High resolution color reflectance and hematite and goethite record of ODP Hole 184-1143A in the South China Sea

Precipitation has a larger variability than temperature in tropical monsoon regions, thus it is an important climate variable. However, reconstructions of long-term rainfall histories are scarce because of the lack of reliable proxies. Here we document that iron oxide minerals, specifically the ratio of hematite to goethite (Hm/Gt), is a reasonable precipitation proxy. Using diffuse reflectance spectrophotometry, we measured samples from Ocean Drilling Program (ODP) 1143 drilling site (9°21.72'N, 113°17.11'E, 2777 m water depth) for hematite and goethite, whose formation processes are favored by opposing climate conditions. In order to determine the content of hematite and goethite we produced a set of calibration samples by removing the iron oxides to generate the natural matrix to which hematite and goethite in known percentages were added. From these calibration samples we developed a transfer function for determining hematite and goethite concentration from a sample's spectral reflectance. Applying this method to ODP 1143 sediments (top 34 m of a 510 m core with sampling interval of 10 cm) we were able to reconstruct a continuous precipitation history for SE Asia of the past 600 kyr using the Hm/Gt ratio as a proxy of the precipitation variability of Asian monsoon. The reliability of this Hm/Gt proxy is corroborated by its consistency with the stalagmite delta18O data from South China. Comparing long-term Hm/Gt records with the surface temperature gradient of equatorial Pacific Ocean, we found that monsoon precipitation and El Niño are correlated for the last 600 kyr. The development of El Niño-like conditions decreased SE Asia precipitation, whereas precipitation increases in response to La Niña intensification

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.

Chemistry of interstitial waters of ODP Site 166-718

Preliminary data are presented on dissolved heavy metals in interstitial water samples collected at Site 718 of Ocean Drilling Program Leg 118. The heavy metals at this site are divided into three groups: Group I (B, K, Mn, Ni, Pb, total Si, total P, V) behaves like Mg, which decrease with depth; Group II (Ba, Cu, Sr, Ti) behaves like Ca, which increases with depth; and Group 111 (Cd, Co, Cr, Fe, Na, Mo, Zn) contains metals that are independent of depth. Mg decreases with depth from 50 mM at the seafloor to 21 mM at 900 mbsf. Mn in the sulfate reduction zone (1.0 to 2.8 ppm) is more highly concentrated than in the methane fermentation zone (0.23 to 0.50 ppm), except for Section 116-718-1H-1. A similar behavior is also observed for V and Pb. Ni, B, and K decrease non-uniformly with depth. Ca and Sr increase with depth at the same rates, indicating the dissolution of inorganic calcium carbonate by anaerobic oxidation of organic matter (Sayles, 1981, doi:10.1016/0016-7037(81)90132-0). The distribution of Ba with depth is very similar to those of Ca and Sr. Cu and Ti profiles trend to increase non-uniformly with depth. Fe is constant with depth. The sharp decrease in total silicate concentration at the seafloor probably indicates a decrease in the decomposition of siliceous biological matter (e.g., diatoms) and production of opal. The constant levels of Group 111, except for Na and Fe, may reveal equal sources of supply from surface seawater and the Himalayas over time.

