Physical oceanography in the Western Bransfield Strait and Gerlache Strait during Austral summer 1995/96

n the framework of the FRUELA project, two oceanographic surveys were conducted by R/V Hespérides in the eastern Bellingshausen Sea, western basin of the Bransfield Strait and Gerlache Strait area during December 1995 and January 1996. The main hydrographic structures of the study domain were the Southern Boundary of the ACC and the Bransfield Front. The characteristics and zonation of local water masses are discussed in terms of temperature, salinity, dissolved oxygen, nutrient and inorganic carbon concentrations. Concentration intervals for water mass labelling, on the basis of chemical parameters in addition to the common theta/S-based classification, are defined. Silicate seems to be a very good discriminator for local water masses.

Oxygen isotopes, IRD and SST reconstruction for the interval MIS31-19 of IODP Site 306-U1314

Early and Mid-Pleistocene climate, ocean hydrography and ice sheet dynamics have been reconstructed using a high-resolution data set (planktonic and benthic d18O time series, faunal-based sea surface temperature (SST) reconstructions and ice-rafted debris (IRD)) record from a high-deposition-rate sedimentary succession recovered at the Gardar Drift formation in the subpolar North Atlantic (Integrated Ocean Drilling Program Leg 306, Site U1314). Our sedimentary record spans from late in Marine Isotope Stage (MIS) 31 to MIS 19 (1069-779 ka). Different trends of the benthic and planktonic oxygen isotopes, SST and IRD records before and after MIS 25 (~940 ka) evidence the large increase in Northern Hemisphere ice-volume, linked to the cyclicity change from the 41-kyr to the 100-kyr that occurred during the Mid-Pleistocene Transition (MPT). Beside longer glacial-interglacial (G-IG) variability, millennial-scale fluctuations were a pervasive feature across our study. Negative excursions in the benthic d18O time series observed at the times of IRD events may be related to glacio-eustatic changes due to ice sheets retreats and/or to changes in deep hydrography. Time series analysis on surface water proxies (IRD, SST and planktonic d18O) of the interval between MIS 31 to MIS 26 shows that the timing of these millennial-scale climate changes are related to half-precessional (10 kyr) components of the insolation forcing, which are interpreted as cross-equatorial heat transport toward high latitudes during both equinox insolation maxima at the equator.

Nitrogen isotopic and opal record of sediments in the Gulf of California

High-resolution records of the nitrogen isotopic composition of organic matter (d15Norg), opal content, and opal accumulation rates from the central Gulf of California reveal large and abrupt variations during deglaciation and gradual Holocene changes coincident with climatic changes recorded in the North Atlantic. Homogenous sediments with relatively low d15Norg values and low opal content were deposited at the end of the last glacial period, during the Younger-Dryas event, and during the middle to late Holocene. In contrast, laminated sediments deposited in the two deglacial stages are characterized by very high d15Norg values (>14 per mil) and opal accumulation rates (29-41 mg/cm**2/yr). Abrupt shifts in d15Norg were driven by widespread changes in the extent of suboxic subsurface waters supporting denitrification and were amplified in the central gulf record due to variations in upwelling, vertical mixing, and/or the latitudinal position of the Intertropical Convergence Zone.

Carbon and Sulphur concentrations of sediments from Peru, Oman and Benguela upwelling environments

Abundances of organic carbon, sulfur, and reactive iron in sediments of three upwelling environments (Peru, Oman and Benguela) suggest that organic carbon/reduced sulfur ratios (C/S-ratios) in this category of marine sediments deviate considerably from previously established empirical ratios in normal marine sediments. To clarify the discrepancies, we investigated those components of the diagenetic system that limit the formation of pyrite: sulfate concentrations and reduction rates in pore waters, availability of reactive iron, and the quantity and quality of organic matter. All three limitations are evident in our sample pools. The results suggest that C/S-ratios in recent and fossil marine sediments rich in organic matter may be unsuitable as paleoenvironmental indicators.

