Geothermal measurements at ODP Leg 127 and 128 sites

Downhole temperature and thermal conductivity measurements in core samples recovered during Legs 127 and 128 in the Japan Sea resulted in five accurate determinations of heat flow through the seafloor and accurate estimates of temperature vs. depth over the drilled sections. The heat flows measured at these sites are in excellent agreement with nearby seafloor measurements. Drilling sampled basaltic rocks that form the acoustic basement in the Yamato and Japan basins and provided biostratigraphic and isotopic estimates of the age of these basins. The preliminary age estimates are compared with predicted heat flow values for two different thermal models of the lithosphere. A heat flow determination from the crest of the Okushiri Ridge yielded an anomalously high heat flow of 156 mW/m**2. This excessive heat flow value may have resulted from frictional heating on an active reverse fault that bounds the eastern side of the Ridge. Accurate estimates of sedimentation rates and temperatures in the sedimentary section combined with models of basin formation provide an opportunity to test thermochemical models of silica diagenesis. The current location of the opal-A/opal CT transition in the sedimentary section is determined primarily by the thermal history of the layer in which the transition is now found. Comparison of the ages and temperatures of the layer where the opal-A/opal-CT is found today is compatible with an activation energy of 14 to 17 kcal/mole.

Analysis and compound of carbon sequestration and serpentinization from the Iberian Margin and the Northern Apennine

Fluid circulation in peridotite-hosted hydrothermal systems influences the incorporation of carbon into the oceanic crust and its long-term storage. At low to moderate temperatures, serpentinization of peridotite produces alkaline fluids that are rich in CH4 and H2. Upon mixing with seawater, these fluids precipitate carbonate, forming an extensive network of calcite veins in the basement rocks, while H2 and CH4 serve as an energy source for microorganisms. Here, we analyzed the carbon geochemistry of two ancient peridotite-hosted hydrothermal systems: 1) ophiolites cropping out in the Northern Apennines, and 2) calcite-veined serpentinites from the Iberian Margin (Ocean Drilling Program (ODP) Legs 149 and 173), and compare them to active peridotite-hosted hydrothermal systems such as the Lost City hydrothermal field (LCHF) on the Atlantis Massif near the Mid-Atlantic Ridge (MAR). Our results show that large amounts of carbonate are formed during serpentinization of mantle rocks exposed on the seafloor (up to 9.6 wt.% C in ophicalcites) and that carbon incorporation decreases with depth. In the Northern Apennine serpentinites, serpentinization temperatures decrease from 240 °C to < 150 °C, while carbonates are formed at temperatures decreasing from ~ 150 °C to < 50 °C. At the Iberian Margin both carbonate formation and serpentinization temperatures are lower than in the Northern Apennines with serpentinization starting at ~ 150 °C, followed by clay alteration at < 100 °C and carbonate formation at < 19-44 °C. Comparison with various active peridotite-hosted hydrothermal systems on the MAR shows that the serpentinites from the Northern Apennines record a thermal evolution similar to that of the basement of the LCHF and that tectonic activity on the Jurassic seafloor, comparable to the present-day processes leading to oceanic core complexes, probably led to formation of fractures and faults, which promoted fluid circulation to greater depth and cooling of the mantle rocks. Thus, our study provides further evidence that the Northern Apennine serpentinites host a paleo-stockwork of a hydrothermal system similar to the basement of the LCHF. Furthermore, we argue that the extent of carbonate uptake is mainly controlled by the presence of fluid pathways. Low serpentinization temperatures promote microbial activity, which leads to enhanced biomass formation and the storage of organic carbon. Organic carbon becomes dominant with increasing depth and is the principal carbon phase at more than 50-100 m depth of the serpentinite basement at the Iberian Margin. We estimate that annually 1.1 to 2.7 × 1012 g C is stored within peridotites exposed to seawater, of which 30-40% is fixed within the uppermost 20-50 m mainly as carbonate. Additionally, we conclude that alteration of oceanic lithosphere is an important factor in the long-term global carbon cycle, having the potential to store carbon for millions of years.

