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.

Chemistry of smectite and volcanic glass of ODP Site 159-959

The Ocean Drilling Program (ODP) Site 959 was drilled in the northern border of the Côte d'Ivoire-Ghana Ridge at a water depth of 2100 m. Pleistocene total thickness does not exceed 20 m. Winnowing processes resulted in a low accumulation rate and notable stratigraphic hiatuses. During the Late Pleistocene, bottom circulation was very active and controlled laminae deposition (contourites) which increased the concentration of glauconitic infillings of foraminifera, and of volcanic glass and blue-green grains more rarely, with one or several subordinate ferromagnesian silicates. Volcanic glass generally was X-ray amorphous and schematically classified as basic to intermediate (44-60% SiO2). Opal-A or opal-CT suggested the beginning of the palagonitisation process, and previous smectitic deposits may have been eroded mechanically. The blue-green grains presented two main types of mineralogic composition: (1) neoformed K, Fe-smectite associated with zeolite (like phillipsite) and unequal amounts of quartz and anorthite; (2) feldspathic grains dominated by albite but including quartz, volcanic glass and smectites as accessory components. They were more or less associated with the volcanic glass. On the basis of their chemical composition, the genetic relationship between the blue-green grains and the volcanic glass seemed to be obvious although some heterogeneous grains seemed to be primary ignimbrite and not the result of glass weathering. The most reasonable origin of these pyroclastic ejecta would be explosive events from the Cameroon Volcanic Ridge, especially from the Sao Thome and Principe Islands and Mount Cameroon area. This is supported both by grain geochemistry and the time of volcanic activity, i.e. Pleistocene. After westward wind transport (some 1200 km) and ash fall-out, the subsequent winnowing by bottom currents controlled the concentration of the volcanic grains previously disseminated inside the hemipelagic sediment. Palagonitisation, and especially phillipsite formation, may result from a relatively rapid reaction during burial diagenesis (<1 m.y.), in deep-sea deposits at relatively low sedimentation rate. However, it cannot be excluded that the weathering had begun widely on the Cameroon Ridge before the explosive event.

Neogene ice-rafted debris record of ODP Hole 120-751A

One of the primary objectives of Leg 120 was to obtain a high-resolution Neogene stratigraphic section from the Kerguelen Plateau. Site 751, located in the central part of the Raggatt Basin on the Southern Kerguelen Plateau in 1633.8 m of water (57°43.56'S; 79°48.89'E), was selected as the dedicated Neogene site for this objective. High-resolution sampling at Site 751 was used to delineate in detail the Neogene ice-rafted debris (IRD) occurrences on the Kerguelen Plateau. The oldest IRD found at Site 751 was approximately 9.9 Ma, and it was not until approximately 8.5 Ma that significant concentrations of IRD were detected. The first major IRD event at this site occurred in the uppermost Miocene between 6.0 and 5.5 Ma. During this time period, a general climatic cooling and glacial expansion occurred on Antarctica. The late Miocene IRD event was followed by a continuous episode of elevated IRD deposition in the lowermost Pliocene between 4.5 and 4.1 Ma. The 0.4-m.y. duration and the timing of the early Pliocene IRD event on the Kerguelen Plateau corresponds with IRD fluxes observed on the Falkland Plateau and in the Weddell Abyssal Plain. This correspondence of data indicates that a major global climatic event occurred during the early Pliocene. The East Antarctic Ice Sheet may have experienced deglaciation between 4.5 and 4.1 Ma and, as a result, released large volumes of sediment-laden ice into the Southern Ocean.

Late Quaternary planktonic foraminifera of ODP Hole 175-1087A

Planktonic foraminiferal assemblages from the upper Pleistocene part of Hole 1087A (0 to 12.1 meters below seafloor) are investigated to assess the role of global and local climate changes on surface circulation in the southern Benguela region. The benthic stable isotope record indicates that the studied interval is representative of the last four climatic cycles, that is, down to marine isotope Stage (MIS) 12. The species assemblages bear a clear transitional to subpolar character, with Neogloboquadrina pachyderma (d), Globorotalia inflata, and Globigerina bulloides, in order of decreasing abundance, as the dominant taxa. This species association presently characterizes the mixing domain of old upwelled and open ocean waters, seaward of the Benguela upwelling cells. Abundance variation of the dominant foraminiferal species roughly follows a glacial-interglacial pattern down to MIS 8, suggesting an alternation of upwelling strength and associated seaward extension of the belt of upwelled water as a response to global climate changes. This pattern is interrupted from ~250 ka down to MIS 12, where the phase relationship with global climate is ill defined and might be interpreted as a local response of the southern Benguela region to the mid-Brunhes event. Of particular interest is a single pulse of newly upwelled waters at the location of Site 1087 during early MIS 9 as indicated by a peak abundance of sinistral N. pachyderma (s). Variable input of warm, salty Indian Ocean thermocline waters into the southeast Atlantic, a key component of the Atlantic heat conveyor, is indicated by abundance changes of the tropical taxon Globorotalia menardii. From this tracer, we suggest that interocean exchange was hardly interrupted throughout the last 460 k.y., but was most effective at glacial terminations, particularly during Terminations I and II, as well as during the upper part of MIS 12. This maximum input of Indian Ocean waters around the southern tip of Africa is associated with the reseeding of G. menardii in the tropical Atlantic.