Quantitative coccolithophore analyses in sediment core MD01-2446

Quantitative coccolithophore analyses were performed in core MD01-2446, located in the mid-latitude North Atlantic, to reconstruct climatically induced sea-surface water conditions throughout Marine Isotope Stages (MIS) 14-9. The data are compared to new and available paleoenvironmental proxies from the same site as well as other nearby North Atlantic records that support the coccolithophore signature at glacial-interglacial to millennial climate scale. Total coccolithophore absolute abundance increases during interglacials but abruptly drops during the colder glacial phases and deglaciations. Coccolithophore warm-water taxa (wwt) indicate that MIS11c and MIS9e experienced warmer and more stable conditions throughout the whole photic zone compared to MIS13. MIS11 was a long-lasting warmer and stable interglacial characterized by a climate optimum during MIS11c when a more prominent influence of the subtropical front at the site is inferred. The wwt pattern also suggests distinct interstadial and stadial events lasting about 4-10 kyr. The glacial increases of Gephyrocapsa margereli-G. muellerae 3-4 µm along with higher values of Corg, additionally supported by the total alkenone abundance at Site U1313, indicate more productive surface waters, likely reflecting the migration of the polar front into the mid-latitude North Atlantic. Distinctive peaks of G. margereli-muellerae (> 4 µm), C. pelagicus pelagicus, Neogloboquadrina pachyderma left coiling, and reworked nannofossils, combined with minima in total nannofossil accumulation rate, are tracers of Heinrich-type events during MIS12 and MIS10. Additional Heinrich-type events are suggested during MIS12 and MIS14 based on biotic proxies, and we discuss possible iceberg sources at these times. Our results improve the understanding of mid-Brunhes paleoclimate and the impact on phytoplankton diversity in the mid-latitude North Atlantic region.

Temperature reconstruction of sediment core SO9-93KL

The Toba volcanic event, one of the largest eruptions during the Quaternary, is documented in marine sediment cores from the northeastern Arabian Sea. On the crest of the Murray Ridge and along the western Indian continental margin, we detected distinct concentration spikes and ash layers of rhyolithic volcanic shards near the marine isotope stage 5-4 boundary with the chemical composition of the "Youngest Toba Tuff". Time series of the Uk'37-alkenone index, planktic foraminiferal species, magnetic susceptibility, and sediment accumulation rates from this interval show that the Toba event occurred between two warm periods lasting a few millennia. Using Toba as an instantaneous stratigraphic marker for correlation between the marine- and ice-core chronostratigraphies, these two Arabian Sea climatic events correspond to Greenland interstadials 20 and 19, respectively. Our data sets thus depict substantial interstadial/stadial fluctuations in sea-surface temperature and surface-water productivity. We show that variable terrigenous (eolian) sediment supply played a crucial role in transferring and preserving the productivity signal in the sediment record. Within the provided stratigraphic resolution of several decades to centennials, none of these proxies shows a particular impact of the Toba eruption. However, our results are additional support that Toba, despite its exceptional magnitude, had only a minor impact on the evolution of low-latitude monsoonal climate on centennial to millennial time scales.

Geochemistry and paleontology of sediment core GeoB3359-3

A geochemical and paleontological reconstruction of paleoproductivity, upwelling intensity and sea surface temperature (SST) off central Chile at 35°S (GeoB3359-3) reveals marked changes from the Last Glacial Maximum (LGM) through the Early Holocene. Surface-water productivity was determined by the interaction between the atmospheric (the Southern Westerlies) and oceanographic (the Antarctic Circumpolar Current, ACC) systems from the LGM through early Termination I (TI). The northward shift of the climate zones during the LGM brought the ACC, as the main macronutrient source, closer to the GeoB3359-3, SST lowered, and surface water productivity and accumulation rates of biogenic components enhanced. With the poleward return of the Southern Westerlies and the ACC, the subtropical high-pressure system became the dominant atmospheric component southward till 35°S during the late TI and Early Holocene and caused surface water productivity to increase through enhanced upwelling.

Glaciological characteristics and carbon dioxide record of ice cores from Antarctica

We report a decadally resolved record of atmospheric CO2 concentration for the last 1000 years, obtained from the West Antarctic Ice Sheet (WAIS) Divide shallow ice core. The most prominent feature of the pre-industrial period is a rapid ~7 ppm decrease of CO2 in a span of ~20-50 years at ~1600 A.D. This observation confirms the timing of an abrupt atmospheric CO2 decrease of ~10 ppm observed for that time period in the Law Dome ice core CO2 records, but the true magnitude of the decrease remains unclear. Atmospheric CO2 variations over the time period 1000-1800 A.D. are statistically correlated with northern hemispheric climate and tropical Indo-Pacific sea surface temperature. However, the exact relationship between CO2 and climate remains elusive due to regional climate variations and/or uneven geographical data density of paleoclimate records. We observe small differences of 0 ~2% (0 ~ 6 ppm) among the high-precision CO2 records from the Law Dome, EPICA Dronning Maud Land and WAIS Divide Antarctic ice cores. However, those records share common trends of CO2 change on centennial to multicentennial time scales, and clearly show that atmospheric CO2 has been increasing above preindustrial levels since ~1850 A.D.

