Stable foraminifer isotope composition and age of ODP Sites 184-1147 and 184-1148, South China Sea

Sites 1147 (18°50.11'N, 116°33.28'E; water depth = 3246 m) and 1148 (18°50.17'N, 116°33.94'E; water depth = 3294 m) are located on the lowermost continental slope off southern China near the continent/ocean crust boundary of the South China Sea Basin. Site 1147 is located upslope ~0.45 nmi west of Site 1148. Three advanced piston corer holes at Site 1147 and two extended core barrel holes at Site 1148 were cored and combined into a composite (spliced) stratigraphic section, which provided a relatively continuous profile for the lower Oligocene to Holocene (Wang, Prell, Blum, et al., 2000, doi:10.2973/odp.proc.ir.184.2000; Jian, et al., 2001, doi:10.1007/BF02907088) for studying stratigraphy and paleoceanography. A total of 1047 planktonic foraminifers stable isotope measurements were performed on 975 samples covering the upper 409.58 meters composite depth (mcd) at ~42-cm intervals (Tables T1, T2), and a total of 1864 benthic foraminifers measurements were performed on 1650 samples in the upper 837.11 mcd at ~51-cm intervals (Tables T3, T4). We significantly improved the time resolution of the benthic stable isotope record in the upper 476.68 mcd by reducing the average sample spacing to ~29 cm. This translates into an average sampling resolution of ~16 k.y. for the Miocene sequence and ~8 k.y. for the Pliocene-Holocene interval, assuming a change in sedimentation rates from ~1.8 to ~3.5 cm/k.y., as suggested by shipboard stratigraphy. These data sets provide the basis for upcoming studies to establish an oxygen isotope stratigraphy and examine the Neogene evolution of deep and surface water signatures (temperature, salinity, and nutrients) in the South China Sea.

Oxygen and carbon isotope composition of Globoquadrina venezuelana and Oridorsalis umbonatus at DSDP Leg 85 Holes

Oxygen and carbon isotope stratigraphies are given for the planktonic foraminifer Globoquadrina venezuelana (a deep-dwelling species) at three DSDP sites located along a north-south transect at approximately 133°W across the Pacific equatorial high-productivity zone. The records obtained at Sites 573 and 574 encompass the lower Miocene. At Site 575 the record includes the middle Miocene and extends into the lowermost lower Miocene. The time resolution of the planktonic foraminifer isotope record varies from 50,000 to 500,000 yr. The benthic foraminifer Oridorsalis umbonatus was analyzed for isotope composition at a few levels of Site 575. Isotope stratigraphies for all three sites are compared with carbonate, foraminifer preservation, and grain size records. We identified a number of chemostratigraphic signals that appear to be synchronous with previously recognized signals in the western equatorial Pacific and the tropical Indian Ocean, and thus provide useful tools for chronostratigraphic correlations. The sedimentary sequence at Site 573 is incomplete and condensed, whereas the sequences from Sites 574 and 575 together provide a complete lower Miocene record. The expanded nature of this record, which was recovered with minimum disturbance and provides excellent calcareous and siliceous biostratigraphic control, offers a unique opportunity to determine the precise timing of early Miocene events. Paleomagnetic data from the hydraulic piston cores at Site 575 for the first time allow late early Miocene paleoceanographic events to be tied directly to the paleomagnetic time scale. The multiple-signal stratigraphies provide clues for paleoceanographic reconstruction during the period of preconditioning before the major middle Miocene cooling. In the lowermost lower Miocene there is a pronounced shift toward greater d13C values (by -1%) within magnetic Chron 16 (between approximately 17.5 and 16.5 Ma). The "Chron 16 Carbon Shift" coincides with the cessation of an early Miocene warming trend visible in the d18O signals. Values of d13C remain high until approximately 15 Ma, then decrease toward initial (early Miocene) values near 13.5 Ma. The broad lower to middle Miocene d13C maximum appears to correlate with the deposition of organic-carbon-rich sediments around the margin of the northern Pacific in the Monterey Formation of California and its lateral equivalents. The sediments rimming the Pacific were probably deposited under coastal upwelling conditions that may have resulted from the development of a strong permanent thermocline. Deposition in the upwelling areas occurred partly under anaerobic conditions, which led to the excess extraction of organic carbon from the ocean. The timing of the middle Miocene cooling, which began after the Chron 16 Carbon Shift, suggests that the extraction of organic carbon preconditioned the ocean-atmosphere system for subsequent cooling. A major carbonate dissolution event in the late early Miocene, starting at approximately 18.7 Ma, is associated with the enrichment in 13C. The maximum dissolution is coeval with the Chron 16 Carbon Shift. It corresponds to a prominent acoustic horizon that can be traced throughout the equatorial Pacific.