Stable carbon isotopic composition of biomarker lipids of DSDP Hole 31-302

The middle Paleocene through early Eocene long-term gradual warming was superimposed by several transient warming events, such as the Paleocene-Eocene Thermal Maximum (PETM) and Eocene Thermal Maximum 2 (ETM2). Both events show evidence for extreme global warming associated with a major injection of carbon into the ocean-atmosphere system, but the mechanisms of carbon injection and many aspects of the environmental response are still poorly understood. In this study, we analyzed the concentration and stable carbon isotopic (d13C) composition of several sulfur-bound biomarkers derived from marine photoautotrophs, deposited in the Arctic Ocean at ~85°N, during ETM2. The presence of sulfur-bound biomarkers across this event points toward high primary productivity and anoxic bottom water conditions. The previously reported presence of isorenieratene derivatives indicates euxinic conditions in the photic zone, likely caused by a combination of enhanced primary productivity and salinity stratification. The negative carbon isotope excursion measured at the onset of ETM2 for several biomarkers, ranges between 3 per mil and 4.5 per mil, much larger than the ~1.4 per mil recorded in marine carbonates elsewhere, suggesting substantial enhanced isotopic fractionation by the primary producers likely due to a significant rise in pCO2. In the absence of biogenic carbonates in the ETM2 section of our core we use coeval planktonic d13C from elsewhere to estimate surface water d13C in the Arctic Ocean and then apply the relation between isotopic fractionation and pCO2, originally calibrated for haptophyte alkenones, to three selected organic biomarkers (i.e., S-bound phytane, C35 hopane, and a C25 highly branched isoprenoid). This yields pCO2 values potentially in the range of four times preindustrial levels. However, these estimates are uncertain because of a lack of knowledge on the importance of pCO2 on photosynthetic isotopic fractionation.

The occurrence and significance of Pleistocene and Upper Pliocene sapropels

Numerous sapropels and sapropelic strata from Upper Pliocene and Pleistocene hemipelagic sediments of the Tyrrhenian Sea show that intermittent anoxia, possibly related to strongly increased biological productivity, was not restricted to the eastern Mediterranean basins and may be a basin-wide result of Late Pliocene-Pleistocene climatic variability. Even though the sapropel assemblage of the Tyrrhenian Sea clearly originates from multiple processes such as deposition under anoxic conditions or during spikes in surface water productivity and lateral transport of organic-rich suspensates, many "pelagic sapropels" have been recognized. Stratigraphic ages calculated for the organic-rich strata recovered during ODP Leg 107 indicate that the frequency of sapropel formation increased from the lowermost Pleistocene to the base of the Jaramillo magnetic event, coinciding with a period when stable isotope records of planktonic foraminifera indicate the onset of climatic cooling in the Mediterranean. A second, very pronounced peak in sapropel formation occurred in the Middle to Late Pleistocene (0.73-0.26 Ma). Formainifers studied in three high-resolution sample sets suggest that changes in surface-water temperature may have been responsible for establishing anoxic conditions, while salinity differences were not noted in the faunal assemblage. However, comparison of sapropel occurrence at Site 653 with the oxygen isotopic record of planktonic foraminifers established by Thunell et al. (1990, doi:10.2973/odp.proc.sr.107.155.1990) indicates that sapropel occurrences coincide with negative d18O excursions in planktonic foraminifers in thirteen of eighteen sapropels recognized in Hole 653A. A variant of the meltwater hypothesis accepted for sapropel formation in the Late Pleistocene eastern Mediterranean may thus be the cause of several "anoxic events" in the Tyrrhenian as well. Model calculations indicate that the amount of oxygen advection from Western Mediterranean Deep Water exerts the dominant control on the oxygen content in deep water of the Tyrrhenian Sea. Inhibition of deep-water formation in the northern Adriatic and the Balearic Basin by increased meltwater discharge and changing storm patterns during climatic amelioration may thus be responsible for sapropel formation in the Tyrrhenian Sea.