Quartz content, accumulation and sedimentation rates at DSDP Hole 91-595A

The quartz contents of sediments from Hole 595A, determined by X-ray diffractometry, serve as an indicator of eolian transport of terrigenous material to the central southern Pacific. The quartz contents are very small and, within limits of analytical resolution, vary only slightly from the Cretaceous to the present. However, the accumulation rate of the eolian quartz does change significantly. The quartz accumulation reflects the changing position of the site with respect to the terrigenous source areas and the variations in wind systems through time.

Mass accumulation rates, age, sedimentation rate and bulk density at the East Pacific Rise

The rate at which hydrothermal precipitates accumulate, as measured by the accumulation rate of manganese, can be used to identify periods of anomalous hydrothermal activity in the past. From a preliminary study of Sites 597 and 598, four periods prior to 6 Ma of anomalously high hydrothermal activity have been identified: 8.5 to 10.5 Ma, 12 to 16 Ma, 17 to 18 Ma, and 23-to-27 Ma. The 18-Ma anomaly is the largest and is associated with the jump in spreading from the fossil Mendoza Ridge to the East Pacific Rise, whereas the 23-to-27-Ma anomaly is correlated with the birth of the Galapagos Spreading Center and resultant ridge reorganization. The 12-to-16-Ma and 8.5-to-10.5-Ma anomalies are correlated with periods of anomalously high volcanism around the rim of the Pacific Basin and may be related to other periods of ridge reorganization along the East Pacific Rise. There is no apparent correlation between periods of fast spreading at 19°S and periods of high hydrothermal activity. We thus suggest that periods when hydrothermal activity and crustal alteration at mid-ocean ridges are the most pronounced may be periods of large-scale ridge reorganization.

Accumulation rates of bulk sediment and total organic carbon for Ontong Java Plateau sediments

Organic geochemical investigations were performed on sediments of Leg 130 to reconstruct the depositional environment of the Ontong Java Plateau. The Miocene to Quaternary sediments collected during the drilling campaign are characterized by extremely low organic carbon contents. As indicated by C/N ratios and Rock-Eval data, most of the organic matter is probably of marine origin. Based on mass-accumulation rates of organic carbon, the paleoproductivity for the Miocene-Pliocene and the late Pliocene-Pleistocene time intervals as well as the modern surface-water production were estimated. The productivity values of the surface sediments (25-59 gC/m2/yr) reflect the various influences of the equatorial upwelling cell on the different sites. The accumulation rates of organic carbon are generally low; however, they show a distinct increase at 8 Ma and a decrease at 2 Ma.

CaCO3, density, sedimentation and accumulation rates at DSDP Hole 68-503B

Pelagic sediments from DSDP Hole 5O3B contain, in their carbonate abundance data, a clear record of glacial-interglacial cycles. The eolian component of those sediments was analyzed over the past four carbonate cycles, and the mass accumulation rate (MAR) and grain size of the eolian component was determined. Eolian MARs range from 24 to 169 mg/cm**2/10**3y. and commonly are higher by a factor of three to five during times of glacial retreat. Reduced contribution during periods of glaciation most likely reflects glacial-age humidity in the American source. Grain-size values (phi50) range from 8.25 to a minimum of 8.79phi-a variation in grain mass by a factor of 3.1. Larger grains reflect more vigorous atmospheric circulation, but sizes do not covary with the carbonate or eolian accumulation curves. These data suggest that the intensity of atmospheric circulation in the tropics may reflect the 42,000 y.-tilt cycle rather than the 100,000 y.-cycle of glacial advance.

