Distribution of Cycladophora davisiana davisiana in upper Pliocene and Pleistocene sediments of the North Atlantic and Labrador Sea

Upper Pliocene and Pleistocene abundance fluctuations of the radiolarian Cycladophora davisiana (Ehrenberg) davisiana (Petrushevskaya) are documented from North Atlantic (Site 609) and Labrador Sea (Site 646B) to provide the first long-term correlation of its abundance fluctuations to oxygen isotope stages 1-114. Also examined are temporal and regional fluctuations in abundances C. d. davisiana and the global dispersal routes of the species. The first occurrence of C. d. davisiana in the eastern North Atlantic Ocean (Site 609) occurred between 2.586 and 2.435 Ma (oxygen isotope stages 109.66-102.19). During the early Matuyama Chron, prior to oxygen isotope stage 63, C. d. davisiana abundances were less than 1% and never greater than 12%, while abundances of greater than 5% are found in stages 65.71-73, 74, and 83-84. The initial major abundance peak (35.7%) of C. d. davisiana was noted near the stage 63/62 boundary. Abundance peaks of greater than 15%, between oxygen isotope stages 35 and 63, are limited to stages 63.02, 58.07, 55.07-54.26, and 50.76-50.22. These represent the only such abundance peaks detected during the first c. 1.5 million years of the species within the North Atlantic. The character of C. d. davisiana abundance fluctuations in Site 609 changes after oxygen isotope stage 35; average abundances are greater (7.7% vs. 4.3%) and abundance maxima of more than 15% are more frequent. Many, but not all, peak abundances of C. d. davisiana occur in glacial stages (e.g., 8, 14, 18, 20, 26, 30, 34, 50, 54, and 58). Increased abundances of the species are also noted in weak interglacial stages (e.g., stages 3, 23, 39, and 41), and significant cool periods of robust interglacial periods (e.g., late stage 11). Sample spacing is adequate in some stages to note some rapid changes in abundance near stage transitions (e.g., stages 4/5, 25/26, 62/63). The sample density in Holes 609 and 611 and the upper portion of 646B is sufficient to detect a synchroneity of many abundance maxima and minima among sites. Some abundance peaks are undetected in one or more of the two holes, warranting further sampling to obtain a more accurate record of regional abundance fluctuations. Prior to stage 36, few ages of Hole 611 peaks are the same as those in the more precisely dated Hole 609. The highest abundances of C. d. davisiana were noted in Labrador Sea Hole 646B where the earliest known occurrence of the species is documented (3.08-2.99 Ma). C. d. davisiana is inferred to have evolved in the Labrador Sea (or Arctic), and migrated next through the Arctic into the North Pacific (2.62-2.64 Ma, stage 114) before migrating into the Norwegian Sea (2.63-2.53 Ma) and North Atlantic (2.59-2.44 Ma, stages 109-102). Additional migration of C. d. dauisiana into the southern South Atlantic (Site 704) occurred much later (2.06 Ma, stage 83).

Age estimates of Discoaster datums and corresponding isotope stages (Table 2)

High-resolution records (2 7 kyr) of Upper Pliocene Discoaster abundances obtained from six ODP/DSDP sites are assessed independently using oxygen isotope stratigraphy. Four Atlantic Ocean sites (DSDP Sites 552 and 607, and ODP Sites 659 and 662) comprise a transect from 56°N to 1°S and provide a record of latitudinal variations in Diseoaster biogeography. Low-latitude sites in the Atlantic (ODP Site 662), Pacific (ODP Site 677), and Indian (ODP Site 709) oceans provide additional information about variability in Discoaster abundance patterns within the equatorial region. A common chronology, based on the astronomical time scale developed for ODP Site 677, has been established for all the sites. By integrating oxygen isotope data and Discoaster abundance records at each site we are able to independently evaluate the temporal and spatial distribution of D. brouweri and D. triradiatus in the 500 kyr prior to the extinction of the discoasters near the base of the Olduvai subchron. Major decreases in abundance are evident during some of the more intense late Pliocene glacial events. In particular, glacial isotope stages 82, 96, 98 and 100 are associated with distinct abundance minima. At these times, large-scale changes in surface hydrographic conditions appear to have suppressed Discoaster numbers on a global scale. The increase in abundance of D. triradiatus, which precedes the extinction of the discoasters by around 200 kyr, may also be related to the intensification of environmental pressures that accompanied the build-up of Northern Hemisphere ice sheets during the late Pliocene. In spite of contrasting geographic and oceanographic settings, the various D. brouweri and D. triradiatus records are remarkably similar. This demonstrates that the acme and extinction events are excellent biostratigraphic datums. The simultaneous extinction of D. brouweri and D. triradiatus at 1.95 Ma were synchronous events at both a regional scale within the Atlantic, and on a global scale between the three major oceans. However, the start of the D. triradiatus acme appears to have been diachronous, occurring some 40 kyr earlier in the Atlantic than in the Indo-Pacific, and hence the stratigraphic usefulness of this datum is regional rather than global.

