Composition of pore water and diagenetic carbonates of ODP Site 123-765

Degradation of organic matter in slightly organic-rich (1 wt% organic carbon) Neogene calcareous turbidites of the Argo Basin at Site 765 by sulfate reduction results in pore-water phosphate, ammonium, manganese, and carbonate alkalinity maxima. Pore-water calcium and magnesium decrease in the uppermost 100 meters below seafloor (mbsf) in response to the precipitation of calcian dolomite with an average composition of Ca1.15Mg0.83Fe0.02(CO3)2. Clear, euhedral dolomite rhombs range from <1 to 40 µm in diameter and occur in trace to minor amounts (<1-2 wt%) in Pleistocene to Pliocene sediment (62-210 mbsf) The abundance of dolomite increases markedly (2-10 wt%) in Miocene sediment (210-440 mbsf). The dolomite is associated with diagenetic sepiolite and palygorskite, as well as redeposited biogenic low-Mg calcite and aragonitic benthic foraminifers. Currently, dolomite is precipitating at depth within the pore spaces of the sediment, largely as a result of aragonite dissolution. The rate of aragonite dissolution, calculated from the pore-water strontium profile, is sufficient to explain the amount of dolomite observed at Site 765. A foraminiferal aragonite precursor is further supported by the carbon and oxygen isotopic compositions of the dolomite, which are fairly close to the range of isotopic compositions observed for Miocene benthic foraminifers. Dolomite precipitation is promoted by the degradation of organic matter by sulfate-reducing bacteria because the lower pore-water sulfate concentration reduces the effect of sulfate inhibition on the dolomite reaction and because the higher carbonate alkalinity increases the degree of saturation of the pore waters with dolomite. Organic matter degradation also results in the precipitation of pyrite and trace amounts of apatite (francolite), and the release of iron and manganese to the pore water by reduction of Fe and Mn oxides. Spherical, silt-sized aggregates of microcrystalline calcian rhodochrosite occur in trace to minor amounts in Lower Cretaceous sediment from 740 to 900 mbsf at Site 765. A negative carbon isotopic composition suggests that the rhodochrosite formed early in the sulfate reduction zone, but a depleted oxygen isotopic composition suggests that the rhodochrosite may have recrystallized at deeper burial depths.

Sea surface temperature estimation for MIS 16 of IODP Site 306-U1313

Hudson Strait (HS) Heinrich Events, ice-rafting events in the North Atlantic originating from the Laurentide ice sheet (LIS), are among the most dramatic examples of millennial-scale climate variability and have a large influence on global climate. However, it is debated as to whether the occurrence of HS Heinrich Events in the (eastern) North Atlantic in the geological record depends on greater ice discharge, or simply from the longer survival of icebergs in cold waters. Using sediments from Integrated Ocean Drilling Program (IODP) Site U1313 in the North Atlantic spanning the period between 960 and 320 ka, we show that sea surface temperatures (SSTs) did not control the first occurrence of HS Heinrich(-like) Events in the sedimentary record. Using mineralogy and organic geochemistry to determine the characteristics of ice-rafting debris (IRD), we detect the first HS Heinrich(-like) Event in our record around 643 ka (Marine Isotope Stage (MIS) 16), which is similar as previously reported for Site U1308. However, the accompanying high-resolution alkenone-based SST record demonstrates that the first HS Heinrich(-like) Event did not coincide with low SSTs. Thus, the HS Heinrich(-like) Events do indicate enhanced ice discharge from the LIS at the end of the Mid-Pleistocene Transition, not simply the survivability of icebergs due to cold conditions in the North Atlantic.

Isotopic analyses of DSDP Hole 45-395A samples from veins containing gypsum (Table 1)

Gypsum and halite crystals, together with saponite and phillipsite, were found in a vein in a basalt sill 625 m below the sea floor at DSDP Site 395A, located 190 km west of the crest of the Mid-Atlantic Ridge. The delta34S value of the gypsum (+19.4?) indicates a seawater source for the sulfate. The delta18O values of the saponite (+19.9?) and phillipsite (+18.1?) indicate either formation from normal seawater at about 55°C or formation from delta18O-depleted seawater at a lower temperature. The gypsum (which could be secondary after anhydrite) was formed by reaction between Ca[2+] released from basalt and SO4[2-] in circulating seawater. The halite could have formed when water was consumed by hydration of basalt under conditions of extremely restricted circulation. A more probable mechanism is that the gypsum was originally precipitated as anhydrite at temperatures above 60°C. As the temperature dropped the anhydrite converted to gypsum. The conversion would consume water, which could cause halite precipitation, and would cause an increase in the volume of solids, which would plug the vein and prevent subsequent dissolution of the halite.