Organic matter in sediments of ODP Site 131-808

Microscopic studies reveal a predominance of terrestrial organic matter in sediments of Site 808. Terrestrial vitrinite and inertinite are more abundant (73% to 100%) than marine organic matter (alginite, 0% to 27%), which increases from open oceanic deposits of the Shikoku Basin sediments to sediments of the outer trench wedge. The abundance of terrestrial organic matter is also reflected through carbon isotope values of -23 per mil to -25.9 per mil. Mass accumulation rates of organic carbon are low in hemipelagic sediments of the Shikoku Basin (<0.2 g/cm**2/k.y.) but increase significantly in sediments of the Nankai Trench (0.2 to 1.7 g/cm**2/k.y.). Although the organic mass accumulation is high in sediments of the Nankai Trench, a comparison of sedimentation rates and total organic carbon suggests relative dilution of organic carbon through turbidite flows. Calculated marine paleoproductivity of organic carbon is low in sediments of the open ocean (Shikoku Basin) and increases closer to the shore (Nankai Trench). Thermal evolution of organic matter is obtained from vitrinite reflectance measurements. Two populations of vitrinites have been observed between 600 and 1234 mbsf. Reflectance values change with increasing depth and temperature in both groups of vitrinite (0.3% to 0.68% in group 1; 0.6% to 1% in group 2).

Salinity estimation from Gulf of Guinea, sediment core MD03-2707

The isotopic composition of surface seawater is widely used to infer past changes in sea surface salinity using paired foraminiferal Mg/Ca and d18O from marine sediments. At low latitudes, paleosalinity reconstructions using this method have largely been used to document changes in the hydrological cycle. This method usually assumes that the modern seawater d18O (d18Osw)/salinity relationship remained constant through time. Modelling studies have shown that such assumptions may not be valid because large-scale atmospheric circulation patterns linked to global climate changes can alter the seawater d18Osw/salinity relationship locally. Such processes have not been evidenced by paleo-data so far because there is presently no way to reconstruct past changes in the seawater d18Osw/salinity relationship. We have addressed this issue by applying a multi-proxy salinity reconstruction from a marine sediment core collected in the Gulf of Guinea. We measured hydrogen isotopes in C37:2 alkenones (dDa) to estimate changes in seawater dD. We find a smooth, long-term increase of ~10 per mil in dDa between 10 and 3 kyr BP, followed by a rapid decrease of ~10 per mil in dDa between 3 kyr BP and core top to values slightly lighter than during the early Holocene. Those features are inconsistent with published salinity estimations based on d18Osw and foraminiferal Ba/Ca, as well as nearby continental rainfall history derived from pollen analysis. We combined dDa and d18Osw values to reconstruct a Holocene record of salinity and compared it to a Ba/Ca-derived salinity record from the same sedimentary sequence. This combined method provides salinity trends that are in better agreement with both the Ba/Ca-derived salinity and the regional precipitation changes as inferred from pollen records. Our results illustrate that changes in atmospheric circulation can trigger changes in precipitation isotopes in a counter-intuitive manner that ultimately impacts surface salinity estimates based on seawater isotopic values. Our data suggest that the trends in Holocene rainfall isotopic values at low latitudes may not uniquely result from changes in local precipitation associated with the amount effect.

Calcareous microfossil isotopes and Miocene datum events of ODP Holes 189-1170A and 189-1172A