On the fine structure of oxygen distribution in surface water layers in the area between Iceland and Faroe Islands

During the international "Overflow-Expedition'' 1973 on R.V. "Meteor" oxygen concentrations in surface layers were measured in order to determine the oxygen gradients within the first two meters and to add some informations to the mechanisms of oxygen exchange at the air-sea interface. These investigations may be interesting also with regard to longterm- observations of the oxygen distribution in the Atlantic, especially the problem of the A.O.U. (apparent oxygen utilization) determination. To measure oxygen gradients a special sampler was built which is able to take water samples each 20 cm of the first 2 meters. These data were supplemented by further samples down to 150 m, taken by conventional water samplers, from which samples were also taken to measure N2/O2-relations. By comparing these relations with theoretical relations in air-saturated water the influence of biological production and consumption on the oxygen contents in water could be estimated. A simple glass apparatus was built to extract gas from the water samples, and hereafter the N2/O2-relations were determined by mass spectrometry. Most distributions of the oxygen anomaly show a negative oxygen balance which varies largely, probably due to strong mixing processes in the Iceland-Faroe ridge area. The distribution of surface oxygen saturation values are of two different types. The values of the stations 260, 262 and 270 stem from mixed water and show homogeneous supersaturations, as can be found instantly when whitecaps appear. The values of 9 other stations are from water, sampled during calm periods which has been mixed and supersaturated before. They show a decreasing oxygen saturation towards the sea surface and often undersaturation in the upper decimeters up to 98 % and even 91 %. So at the air-sea interface even less initial oxygen saturation than 100 % can be found after supersaturation during heavy weather periods.

Temperatures, salinity, and stable oxygen isotope values of bottom water and of the mollusc shell Arctica islandica from Nantucket Shoals

The carbonate shell of the bivalve Arctica islandica has been recognized, for more than a decade, as a potentially important marine geochemical biorecorder owing to this species' great longevity (200+ years) and wide geographic distribution throughout the northern North Atlantic Ocean, a region vital to global climate and ocean circulation. However, until now this potential has not been realized owing to the difficulty of precisely sampling the shell of this slow growing species. Using newly available automated microsampling techniques combined with micromass stable isotope mass spectrometry, a stable oxygen isotope record (1956-1957 and 1961-1970) has been obtained from a live-captured, 38-year-old A. islandica specimen collected near the former position of the Nantucket Shoals Lightship (41°N. 69°W). The shell's delta18O signal is compared with an expected signal derived from ambient bottom temperature and salinity data recorded at the lightship for the same period. The results show that A islandica's delta18O record (1) is in phase with its growth banding, confirming the annual periodicity of this species' growth bands, (2) is in oxygen isotopic equilibrium with the ambient seawater, (3) shows a consistent shell growth shutdown temperature of ~6°C. which translates into an ~8-month (May-December) shell growth period at this location, and (4) records the ambient bottom temperature with a precision of ~ +/-1.2°C. These results add important information on the life history of this commercially important shellfish species and demonstrate that A. islandica shells can be used to reconstruct inter- and intra-annual records of the continental shelf bottom temperature.

Os and Re concentrations and Os isotope ratios for DSDP and ODP metalliferous sediments from the Pacific Ocean (Table 1)

