The alkali element and boron geochemistry of ODP Site 169-1038 in the Escanaba Trough, East Pacific

A suite of conjugate pore fluid and sediment samples were collected during Leg 169 of the ODP from within the clastic sedimentary sequences which host massive sulphides at Central Hill, Escanaba Trough (ODP Site 1038). We report the alkali element and boron, and Li and B isotope data for these samples. Relative to a reference site (Site 1037) located outside the zone of high heat flow, pore fluids from Site 1038 show a wide variation in Cl (300-800 mM), and have far higher concentrations of Li (up to 6.2 mM), B (up to 9.7 mM), Cs (up to 5.0 mM), and Rb (up to 97 mM). We show that the pore fluids are derived from hydrothermal circulation that has extended into the basement oceanic crust, with input of the alkali elements and B as the rising hydrothermal fluids interact geochemically with the overlying clastic sediments. There is, however, no marked depletion of these elements in the conjugate sediments, suggesting that there has been advective transport of fluids away from the primary hydrothermal reaction site. This is supported by modelling of the Li and B isotope systematics of the pore fluids, which shows that they record extensive formation of secondary minerals during cooling of the fluids from ~350 to ~20ºC. Precipitation of metal-rich sulphides would have occurred prior to the formation of these minerals, thus, the pore fluid Li and B isotope data can place important constraints on the locus of sulphide deposition beneath the seafloor at Escanaba.

Stable isotope record of planktonic foraminifera of the South Atlantic

The central problem of late Quaternary circulation in the South Atlantic is its role in transfer of heat to the North Atlantic, as this modifies amplitude, and perhaps phase, of glacialinterglacial fluctuations. Here we attempt to define the problem and establish ways to attack it. We identify several crucial elements in the dynamics of heat export: (1) warm-water pile-up (and lack thereof) in the Western equatorial Atlantic, (2) general spin-up (or spin-down) of central gyre, tied to SE trades, (3) opening and closing of Cape Valve (Agulhas retroflection), (4) deepwater E-W asymmetry. Means for reconstruction are biogeography, stable isotopes, and productivity proxies. Main results concern overall glacial-interglacial contrast (less pile-up, more spin-up, Cape Valve closed, less NADW during glacial time), dominance of precessional signal in tropics, phase shifts in precessional response. To generate working hypotheses about the dynamics of surface water circulation in the South Atlantic we employ Croll's paradigm that glacial - interglacial fluctuations are analogous to seasonal fluctuations. Our general picture for the last 300 kyrs is that, as concerns the South Atlantic, intensity of surface water (heat) transport depends on the strength of the SE trades. From various lines of evidence it appears that strenger SE trades appeared during glacials and cold substages during interglacials, analogous to conditions in southern winter (August).

Background heat flow values in the region of the Sea of Okhotsk

Particular features of tectonic structure and anomalous distribution of geothermal, geomagnetic, and gravity fields in the region of the Sea of Okhotsk are considered. On the basis of heat flow data, ages of large-scale structures in the Sea of Okhotsk are estimated at 65 Ma for the Central Okhotsk Rise and 36 Ma for the South Okhotsk Basin. Age of the South Okhotsk Basin is confirmed by data on kinematics and corresponds to 50 km thickness of the lithosphere. This is in accordance with thickness value obtained by magnetotelluric soundings. Comparative analysis of model geothermal background and measured heat flow values on the Akademii Nauk Rise is performed. Analysis points to abnormally high (~20%) measured heat flow agrees with high negative gradient of gravity anomalies. Estimates of deep heat flow and basement age of riftogenic basins in the Sea of Okhotsk were carried out in the following areas: Deryugin Basin (18 Ma, Early Miocene), TINRO Basin (12 Ma, Middle Miocene), and West Kamchatka Basin (23 Ma, Late Oligocene). Temperatures at boundaries of the main lithological complexes of the sedimentary cover are calculated and zones of oil and gas generation are defined. On the basis of geothermal, magnetic, structural, and other geological-geophysical data a kinematic model of the region of the Sea of Okhotsk for period of 36 Ma was calculated and constructed.