Vein mineral ages of oceanic crust from the Atlantic Ocean

Vein smectites with large Rb/Sr enrichments from extensively altered basaltic oceanic crust in Deep Sea Drilling Project hole 417A in the western Atlantic define a highly constrained Rb/Sr isochron age of 108 +/- 3 m.y. This age is identical to a less well constrained age of 108 +/- 17 m.y. for vein smectites with lower Rb/Sr enrichments from adjacent hole 418A and to the 108 m.y. age of crust formation derived by paleontological and magnetic anomaly correlation. Reasonable agreement exists between the 87Sr/86Sr ratio of vein calcites from both sites and the seawater 87Sr/86Sr ratio at the time. Pervasive low-temperature alteration in the contrasting environments of sites 417 and 418 appears to be coeval and essentially coincident with basement formation. Alteration may be used to advantage in determining ages of old oceanic crust.

Temperature reconstruction of sediment cores from the northeastern Arabian Sea

In order to reconstruct the monsoonal variability during the late Holocene we investigated a complete, annually laminated sediment record from the oxygen minimum zone (OMZ) off Pakistan for oxygen isotopes of planktic foraminifera and alkenone-derived sea surface temperatures (SST). Significant SST changes of up to 3°C which cannot be explained by changes in the alkenone-producing coccolithophorid species (inferred from the Gephyrocapsa oceanica / Emiliania huxleyi ratio) suggest that SST changes are driven by changes in the monsoon strength. Our high-(decadal)-resolution data indicate that the late Holocene in the northeastern Arabian Sea was not characterized by a stable uniform climate, as inferred from the Greenland ice cores, but by variations in the dominance of the SW monsoon conditions with significant effects on temperatures. Highest SST fluctuations of up to 3.0°C and 2.5°C were observed for the time interval from 4600 to 3300 years B.P. and during the past 500 years. The significant, short-term SST changes during the past 500 years might be related to climatic instabilities known from the northern latitudes ("Little Ice Age") and confirm global effects. Surface salinity values, reconstructed from delta18O records after correction for temperature-related oxygen isotope fractionation, suggest that in general, the past 5000 years were characterized by higher-than-recent evaporation and more intense SW monsoon conditions. However, between 4600 and 3700 years B.P., evaporation dropped, SW monsoon weakened, and NE monsoon conditions were comparatively enhanced. For the past 1500 years we infer strongly fluctuating monsoon conditions. Comparisons of reconstructed salinity records with ice accumulation data from published Tibetan ice core and Tibetan tree ring width data reveal that during the past 2000 years, enhanced evaporation in the northeastern Arabian Sea correlates with periods of increased ice accumulation in Tibet, and vice versa. This suggests a strong climatic relationship between both monsoon-controlled areas.

Paleoclimate reconstructions from sediment core MD01-2378

On the basis of the radiocarbon (14C) plateau-tuning method a new age model for Timor Sea Core MD01-2378 was established. It revealed a precise centennial-scale phasing of climate events in the ocean, cryo-, and atmosphere during the last deglacial and provides important new insights into causal linkages controlling events of global climate change. At Site MD01-2378 reservoir ages of surface waters dropped from 1600 yr prior to 20 cal ka to 250-500 yr after 18.8 cal ka. This evidence was crucial for generating a high-resolution age model for deglacial events in the Indo-Pacific Warm Pool. Sea-surface temperatures (SST) started to change near 18.8 cal ka, that is ~500 yr after the start of, presumably northern hemispheric, deglacial melt and sea level rise as shown by the benthic foraminiferal oxygen isotope ratio (d18O). However, the SST rise occurred 500-1000 yr prior to the onset of deglacial Antarctic warming and the first major rise in atmospheric carbon dioxide at about 18 ka. The increase in SST may partly reflect reduced seasonal upwelling of cold subsurface waters along the eastern margin of the Indian Ocean, which is reflected by a doubling of the thermal gradient between the sea surface and the thermocline, a halving of chlorin productivity from 19 to 18.5 cal ka, and in particular, by the strong decrease in surface water reservoir ages. Two significant increases in deglacial Timor Sea surface salinities from 19-18.5 and 15.5-14.5 cal ka, may partly reflect the deglacial increase in the distance of local river mouths, partly an inter-hemispheric millennial-scale see-saw in tropical monsoon intensity, possibly linked to a deglacial increase in the dominance of Pacific El Niño regimes over Heinrich stadial 1.

Reconstruction of sea surface temperature and salinity of the Arabian Sea

High-frequency suborbital variations (Dansgaard-Oeschger cycles) characterize the climatic history of the Northern Hemisphere as observed in Greenland ice cores, deep-sea sediments of the North Atlantic, the Californian borderland, the Arabian Sea, the South China Sea, and the Chinese loess area. Paleoceanographic data from core KL126 from the Bay of Bengal in combination with data from the other Asian monsoonal areas indicate that the feedback processes involving snow and dust of the Tibetan Plateau vary the summer monsoon capacity to transport moisture into central South Asia and into the atmosphere. We postulate that the summer monsoon initiates, amplifies, and terminates these cycles in the Northern Hemisphere.

Contents of organic compounds in sediments from DSDP Legs 1 and 4

Fifteen sediment samples were studied from five drill sites recovered by the Glomar Challenger on Legs I and IV in the Gulf of Mexico and western Atlantic. This study concentrated on compounds derived from biogenic precursors, namely: (1) hydrocarbons, (2) fatty acids, (3) pigments and (4) amino acids. Carbon isotope (dC13) data [values <(-26)?, relative to PDB], long-chain n-alkyl hydrocarbons (>>C27) with odd carbon numbered molecules dominating even carbon numbered species, and presence of perylene proved useful as possible indicators for terrigenous contributions to the organic matter in some samples. Apparently land-derived organic matter can be transported for distances over 1000 km into the ocean and their source still recognized. The study was primarily designed to investigate: (i) the sources of the organic matter present in the sediment, (ii) their stability with time of accumulation and (iii) the conditions necessary for in situ formation of new compounds.

Strontium isotopic data for calcareous ooze, chalk, and pore waters from DSDP Hole 590B

A detailed study of strontium isotope variations in Neogene marine carbonate sediments from Deep Sea Drilling Project Site 590B, using techniques that allow the 87Sr/86Sr ratio to be determined to better than +/-0.00001, gives a high-resolution record of the Sr isotopic evolution of seawater. The data show that the rate of change of the marine 87Sr/86Sr ratio has varied significantly even on time scales as short as 1 m.y. Periods of particularly rapid growth appear to follow major marine regressions and probably reflect an increase in the delivery of radiogenic Sr from the continents coupled with a decreased submarine carbonate dissolution rate (greater carbonate compensation depth). Periods of relatively slowly changing 87Sr/86Sr follow major marine transgressions. On the basis of correlations with the marine oxygen isotope record and the times of major continental glacier growth, it is inferred that the effects of sea-level variations are modified by climatic factors that affect the intensity of continental weathering and runoff. The effects of sea-floor generation rate variations are not discernible for the Neogene. The maximum attainable stratigraphic resolution using Sr isotopes is between 0.1 and 2 m.y. for this time period.