We report new 187Os/186Os data and Re and Os concentrations in metalliferous sediments from the Pacific to construct a composite Os isotope seawater evolution curve over the past 80 m.y. Analyses of four samples of upper Cretaceous age yield 187Os/186Os values of between 3 and 6.5 and 187Re/186Os values below 55. Mass balance calculations indicate that the pronounced minimum of about 2 in the Os isotope ratio of seawater at the K-T boundary probably reflects the enormous input of cosmogenic material into the oceans by the K-T impactor(s). Following a rapid recovery to 187Os/186Os of 3.5 at 63 Ma, data for the early and middle part of the Cenozoic show an increase in 187Os/186Os to about 6 at 15 Ma. Variations in the isotopic composition of leachable Os from slowly accumulating metalliferous sediments show large fluctuations over short time spans. In contrast, analyses of rapidly accumulating metalliferous carbonates do not exhibit the large oscillations observed in the pelagic clay leach data. These results together with sediment leaching experiments indicate that dissolution of non-hydrogenous Os can occur during the hydrogen peroxide leach and demonstrate that Os data from pelagic clay leachates do not always reflect the Os isotopic composition of seawater. New data for the late Cenozoic further substantiate the rapid increase in the 187Os/186Os of seawater during the past 15 Ma. We interpret the correlation between the marine Sr and Os isotope records during this time period as evidence that weathering within the drainage basin of the Ganges-Brahmaputra river system is responsible for driving seawater Sr and Os toward more radiogenic isotopic compositions. The positive correlation between 87Sr/86Sr and U concentration, the covariation of U and Re concentrations, and the high dissolved Re, U and Sr concentrations found in the Ganges-Brahmaputra river waters supports this interpretation. Accelerating uplift of many orogens worldwide over the past 15 Ma, especially during the last 5 Ma, could have contributed to the rapid increase in 187Os/186Os from 6 to 8.5 over the past 15 Ma. Prior to 15 Ma the marine Sr and Os record are not tightly coupled. The heterogeneous distribution of different lithologies within eroding terrains may play an important role in decoupling the supplies of radiogenic Os and Sr to the oceans and account for the periods of decoupling of the marine Sr and Os isotope records.

Strontium isotope geochemistry of ODP Leg 113 sites

The concentration of dissolved Sr and the distribution of 87Sr/86Sr isotope ratios in Leg 113 interstitial waters may be interpreted in terms of mixing of Sr from four different reservoirs: indigenous seawater, marine carbonate minerals, and basaltic and siliceous detrital material. The input to the pore water from these reservoirs is determined by the reactivity of the reservoir rather than its size. The presence of strontium derived from siliceous detrital material is unequivocally demonstrated in the pore waters of the hemipelagic deposits, and is also significant in the calcareous Maud Rise sediments due to the unusually low degree of carbonate recrystallization. Also, alteration of basic volcanic material is important at several sites.

Stable carbon and oxygen isotope ratios of Miocene to Recent foraminifera from the Rio Grande Rise, South Atlantic

Miocene to Recent species of planktic foraminifera in the Globorotalia (Globoconella) lineage evolved entirely within the thermocline. All species are most abundant within subtropical-temperate watermasses throughout their history. The near stasis in distribution within the thermocline and the subtropical convergence suggests the major morphological changes in Globorotalia (Globoconella) may have occurred through habitat subdivision rather than by vicariant shifts into new watermasses. At the Rio Grande Rise, in the South Atlantic, modern G. inflata is 0.66-0.84? more positive for delta18O than the most enriched coexisting Globigerinoides sacculifer and probably grows in the mid thermocline deeper than 325 m. All extinct globoconellid species have mean delta18O ratios 0.5-0.8? more positive than Globigerinoides trilobus and G. sacculifer and probably lived within the thermocline as well. Major events in skeletal evolution are poorly correlated with changes in delta18O in this group. These include evolutionary transitions to compressed, smooth-walled tests and acquisition of keels. In addition, morphological reversals from the umbilically-inflated G. conomiozea to biconvex G. pliozea and to unkeeled G. puncticulata occur in the absence of changes in delta18O signature. Instead, the ranges of delta18O between different species almost completely overlap once corrected for temporal changes in delta18O of sea water. Foraminifera morphologies have been widely considered to evolve in response to changes in watermasses or depth habitats. However, the variety of skeletal shapes in the globoconellid lineage apparently are not adaptations to a progressive radiation from the surface mixed layer into deeper waters.