Mid-Pleistocene benthic foraminiferal record in the Southwest Pacific

Benthic foraminiferal faunas from three bathyal sequences provide a proxy record of oceanographic changes through the mid-Pleistocene transition (MPT) on either side of the Subtropical Front (STF), east of New Zealand. Canonical correspondence analyses show that factors related to water depth, latitude and climate cycles were more significant than oceanographic factors in determining changes in faunal assemblage composition over the last 1 Ma. Even so, mid-Pleistocene faunal changes are recognizable and can be linked to inferred palaeoceanographic causes. North of the largely stationary STF the faunas were less variable than to the south, perhaps reflecting the less extreme glacial-interglacial fluctuations in the overlying Subtropical Surface Water. Prior to Marine Isotope Stage (MIS) 21 and after MIS 15, the northern faunas had fairly constant composition, but during most of the MPT faunal composition fluctuated in response to climate-related food-supply variations. Faunal changes through the MPT suggest increasing food supply and decreasing dissolved bottom oxygen. South of the STF, beneath Subantarctic Surface Water, mid-Pleistocene faunas exhibited strong glacial-interglacial fluctuations, inferred to be due to higher interglacial nutrient supply and lower oxygen levels. The most dramatic faunal change in the south occurred at the end of the MPT (MIS 17- 12). with an acme of Abditodentrix pseudothalmanni, possibly reflecting higher carbon flux and lower bottom oxygen. This study suggests that the mid-Pleistocene decline and extinction of a group of elongate, cylindrical deep-sea foraminifera may have been related to decreased bottom oxygen concentrations as aresult of slower deep-water currents.

Geochemistry and soil characteristics of sediment core GeoB4234

Stable isotopes of sedimentary nitrogen and organic carbon are widely used as proxy variables for biogeochemical parameters and processes in the water column. In order to investigate alterations of the primary isotopic signal by sedimentary diagenetic processes, we determined concentrations and isotopic compositions of inorganic nitrogen (IN), organic nitrogen (ON), total nitrogen (TN), and total organic carbon (TOC) on one short core recovered from sediments of the eastern subtropical Atlantic, between the Canary Islands and the Moroccan coast. Changes with depth in concentration and isotopic composition of the different fractions were related to early diagenetic conditions indicated by pore water concentrations of oxygen, nitrate, and ammonium. Additionally, the nature of the organic matter was investigated by Rock-Eval pyrolysis and microscopic analysis. A decrease in ON during aerobic organic matter degradation is accompanied by an increase of the 15N/14N ratio. Changes in the isotopic composition of ON can be described by Rayleigh fractionation kinetics which are probably related to microbial metabolism. The influence of IN depleted in 15N on the bulk sedimentary (TN) isotope signal increases due to organic matter degradation, compensating partly the isotopic changes in ON. In anoxic sediments, fixation of ammonium between clay lattices results in a decrease of stable nitrogen isotope ratio of IN and TN. Changes in the carbon isotopic composition of TOC have to be explained by Rayleigh fractionation in combination with different remineralization kinetics of organic compounds with different isotopic composition. We have found no evidence for preferential preservation of terrestrial organic carbon. Instead, both TOC and refractory organic carbon are dominated by marine organic matter. Refractory organic carbon is depleted in 13C compared to TOC.

Sedimentology and planktonioc foraminifera of core SU90-08

The observation by Heinrich (1988) that, during the last glacial period, much of the input of ice-rafted detritus to the North Atlantic sediments may have occurred as a succession of catastrophic events, rekindled interest on the history of the northern ice sheets over the last glacial period. In this paper, we present a rapid method to study the distribution of these events (both in space and time) using whole core low-field magnetic susceptibility. We report on approximately 20 cores covering the last 150 to 250 kyr. Well-defined patterns of ice-rafted detritus appear during periods of large continental ice-sheet extent, although these are not always associated within their maxima. Most of the events may be traced across the North Atlantic Ocean. For the six most recent Heinrich layers (HL), two distinct patterns exist: HL1, HL2, HL4, HL5 are distributed along the northern boundary of the Glacial Polar Front, over most of the North Atlantic between ~40° and 50°N; HL3 is more restricted to the central and eastern part of the northern Atlantic. The Nd-Sr isotopic composition of the material constituting different Heinrich events indicates the different provenance of the two patterns: HL3 has a typical Scandinavia-Arctic-Icelandic 'young crust' signature, and the others have a large component of northern Quebec and northern West Greenland 'old crust' material. These isotopic results, obtained on core SU-9008 from the North American basin, are in agreement with the study by Jantschik and Huon (1992), who used K-Ar dating of silt- and clay-size fractions of an eastern basin core (ME-68-89). These data confirm the large spatial scale of these events, and the enormous amount of ice-rafted detritus they represent.

Sedimentology and stable isotope record of DSDP Hole 68-503B

Samples from the upper portion of a cyclic pelagic carbonate sediment sequence in Deep-Sea Drilling Project (DSDP) hole 503B (4.0°N, 95.6°W) are the first group to be analyzed for paleoceanographic and paleoclimatic proxy-indicators of ice volume, deep ocean and surface water circulation, and atmospheric circulation in order to resolve the complex origin of the cyclicity. Temporal resolution is taken from the delta18O time scale, most other parameters are calculated in terms of their mass flux to the seafloor. CaCO3 percent in the sediments fluctuates in the well-known Pacific pattern and is higher during glacial times. The fluxes of opal and organic carbon have patterns similar to each other and show a variability of a factor of 2.5 to 4. The longer organic carbon record shows flux maxima during both glacial and interglacial times. The accumulation patterns of both opal and organic carbon suggest that the variability in surface water productivity and/or seafloor preservation of those materials is not simply correlated to glacial or interglacial periods. Eolian dust fluxes are greater during interglacial periods by factors of 2 to 5, indicating that eolian source regions in central and northern South America were more arid during interglacial periods. The record of eolian grain size provides a semiquantitative estimation of the intensity of the transporting winds. The eolian data suggest more intense atmospheric circulation during interglacial periods, opposite to the anticipated results. We interpret this observation as recording the southerly shift of the intertropical convergence zone to the latitude of hole 503B during glaciations.