Seawater carbonate chemistry, primary production, biomass and calcification of plankton and bacteria, 2009

Production (abundance and biomass) and net calcification rates of the coccolithophorid Pleurochrysis carterae under different partial pressures of CO2 (pCO2) were examined using short (15, 24 and 39 h), long (7 d) and dark (7 d) incubation experiments. Short incubations were conducted at ambient, 500 and 820 ppm pCO2 levels in natural seawater that was enriched with nutrients and inoculated with P. carterae. Long incubations were conducted at ambient and 1200 ppm pCO2 levels in natural seawater (0.2 µm filtered as well as unfiltered) that was enriched with nutrients and inoculated with P. carterae. Dark incubations were conducted at ambient and 1200 ppm pCO2 in unfiltered seawater that was inoculated with P. carterae. The abundance and biomass of coccolithophorids increased with pCO2 and time. The abundance and biomass of most noncalcifying phytoplankton also increased, and were hardly affected by CO2 inputs. Net calcification rates were negative in short incubations during the pre-bloom phase regardless of pCO2 levels, indicating dissolution of calcium carbonate. Further, the negative values of net calcification in short incubations became less negative with time. Net calcification rates were positive in long incubations during blooms regardless of pCO2 level, and the rate of calcification increased with pCO2. Our results show that P. carterae may adapt to increased (~1200 ppm) pCO2 level with time, and such increase has little effect on the ecology of noncalcifying groups and hence in ecosystem dynamics. In dark incubations, net calcification rates were negative, with the magnitude being dependent on pCO2 levels.

Rubidium and Strontium chemistry of oceanic crust

Times of vein mineral deposition in the ocean crust have been determined both by Rb-Sr isochron ages of vein smectites and by comparison of 87Sr/86Sr ratios of vein calcites with the known variations of seawater 87Sr/86Sr ratio with time. Results from drilling sites 105, 332B and 418A, Atlantic Ocean, which have basement formation ages of 155 m.y., 3.5 m.y., and 110 m.y., respectively, show that vein deposition is essenrially complete within 5-10 m.y. after formation of the basaltic crust. This provids direct evidence that hydrothermal circulation of sea-water through the oceanic crust is an important process for only 5-10 m.y. after crust formation.

Seawater carbonate chemistry and calcification rate in flume experiment

Ocean acidification (OA) poses a severe threat to tropical coral reefs, yet much of what is know about these effects comes from individual corals and algae incubated in isolation under high pCO2. Studies of similar effects on coral reef communities are scarce. To investigate the response of coral reef communities to OA, we used large outdoor flumes in which communities composed of calcified algae, corals, and sediment were combined to match the percentage cover of benthic communities in the shallow back reef of Moorea, French Polynesia. Reef communities in the flumes were exposed to ambient (400 ?atm) and high pCO2 (1300 ?atm) for 8 weeks, and calcification rates measured for the constructed communities including the sediments. Community calcification was reduced by 59% under high pCO2, with sediment dissolution explaining ~ 50% of this decrease; net calcification of corals and calcified algae remained positive but was reduced by 29% under elevated pCO2. These results show that, despite the capacity of coral reef calcifiers to maintain positive net accretion of calcium carbonate under OA conditions, reef communities might transition to net dissolution as pCO2 increases, particularly at night, due to enhanced sediment dissolution.

Chemistry of biogenic sediments from the equatorial Pacific

To increase our understanding of the mechanisms that control the distribution of Al and Ti within marine sediment, we performed sequential extractions targeting the chemical signatures of the loosely bound, exchangeable, carbonate, oxide, organic, opal, and residual fraction of sediment from a carbonate-dominated regime (equatorial Pacific) and from a mixed opal-terrigenous regime (West Antarctic Peninsula). We observe a systematic partitioning of Al and Ti between sediment phases that is related to bulk Al/Ti. We show that, where we can quantify an Al(excess) component, the dissolved Al is preferentially affiliated with the oxide fraction, resulting in Al/Ti molar ratios of 500-3000. This is interpreted as the result of surface complexation in the water column of dissolved Al onto oxyhydroxides. We also observe a previously undetected Ti(excess) with as much as 80% of the total Ti in the organic fraction, which is most likely a function of metal-organic colloidal removal from the water column. In samples where the excess metals are obscured by the detrital load, the Al and Ti are almost exclusively found in the residual phase. This argues for the paired removal of Al (preferentially by the oxide component) and Ti (preferentially by the organic component) from the water column by settling particulate matter. This research builds upon earlier work that shows changes in the bulk ratio of Al to Ti in carbonate sediment from the central-equatorial Pacific that coincide with changes in the sedimentary bulk accumulation rate (BAR). The ratios that are observed are as much as three times higher than typical shale values, and were interpreted as the result of scavenging of dissolved Al onto particles settling in the water column. Because this non-terrigenous Al(excess) accounts for up to 50% of the total sedimentary Al inventory and correlates best with BAR, the bulk Al/Ti may be a sensitive tracer of particle flux and, therefore, export production. Because we show that the excess metals are the result of scavenging processes, the bulk Al/Ti may be considered a sensitive proxy for this region.