Carbon and deuterium isotopes and C ratios in gas hydrates of ODP Leg 204 sites

Sediments at the southern summit of Hydrate Ridge display two distinct modes of gas hydrate occurrence. The dominant mode is associated with active venting of gas exsolved from the accretionary prism and leads to high concentrations (15%-40% of pore space) of gas hydrate in seafloor or near-surface sediments at and around the topographic summit of southern Hydrate Ridge. These near-surface gas hydrates are mainly composed of previously buried microbial methane but also contain a significant (10%-15%) component of thermogenic hydrocarbons and are overprinted with microbial methane currently being generated in shallow sediments. Focused migration pathways with high gas saturation (>65%) abutting the base of gas hydrate stability create phase equilibrium conditions that permit the flow of a gas phase through the gas hydrate stability zone. Gas seepage at the summit supports rapid growth of gas hydrates and vigorous anaerobic methane oxidation. The other mode of gas hydrate occurs in slope basins and on the saddle north of the southern summit and consists of lower average concentrations (0.5%-5%) at greater depths (30-200 meters below seafloor [mbsf]) resulting from the buildup of in situ-generated dissolved microbial methane that reaches saturation levels with respect to gas hydrate stability at 30-50 mbsf. Net rates of sulfate reduction in the slope basin and ridge saddle sites estimated from curve fitting of concentration gradients are 2-4 mmol/m**3/yr, and integrated net rates are 20-50 mmol/m**2/yr. Modeled microbial methane production rates are initially 1.5 mmol/m**3/yr in sediments just beneath the sulfate reduction zone but rapidly decrease to rates of <0.1 mmol/m**3/yr at depths >100 mbsf. Integrated net rates of methane production in sediments away from the southern summit of Hydrate Ridge are 25-80 mmol/m**2/yr. Anaerobic methane oxidation is minor or absent in cored sediments away from the summit of southern Hydrate Ridge. Ethane-enriched Structure I gas hydrate solids are buried more rapidly than ethane-depleted dissolved gas in the pore water because of advection from compaction. With subsidence beneath the gas hydrate stability zone, the ethane (mainly of low-temperature thermogenic origin) is released back to the dissolved gas-free gas phases and produces a discontinuous decrease in the C1/C2 vs. depth trend. These ethane fractionation effects may be useful to recognize and estimate levels of gas hydrate occurrence in marine sediments.

(Table 1) Composite surface scraping samples and Coscinodiscus nodulifer abundance from DSDP Hole 64-480

Analysis of individual laminae in Recent sediments from the slopes of the Guaymas Basin indicates an increased abundance of the larger sizes (>60 µm) of Coscinodiscus nodulifer when upwelling conditions persist over the depositional site and an increase of the smaller sizes (<60 µm) when nonupwelling conditions exist. The size distributions of C. nodulifer in 10-cm composite samples from Hole 480 show a greater abundance of the large sizes (>60 µm) associated with well-laminated sections and more of the smaller sizes (<60 µm) in samples from the homogeneous sections. We have tentatively correlated the pronounced homogeneous zones with late Pleistocene glacial periods. These results indicate a persistence of nonupwelling conditions over Hole 480 during glacial periods (homogeneous zones) and a predominance of upwelling conditions during interglacial periods (laminated zones). Other evidence from diatom and silicoflagellate floral analysis seems to support these preliminary conclusions.

Helium-3 in different carriers from pelagic clays of DSDP Hole 91-596B and Core LR-44-GPC-3

To better understand the composition, characteristics of helium diffusion, and size distribution of interplanetary dust particles (IDPs) responsible for the long-term retention of extraterrestrial 3He, we carried out leaching, stepped heating, and sieving experiments on pelagic clays that varied in age from 0.5 Ma to ~90 Myr. The leaching experiments suggest that the host phase(s) of 3He in geologically old sediments are neither organic matter nor refractory phases, such as diamond, graphite, Al2O3, and SiC, but are consistent with extraterrestrial silicates, Fe-Ni sulfides, and possibly magnetite. Stepped heating experiments demonstrate that the 3He release profiles from the magnetic and non-magnetic components of the pelagic clays are remarkably similar. Because helium diffusion is likely to be controlled by mineral chemistry and structure, the stepped heating results suggest a single carrier that may be magnetite, or more probably a phase associated with magnetite. Furthermore, the stepped outgassing experiments indicate that about 20% of the 3He will be lost through diffusion at seafloor temperatures after 50 Myrs, while sedimentary rocks exposed on the Earth's surface for the same amount of time would lose up to 60%. The absolute magnitude of the 3He loss is, however, likely to depend upon the 3He concentration profile within the IDPs, which is not well known. Contrary to previous suggestions that micrometeorites in the size range of 50-100 µm in diameter are responsible for the extraterrestrial 3He in geologically old sediments [Stuart, F.M., Harrop, P.J., Knott, S., Turner, G., 1999. Laser extraction of helium isotopes from Antarctic micrometeorites: source of He and implications for the flux of extraterrestrial 3He flux to earth. Geochimica et Cosmochimica Acta, 63, 2653-2665, doi:10.1016/S0016-7037(99)00161-1], our sieving experiment demonstrates that at most 20% of the 3He is carried by particles greater than 50 µm in diameter. The size-distribution of the 3He-bearing particles implies that extraterrestrial 3He in sediments record the IDP flux rather than the micrometeorite flux.

