therefore, export production. Because we show that the excess metals are the result of scavenging processes, the bulk Al/Ti may be considered a sensitive proxy for this region.

Ice-rafted debris record in the Prydz Bay

Ice-rafted debris mass accumulation rates (IRD MAR) at a drill site on the Antarctic continental margin are investigated to evaluate the linkages between East Antarctic Ice Sheet extent and Southern Ocean temperatures in the early to mid-Pliocene. ODP Site 1165 is within 400 km of the Antarctic coastline and in the direct pathway of icebergs released by the Amery Ice Shelf. The Amery Ice Shelf is the largest ice shelf in East Antarctica and it buttresses the Lambert Glacier drainage system, which accounts for 14% of the outflow from the East Antarctic Ice Sheet. IRD MAR were low during peak Southern Ocean warming in the early Pliocene. After a brief precursor, a tenfold increase in IRD MAR at 3.3 Ma marks the termination of the early Pliocene ice sheet minimum, coincident with the M2 glacial. For the mid-Pliocene, a strong correlation exists between the high-amplitude signal in the LR04 benthic stack and IRD MAR, suggesting linkages between East Antarctic ice extent, global ice volume and deep-water temperatures. The IRD record at Site 1165 provides evidence of greater sensitivity of the Lambert Glacier-Amery Ice Shelf system to Southern Ocean warming than is currently predicted by ice sheet models, which may relate to uncertainties in the understanding of ocean heat uptake, poleward heat transport and ice sheet-ocean interactions.

Paleomagnetism and bedding orientations at DSDP Sites 87-582, 87-583 and 87-584

Paleomagnetic measurements of sediment samples provide the magnetostratigraphy at Deep Sea Drilling Sites 582, 583, and 584 in the Nankai Trough and the Japan Trench. Drastic changes in the rate of sediment accumulation are documented by the magnetostratigraphic and biostratigraphic correlations. The changes in the accumulation rate correspond to the supply of sediments and variations in the accretionary process, which are directly related to the tectonic cycles in the geologic evolution of the Japanese island arc. Faults and folds within the drilled sedimentary sequences are oriented by paleomagnetic declination. Their directions and stress patterns are related to the relative plate motion along the trough and trench. The original remanent magnetization of the sediment was modified and remagnetized in the tectonic process of accretion by physical deformation, faulting, and intrusion of dewatering veinlets.