Dinoflagellate cyst assemblages, Mg/Ca based reconstructed temperatures, stable carbon isotopes, and geochemical parameters of sediment core GeoB9508-5

In order to investigate a possible connection between tropical northeast (NE) Atlantic primary productivity, Atlantic meridional overturning circulation (AMOC), and drought in the Sahel region during Heinrich Stadial 1 (HS1), we used dinoflagellate cyst (dinocyst) assemblages, Mg/Ca based reconstructed temperatures, stable carbon isotopes (d13C) and geochemical parameters of a marine sediment core (GeoB 9508-5) from the continental slope offshore Senegal. Our results show a two-phase productivity pattern within HS1 that progressed from an interval of low marine productivity between ~ 19 and 16 kyr BP to a phase with an abrupt and large productivity increase from ~ 16 to 15 kyr BP. The second phase is characterized by distinct heavy planktonic d13C values and high concentrations of heterotrophic dinocysts in addition to a significant cooling signal based on reconstructions of past sea surface temperatures (SST). We conclude that productivity variations within HS1 can be attributed to a substantial shift of West African atmospheric processes. Taken together our results indicate a significant intensification of the North East (NE) trade winds over West Africa leading to more intense upwelling during the last millennium of HS1 between ~ 16 and 15 kyr BP, thus leaving a strong imprint on the dinocyst assemblages and sea surface conditions. Therefore, the two-phase productivity pattern indicates a complex hydrographic setting suggesting that HS1 cannot be regarded as uniform as previously thought.

Stable oxygen isotope record and lithogenic composition of ODP Hole 117-722B

The upper 38 m of Hole 722B sediments (Owen Ridge, northwest Arabian Sea) was sampled at 20 cm intervals and used to develop records of lithogenic percent, mass accumulation rate, and grain size spanning the past 1 m.y. Over this interval, the lithogenic component of Owen Ridge sediments can be used to infer variability in the strength of Arabian Sea summer monsoon winds (median grain size) and the aridity of surrounding dust source-areas (mass accumulation rate; MAR in g/cm**2/k.y). The lithogenic MAR has strong 100, 41, and 23 k.y. cyclicities and is forced primarily by changes in source-area aridity associated with glacial-interglacial cycles. The lithogenic grain size, on the other hand, exhibits higher frequency variability (23 k.y.) and is forced by the strength of summer monsoon winds which, in turn, are forced by the effective sensible heating of the Indian-Asian landmass and by the availability of latent heat from the Southern Hemisphere Indian Ocean. These forcing mechanisms combine to produce a wind-strength record which has no strong relationship to glacial-interglacial cycles. Discussion of the mechanisms responsible for production of primary Milankovitch cyclicities in lithogenic records from the Owen Ridge is presented elsewhere (Clemens and Prell, 1990, doi:10.1029/PA005i002p00109). Here we examine the 1 m.y. record from Hole 722B focusing on different aspects of the lithogenic components including an abrupt change in the monsoon wind-strength record at 500 k.y., core-to-core reproducibility, comparison with magnetic susceptibility, coherency with a wind-strength record from the Pacific Ocean, and combination frequencies in the wind-strength record. The Hole 722B lithogenic grain-size record shows an abrupt change at 500 k.y. possibly indicating decreased monsoon wind-strength over the interval from 500 k.y. to present. The grain-size decrease appears to be coincident with a loss of spectral power near the 41 k.y. periodicity. However, the grain-size decrease is not paralleled in the Globigerina bulloides upwelling record, an independent record of summer monsoon wind-strength (Prell, this volume). These observations leave us with competing hypotheses possibly involving: (1) a decrease in the sensitivity of monsoon windstrength to obliquity forcing, (2) decoupling of the grain size and G. bulloides records via a decoupling of the nutrient supply from wind-driven upwelling, and/or (3) a change in dust source-area or the patterns of dust transporting winds. Comparison of the lithogenic grain size and weight percent records from Hole 722B with those from a nearby core shows that the major and most minor events are well replicated. These close matches establish our confidence in the lithogenic extraction techniques and measurements. Further, reproducibility on a core-to-core scale indicates that the eolian depositional signal is regionally strong, coherent, and well preserved. The lithogenic weight percent and magnetic susceptibility are extremely well correlated in both the time and frequency domains. From this we infer that the magnetically susceptible component of Owen Ridge sediments is of terrestrial origin and transported to the Owen Ridge via summer monsoon winds. Because of the high correlation with the lithogenic percent record, the magnetic susceptibility record can be cast in terms of lithogenic MAR and used as a high resolution proxy for continental aridity. In addition to primary Milankovitch periodicities, the Hole 722B grain-size record exhibits periodicity at 52 k.y. and at 29 k.y. Both periodicities are also found in the grain-size record from piston core RC11-210 in the equatorial Pacific Ocean. Comparison of the two grain-size records shows significant coherence and zero phase relationships over both the 52 and 29 k.y. periodicities suggesting that the strengths of the Indian Ocean monsoon and the Pacific southeasterly trade winds share common forcing mechanisms. Two possible origins for the 52 and 29 k.y. periodicities in the Hole 722B wind-strength record are (1) direct Milankovitch forcing (54 and 29 k.y. components of obliquity) and (2) combination periodicities resulting from nonlinear interactions within the climate system. We find that the 52 and 29 k.y. periodicities show stronger coherency with crossproducts of eccentricity and obliquity (29 k.y.) and precession and obliquity (52 k.y.) than with direct obliquity forcing. Our working hypothesis attributes these periodicities to nonlinear interaction between external insolation forcing and internal climatic feedback mechanisms involving an interdependence of continental snow/ice-mass (albedo) and the hydrological cycle (latent heat availability).