Chemical and isotopic compositions of rocks and minerals from the Palenyi Island, Por'ya Guba (Bay), White Sea

Hypersthene-garnet-sillimanite-quartz enclaves were studied in orthopyroxene-plagioclase and orthopyroxene-clinopyroxene crystalline schists and gneisses from shear zones exposed in the Palenyi Island within the Early Proterozoic Belomorian Mobile Belt. Qualitative analysis of mineral assemblages indicates that these rocks were metamorphosed to the granulite facies (approximately 900°C and 10-11 kbar). Oxygen isotopic composition was determined in rock-forming minerals composing zones of the enclaves of various mineral and chemical composition. Closure temperatures of the isotopic systems obtained by methods of oxygen isotopic thermometry are close to values obtained with mineralogical geothermometers (garnet-orthopyroxene and garnet-biotite) and correspond to the high-temperature granulite facies (860-900°C). Identified systematic variations in d18O values were determined in the same minerals from zones of different mineral composition. Inasmuch as these zones are practically in contact with one another, these variations in d18O cannot be explained by primary isotopic heterogeneity of the protolith. Model calculations of the extent and trend of d18O variations in minerals suggest that fluid-rock interaction at various integral fluid/rock ratios in discrete zones was the only mechanism that could generate the zoning. This demonstrates that focused fluid flux could occur in lower crustal shear zones. Preservation of high-temperature isotopic equilibria of minerals testifies that the episode of fluid activity at the peak of metamorphism was very brief.

Geochemistry from DSDP Holes 67-497 and 67-496

Mineralogical identification, glass chemistry, and instrumental neutron activation analyses of Quaternary volcanic ash layers from Leg 67 Holes 496, 497, and 499 are used to correlate the drill holes and on-land sources. We have identified two units at Hole 496 that correspond to the 23,000-yr.-old Pinos Altos ash (Samples 496-3-4, 55-57 cm and 496-3-5, 74-76 cm); the 84,000-yr.-old Los Chocoyos ash corresponds with Sample 496-5-4, 134-146 cm, but this latter correlation is less certain.

Interstitial-water chemistry and stable isotope record of Prydz Bay sediments

Leg 119 of the Ocean Drilling Program (ODP) provided the first opportunity to study the interstitial-water chemistry of the eastern Antarctic continental margin. Five sites were cored in a northwest-southeast transect of Prydz Bay that extended from the top of the continental slope to within 30 km of the coastline. Geological studies of the cores reveal a continental margin that has evolved through terrestrial, glacial, and glacial-marine environments. Chemical and stable isotopic analyses of the interstitial-waters were performed to determine the types of depositional environments and the diagenetic and hydrologic processes that are operating in this unusual marine environment. Highly compacted glacial sediments provide an effective barrier to the vertical diffusion of interstitial-water solutes. Meteoric water from the Antarctic continent appears to be flowing into Prydz Bay sediments through the sequence of terrestrial sediments that lie underneath the glacial sediments. The large amounts of erosion associated with glacial advances appear to have had the effect of limiting the amount of marine organic matter that is incorporated into the sediments on the continental shelf. Although all of the sites cored in Prydz Bay exhibit depletions in dissolved sulfate with increasing depth, the greatest bacterial activity is associated with a thin layer of diatom ooze that coats the seafloor of the inner bay. Results of alkalinity modeling, thermodynamic calculations, and strontium analyses indicate that (1) ocean bottom waters seaward of Site 740 are undersaturated with respect to both calcite and aragonite, (2) interstitial waters at each site become saturated or supersaturated with respect to calcite and aragonite with increasing depth, (3) precipitation of calcium carbonate reduces the alkalinity of the pore waters with increasing depth, and (4) recrystallization of aragonite to calcite accounts for 24% of the pore-water strontium. Weathering of unstable terrestrial debris and cation exchange between clay minerals and pore fluids are the most probable chemical processes affecting interstitial water cation gradients.

Chemistry of smectite and volcanic glass of ODP Site 159-959

The Ocean Drilling Program (ODP) Site 959 was drilled in the northern border of the Côte d'Ivoire-Ghana Ridge at a water depth of 2100 m. Pleistocene total thickness does not exceed 20 m. Winnowing processes resulted in a low accumulation rate and notable stratigraphic hiatuses. During the Late Pleistocene, bottom circulation was very active and controlled laminae deposition (contourites) which increased the concentration of glauconitic infillings of foraminifera, and of volcanic glass and blue-green grains more rarely, with one or several subordinate ferromagnesian silicates. Volcanic glass generally was X-ray amorphous and schematically classified as basic to intermediate (44-60% SiO2). Opal-A or opal-CT suggested the beginning of the palagonitisation process, and previous smectitic deposits may have been eroded mechanically. The blue-green grains presented two main types of mineralogic composition: (1) neoformed K, Fe-smectite associated with zeolite (like phillipsite) and unequal amounts of quartz and anorthite; (2) feldspathic grains dominated by albite but including quartz, volcanic glass and smectites as accessory components. They were more or less associated with the volcanic glass. On the basis of their chemical composition, the genetic relationship between the blue-green grains and the volcanic glass seemed to be obvious although some heterogeneous grains seemed to be primary ignimbrite and not the result of glass weathering. The most reasonable origin of these pyroclastic ejecta would be explosive events from the Cameroon Volcanic Ridge, especially from the Sao Thome and Principe Islands and Mount Cameroon area. This is supported both by grain geochemistry and the time of volcanic activity, i.e. Pleistocene. After westward wind transport (some 1200 km) and ash fall-out, the subsequent winnowing by bottom currents controlled the concentration of the volcanic grains previously disseminated inside the hemipelagic sediment. Palagonitisation, and especially phillipsite formation, may result from a relatively rapid reaction during burial diagenesis (<1 m.y.), in deep-sea deposits at relatively low sedimentation rate. However, it cannot be excluded that the weathering had begun widely on the Cameroon Ridge before the explosive event.