South Pacific dissolved Nd isotope compositions, rare earth element and nutrient concentrations from SONNE cruise SO213

Despite its enormous extent and importance for global climate, the South Pacific has been poorly investigated in comparison to other regions with respect to chemical oceanography. Here we present the first detailed analysis of dissolved radiogenic Nd isotopes (epsilon-Nd) and rare earth elements (REEs) in intermediate and deep waters of the mid-latitude (~40°S) South Pacific along a meridional transect between South America and New Zealand. The goal of our study is to gain better insight into the distribution and mixing of water masses in the South Pacific and to evaluate the validity of Nd isotopes as a water mass tracer in this remote region of the ocean. The results demonstrate that biogeochemical cycling (scavenging processes in the Eastern Equatorial Pacific) and release of LREEs from the sediment clearly influence the distribution of the dissolved REE concentrations at certain locations. Nevertheless, the Nd isotope signatures clearly trace water masses including AAIW (Antarctic Intermediate Water) (average epsilon-Nd = -8.2 ± 0.3), LCDW (Lower Circumpolar Deep Water) (average epsilon-Nd = -8.3 ± 0.3), NPDW (North Pacific Deep Water) (average epsilon-Nd = -5.9 ± 0.3), and the remnants of NADW (North Atlantic Deep Water) (average epsilon-Nd = -9.7 ± 0.3). Filtered water samples taken from the sediment-water interface under the deep western boundary current off New Zealand suggest that boundary exchange processes are limited at this location and highlight the spatial and temporal variability of this process. These data will serve as a basis for the paleoceanographic application of Nd isotopes in the South Pacific.

Quaternary surface-water stable isotope signal from calcareous nannofossils at DSDP Site 90-593, South Tasman Sea

Oxygen isotope values from calcareous nannofossils in four cores spanning the Quaternary from DSDP Site 593 in Tasman Sea are compared with the delta18O signal of planktonic and benthic foraminifers from the same samples. The classic mid-late Quaternary isotope stages are exhibited with stage 12 particularly well developed. When delta18O values of nannofossils are adjusted for coccolithophore vital effects they indicate larger (by 1-6°C) surface to bottom paleotemperature gradients and greater (by 1-3°C) changes in mean sea-surface temperature between full glacial and interglacial conditions than do delta18O values from planktonic foraminifers. Along with the foraminifers, the nannofossils record a bimodal distribution of delta18O between the early and mid-late Quaternary, indicating a significant change in global ice budget. The delta13C of nannofossils also shows a bimodal distribution, but is opposite to that for the foraminifers. Nannofossil delta18O values record a shift of c. -0.8? at isotope stage 8 corresponding to a major reduction in abundance of the previously dominant gephyrocapsids. A shift in delta13C of c. -1.5? also occurs at stage 8, and a shift in delta13C of c. +1.2? at around stage 14. The delta18O shift in nannofossils is at least a Pacific-wide phenomenon; the delta13C shifts are possibly global. The delta13C signal of nannofossils exhibits an antipathetic relationship to that of benthic foraminifers back to isotope stage 18 but no significant correlation beyond this level to the base of the Quaternary. This is interpreted as reflecting local productivity dominating global influences on delta13C since stage 18 at DSDP Site 593. The difference between nannofossil and benthic foraminifer delta13C signals (Delta13C) tends to be maximum during glacial stages and minimum during interglacials throughout the section, showing a strong correlation with the nannofossil delta180 signal. The increased partitioning of 13C between surface and bottom waters during the glacial periods may indicate heightened productivity in surface waters in the southern Tasman Sea at these times.

Stable isotope ratios of foraminifera from last glacial-interglacial sediments of the southern high latitudes

A set of numerical equations is developed to estimate past sea surface temperatures (SST) from fossil Antarctic diatoms. These equations take into account both the biogeographic distribution and experimentally derived silica dissolution. The data represent a revision and expansion of a floral data base used previously and includes samples resulting from progressive opal dissolution experiments. Factor analysis of 166 samples (124 Holocene core top and 42 artificial samples) resolved four factors. Three of these factors depend on the water mass distribution (one Subantarctic and two Antarctic assemblages); factor 4 corresponds to a 'dissolution assemblage'. Inclusion of this factor in the data analysis minimizes the effect of opal dissolution on the assemblages and gives accurate estimates of SST over a wide range of biosiliceous dissolution. A transfer function (DTF 166/34/4) is derived from the distribution of these factors versus summer SST. Its standard error is +/- 1°C in the -1 to +10 °C summer temperature range. This transfer function is used to estimate SST changes in two southern ocean cores (43°S and 55°S) which cover the last climatic cycle. The time scale is derived from the changes in foraminiferal oxygen and carbon isotopic ratios. The reconstructed SST records present strong analogies with the air temperature record over Antarctica at the Vostok site, derived from changes in the isotopic ratio of the ice. This similarity may be used to compare the oceanic isotope stratigraphy and the Vostok time scale derived from ice flow model. The oceanic time scale, if taken at face value, would indicate that large changes in ice accumulation rates occurred between warm and cold periods.