Stable oxygen isotope record and relative abundances of planktonic foraminifera of ODP Hole 117-728A

High resolution stratigraphy based on oxygen isotope ratios of the planktonic foraminifers Neogloboquadrina dutertrei (d'Orbigny), Globigeriniodes ruber (d'Orbigny), and Globigerina bulloides (d'Orbigny), magnetic susceptibility, and calcium carbonate content covers the sedimentary record of ODP Hole 728A drilled on the Oman Margin from approximately 10 k.y. to 525 k.y., comprising isotopic stages 1-13. Below stage 13 isotopic stage boundaries cannot be defined with certainty in our data. Sediment accumulation rates were calculated from the isotopic record of N. dutertrei by matching it with the age model SPECMAP curve. During the glacial periods sediment accumulation rates were higher than during the interglacial periods, reflecting increased input from the shelf during low-stands of sea level and increased eolian input. Periodograms for the past 524 k.y. on oxygen isotope records of N. dutertrei, G. ruber, and G. bulloides, on calcium carbonate content, magnetic susceptibility, and on a foraminiferal fragmentation record show powers matching the Milankovitch periodicities. High powers are concentrated around 103 k.y. In the spectra of oxygen isotope ratios of N. dutertrei, magnetic susceptibility, and foraminiferal fragmentation these are significant at the 80% confidence level with respect to a first order autoregressive model. Power concentrations near 43 k.y., matching obliquity, are present but subdued in all spectra. Power concentrations near 23 k.y., matching precession, are significant in the spectra of the oxygen isotope record of N. dutertrei, magnetic susceptibility, and calcium carbonate content record. Fragmentation of planktonic foraminifers increased during the interglacial periods. This is attributed to dissolution of the tests in an expanded oxygen minimum zone (OMZ), where undersaturation of calcium carbonate is caused by enhanced production in the euphotic zone, which would suggest stronger monsoonal induced upwelling during interglacial periods. Extension of the OMZ could also be increased by outflow of low oxygen marginal basin bottom water.