Downcore oxygen and carbon stable isotope records of planktonic and benthic foraminifers and fine-fraction carbonate from the southern high latitudes provide critical paleohydrographic constraints on the evolution of the Southern Ocean climate. In particular, the potential effects of an intensified Antarctic Circumpolar Current on the thermal isolation and cooling of the southern high latitudes, production of cold deep waters, and, ultimately, accumulation of continental ice on Antarctica in the middle Miocene are matters of interest. Using sediment materials from Ocean Drilling Program Leg 189 Sites 1170 and 1172 off Tasmania, Ennyu and Arthur (2004, doi:10.1029/151GM13) established the surface- and deepwater stable isotope records in the Southern Ocean across the middle Miocene event of the east Antarctic ice sheet expansion and discussed the paleoclimate proxy records in terms of the thermal evolution of the southern high latitudes and its effect on deepwater circulation. This report provides data tables and other supporting information relevant to discussions presented in Ennyu and Arthur (2004, doi:10.1029/151GM13). Items included in this report are (1) the oxygen and carbon stable isotope data measured on the Miocene bulk fine-fraction (i.e., <63 µm, primarily polyspecific nannofossil assemblage) carbonate and planktonic and benthic foraminifers from Holes 1170A and 1172A and (2) the Miocene depth-age models for the two sites.

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.

SHRIMP U-Pb zircon data from Gjelsvikfjella, Dronning Maud Land, East Antarctica

The Maud Belt in Dronning Maud Land (western East Antarctic Craton) preserves a high-grade polyphase tectono-thermal history with two orogenic episodes of Mesoproterozoic (1.2-1.0 Ga) and Neoproterozoic (0.6-0.5 Ga) age. New SHRIMP U-Pb zircon data from southern Gjelsvikfjella in the northeastern part of the belt make it possible to differentiate between a series of magmatic and metamorphic events. The oldest event recorded is the formation of an extensive 1140-1130 Ma volcanic arc. This was followed by 1104 ± 8 Ma granitoids that might represent, together with so far undated mafic dykes, part of a decompression melting-related bimodal suite that reflects the sub-continental Umkondo igneous event. The first high-grade metamorphism is constrained at 1070 Ma. The metamorphic age data are similar to those obtained from other parts of the Maud Belt, but also from the Namaqua-Natal Belt in South Africa, but the preceding arc formation was diachronous in the two belts. This indicates that the two belts did not form a continuous volcanic arc unit as suggested in previous models, but became connected only at the end of the Mesoproterozoic. Intense reworking during the Neoproterozoic, probably as a result of continent-continent collision between components of Gondwana, is indicated by ductile refliation, further high-grade metamorphic recrystallisation and metamorphic zircon overgrowths at approximately 530 Ma. This was followed by late- to post-tectonic magmatism, reflected by 500 Ma granite bodies and 490 Ma aplite dykes as well as a 480 Ma gabbro body.

Porphyrin geochemistry of DSDP Hole 71-511

Late Jurassic-early Cretaceous black shales and an overlying sequence of Albian-Campanian zeolitic claystones from the Falkland Plateau (DSDP/IPOD Leg 71, Site 511) were analyzed for tetrapyrrole pigment type and abundance. The "black shale" sequence was found to be rich in DPEP-series dominated free-base, nickel (Ni) and, to a lesser extent, vanadyl (V = 0) porphyrins. A low level of organic maturity (i.e. precatagenesis) is indicated for these strata as nickel chelation by free-base porphyrins is only 50-75% complete, proceeding down-hole to 627 meters sub-bottom. Electronic and mass spectral data reveal that the proposed benzo-DPEP (BD) and tetrahydrobenzo-DPEP (THBD) series are present in the free-base and Ni species, as well as the more usual occurrence in V = 0 porphyrin arrays. Highly reducing conditions are suggested by an abundance of the PAH perylene, substantial amounts of the THBD/BD series and a redox equilibrium between free-base DPEP and 7,8-dihydro-DPEP series, which exist in a 7:1 molar ratio. The Albian-Campanian claystone strata were found to be tetrapyrrolepoor, and those pigments present were typed as Cu/Ni highly dealkylated (C26 max.) etioporphyrins, thought to be derived via redeposition and oxidation of terrestrial organic matter (OM). Results from the present study are correlated to our past analyses of Jurassic-Cretaceous sediments from Atlantic margins in an effort to relate tetrapyrrole quality and quantity to basin evolution and OM sources in the proto-Atlantic.