Planktonic foraminifera Mg/Ca paleotemperatures and oxygen isotopes of ODP Hole 175-1075C

High resolution planktonic foraminifera Mg/Ca paleotemperatures and oxygen isotopes of seawater of Ocean Drilling Program (ODP) Site 1078 (off Angola) have been reconstructed and reveal insights into the seasonal thermal evolution of the Angola Current (AC), the Angola-Benguela Front (ABF), and the Benguela Current (BC) during the last glacial (50-23.5 ka BP). Special emphasis is put on time intervals possibly associated with the North Atlantic Heinrich Stadials (HS), which are thought to lead to an accumulation of heat in the South Atlantic due to a reduction of the Atlantic Meridional Overturning Circulation (AMOC). Within dating uncertainties, Globigerinoides ruber (pink) Mg/Ca-based sea surface temperature (SST) estimates that represent southern hemisphere summer surface conditions show several warming episodes that coincide with North Atlantic HS, thus supporting the concept of the bipolar thermal seesaw. In contrast, the Mg/Ca-based temperatures of Globigerina bulloides, representing the SST of the ABF/BC system during southern hemisphere winter, show no obvious response to the North Atlantic HS in the study area. We suggest that surface water cooling during the winter season is due to enhanced upwelling or upwelling of colder water masses which has most likely mitigated a warming of the ABF/BC system during HS. We further speculate that the seasonal asymmetry in our SST record results from seasonal differences in the dominance of atmospheric and oceanic teleconnections during periods of northern high latitude cooling.

Biosiliceous sedimentation at ODP Site 128-798

Upper Pliocene through Holocene sediments recovered at Site 798 in the Japan Sea (Oki Ridge) exhibit rhythmic variation in weight percent biogenic opal at intervals of ~5 m and periods equivalent to the 41-k.y. obliquity cycle. Variance at 17 and 100 k.y. is observed prior to 1.3 Ma. These cycles are also clearly defined by log data and correspond to clusters of decimeter-scale dark-colored sediment units alternating with clusters of light-colored units. Opal content varies between 3% and 22% between 0 and 1.3 Ma and from 3% to 43% between 1.3 and 2.6 Ma. Long-term opal accumulation rates average 1.8 g/cm**2/k.y. in the late Pliocene/early Pleistocene and decrease by about 60% at ~1.3 Ma. Rough calculations suggest that opal accumulation rates increased and terrigenous flux decreased during the Holocene relative to the last glacial period. Our age control is not yet sufficient to allow a similar analysis of the 41-k.y. cyclicity in opal content throughout the Pleistocene. Stable isotope results from planktonic foraminifers confirm previous suggestions of a strong surface-water freshening event during isotope stage 2; however, this episode appears to be unique during the Pleistocene. Benthic foraminifers are depleted in 18O during parts of glacial stages 2 and 6 relative to adjacent interglacials, suggesting unusual warming and/or freshening of deep waters. Collectively, the stable isotope and %opal data are consistent with continuing isolation of the Japan Sea during the Quaternary with important transitions occurring at 1.3, 0.7 to 1.0, and 0.2 to 0.3 Ma. Complex relationships among the stable isotope results, %opal data, and sediment characteristics such as color and organic and inorganic carbon content preclude development of a simple model to explain cyclical sedimentation. Opal maxima occur within both light and dark intervals and the processes that control surface-water productivity are at times decoupled from the factors that regulate deep-water dysaerobia. We suggest that water column overturn is controlled largely by regional atmospheric circulation that must also have an as yet poorly understood effect on surface-water fertility.