Organic matter preservation in sediments of the Peru and Oman continental margin

Detailed petrographical and bulk geochemical investigations of organic matter (OM) have been performed on sediments deposited below or close to upwelling areas offshore Peru (ODP-Leg 112; Sites 679, 681, 688) and Oman (ODP-Leg 117; Sites 720, 723, 724) in order to obtain a quantitative understanding of its accumulation and degradation. Microscopical as well as nanoscopical investigations reveal that the OM in sediments affected by upwelling mechanisms mainly (up to 98%) consists of unstructured (amorphous) organic aggregates without any apparent biological structures. In sediments which are not or to a lesser extent affected by upwelling (Site 720) terrestrial OM predominates. Organic carbon (TOC) contents are highly variable and range between 9.8% in sediments deposited below upwelling cells and 0.2% in sediments outside the upwelling zone. The TOC/sulphur ratios of the sediments scatter widely. The samples from the deep-water locations (Sites 688 and 720), show C/S-ratios of "normal" marine sediments, whereas at the other locations no correlation or even a negative correlation between sulphur and TOC concentration exists. In most of the upwelling-influenced sediments OM contains a significant amount of sulphur. The incorporation of sulphur into the OM followed microbial sulphate reduction and occurred in the upper meters of the sedimentary column. Below, OM is still present in vast amounts and relatively hydrogen-rich, but is nevertheless non-metabolizable and becomes the limiting factor for bacterial sulphate reduction. According to mass balance calculations 90-99% of the OM produced in the photic zone was remineralized and 1-3% was consumed by microbial sulphate reduction. The aerobic and anaerobic processes have greatly affected degradation and conservation of OM.

Late Neogene sediment composition and accumulation rates of ODP Hole 120-751A

Through scanning electron microscope analysis of sediment microfabric, we have evaluated variations in high-resolution shipboard physical properties (index properties and shear strength), sediment components (smear slide determinations), and shore-based calcium carbonate and biogenic silica data from Site 751 (Kerguelen Plateau). The stratigraphic section at this site records a change in biogenic ooze composition from predominantly calcareous (nannofossil) to siliceous (diatom) ooze from ~23 Ma to the present, reflecting expansion of Antarctic water masses during the late Neogene. The profound change in physical properties and sediment character at 40.1 mbsf (~5-6 Ma) evidently records the northward movement of the Polar Front and a change in absolute accumulation rates of sediment at this site. Trends in geotechnical properties with depth at Site 751 allowed us to subdivide the sedimentary column into a number of geotechnical units that reflect changes in depositional and postdepositional processes with time. Geotechnical properties are sensitive to changing sedimentary inputs of primarily siliceous and calcareous microfossils. This allows us to study the physical nature of biostratigraphically-identified hiatuses and variations in environmental conditions linked to the migration of the Polar Front across this region. The analysis of geotechnical properties permits a more detailed division of the sedimentary column than is possible from shipboard lithologic descriptions alone. Our study of the sedimentary microfabric indicates that randomly oriented, elongate pennate diatom valves compose the sediments with highest porosity and water content values, and the lowest density values (wet bulk, dry bulk, and grain density). Conversely, sediments composed of nannofossils and disassociated nannofossil crystallites and little or no siliceous remains have the lowest porosity and water content values, and the highest density values. Samples of mixed siliceous/calcareous composition have intermediate physical property values, but these vary according to the nature of the sedimentary matrix and the state of preservation of individual skeletal elements.

Temperature and salinity reconstruction for sediment core GeoB3313-1

We reconstructed changes of temperature, salinity, and productivity within the southern Peru-Chile Current during the last 8000 years from a high-resolution sediment core recovered at 41°S using alkenones, isotope ratios of planktic foraminifera, biogenic opal, and organic carbon. Paleotemperatures and paleosalinities reached maximum values at ~5500 years ago and thereafter declined to modern values, whereas paleoproductivity continuously increased throughout the last 8000 years. We ascribe these long-term Holocene trends primarily to latitudinal shifts of the Antarctic Circumpolar Current (ACC). The concurrence with shifts in the position of the Southern Westerlies points to a common response of atmospheric and oceanographic circulation patterns off southern Chile. Millennial- to centennial-scale fluctuations of paleotemperatures and paleosalinities, on the other hand, lag displacements in the position of the Southern Westerlies but reveal a significant correlation to short-term temperature changes in Antarctica, indicating a high-latitude control of the ACC at these timescales.