Planktic foraminiferal abundance and stable oxygen isotope ratios of sediment cores from the Southern California Borderlands

A variety of evidence suggests that average sea surface temperatures (SSTs) during the last glacial maximum in the California Borderlands region were significantly colder than during the Holocene. Planktonic foraminiferal delta18O evidence and average SST estimates derived by the modern analog technique indicate that temperatures were 6°-10°C cooler during the last glacial relative to the present. The glacial plankton assemblage is dominated by the planktonic foraminifer Neogloboquadrina pachyderma (sinistral coiling) and the coccolith Coccolithus pelagicus, both of which are currently restricted to subpolar regions of the North Pacific. The glacial-interglacial average SST change determined in this study is considerably larger than the 2°C change estimated by Climate: Long-Range Investigation, Mapping, and Prediction (CLIMAP) [1981]. We propose that a strengthened California Current flow was associated with the advance of subpolar surface waters into the Borderlands region during the last glacial.

Stable oxygen isotope ratios in zircons from oceanic crust

Lower ocean crust is primarily gabbroic, although 1-2% felsic igneous rocks that are referred to collectively as plagiogranites occur locally. Recent experimental evidence suggests that plagiogranite magmas can form by hydrous partial melting of gabbro triggered by seawater-derived fluids, and thus they may indicate early, high-temperature hydrothermal fluid circulation. To explore seawater-rock interaction prior to and during the genesis of plagiogranite and other late-stage magmas, oxygen-isotope ratios preserved in igneous zircon have been measured by ion microprobe. A total of 197 zircons from 43 plagiogranite, evolved gabbro, and hydrothermally altered fault rock samples have been analyzed. Samples originate primarily from drill core acquired during Ocean Drilling Program and Integrated Ocean Drilling Program operations near the Mid-Atlantic and Southwest Indian Ridges. With the exception of rare, distinctively luminescent rims, all zircons from ocean crust record remarkably uniform d18O with an average value of 5.2 ± 0.5 per mil (2SD). The average d18O(Zrc) would be in magmatic equilibrium with unaltered MORB [d18O(WR) ~5.6-5.7 per mil], and is consistent with the previously determined value for equilibrium with the mantle. The narrow range of measured d18O values is predicted for zircon crystallization from variable parent melt compositions and temperatures in a closed system, and provides no indication of any interactions between altered rocks or seawater and the evolved parent melts. If plagiogranite forms by hydrous partial melting, the uniform mantle-like d18O(Zrc) requires melting and zircon crystallization prior to significant amounts of water-rock interactions that alter the protolith d18O. Zircons from ocean crust have been proposed as a tectonic analog for >3.9 Ga detrital zircons from the earliest (Hadean) Earth by multiple workers. However, zircons from ocean crust are readily distinguished geochemically from zircons formed in continental crustal environments. Many of the >3.9 Ga zircons have mildly elevated d18O (6.0-7.5 per mil), but such values have not been identified in any zircons from the large sample suite examined here. The difference in d18O, in combination with newly acquired lithium concentrations and published trace element data, clearly shows that the >3.9 Ga detrital zircons did not originate by processes analogous to those in modern mid-ocean ridge settings.

Oxygen and carbon isotope data for benthic foraminifera from DSDP Site 94-608 and ODP Site 208-1264

Small biserial foraminifera were abundant in the early Miocene (ca. 18.9-17.2 Ma) in the eastern Atlantic and western Indian Oceans, but absent in the western equatorial Atlantic Ocean, Weddell Sea, eastern Indian Ocean, and equatorial Pacific Ocean. They have been assigned to the benthic genus Bolivina, but their high abundances in sediments without evidence for dysoxia could not be explained. Apertural morphology, accumulation rates, and isotopic composition show that they were planktic (genus Streptochilus). Living Streptochilus are common in productive waters with intermittent upwelling. The widespread early Miocene high Streptochilus abundances may reflect vigorous but intermittent upwelling, inducing high phytoplankton growth rates. However, export production (estimated from benthic foraminiferal accumulation rates) was low, possibly due to high regeneration rates in a deep thermocline. The upwelled waters may have been an analog to Subantarctic Mode Waters, carrying nutrients into the eastern Atlantic and western Indian Oceans as the result of the initiation of a deep-reaching Antarctic Circumpolar Current, active Agulhas Leakage, and vigorous vertical mixing in the Southern Oceans.