Photosynthetically active radiation during the experiment, and lipid content of sub- and intertidal specimens of the endemic Antarctic red alga, Palmaria decipiens

In coastal waters, Antarctic rhodophytes are exposed to harsh environmental conditions throughout the year, like low water temperatures ranging from -1.8°C to 2°C and high light during the summer season. Photosynthetic performance under these conditions may be affected by slowed down enzymatic reactions and the increased generation of reactive oxygen species. The consequence might be a chronic photoinhibition of photosynthetic primary reactions related to increased fragmentation of the D1 reaction centre protein in photosystem II. It is hypothesized that changes in lipid composition of biomembranes may represent an adaptive trait to maintain D1 turnover in response to temperature variation. The interactive effects of high light and low temperature were studied on an endemic Antarctic red alga, Palmaria decipiens, sampled from two shore levels, intertidal and subtidal, and exposed to mesocosm experiments using two levels of natural solar radiation and two different temperature regimes (2-5°C and 5-10°C). During the experimental period of 23 days, maximum quantum yield of photosynthesis decreased in all treatments, with the intertidal specimens exposed at 5-10°C being most affected. On the pigment level, a decreasing ratio of phycobiliproteins to chlorophyll a was found in all treatments. A pronounced decrease in D1 protein concentration occurred in subtidal specimens exposed at 2-5°C. Marked changes in lipid composition, i.e. the ratio of saturated to unsaturated fatty acids, indicated an effective response of specimens to temperature change. Results provide new insights into mechanisms of stress adaptation in this key species of shallow Antarctic benthic communities.

Whole-rock geochemical analyses of serpentinized harzburgites from ODP Site 209-1270

Small-scale shear zones are present in drillcore samples of abyssal peridotites from the Mid-Atlantic ridge at 15°20'N (Ocean Drilling Program Leg 209). The shear zones act as pathways for both evolved melts and hydrothermal fluids. We examined serpentinites directly adjacent to such zones to evaluate chemical changes resulting from melt-rock and fluid-rock interaction and their influence on the mineralogy. Compared to fresh harzburgite and melt-unaffected serpentinites, serpentinites adjacent to melt-bearing veins show a marked enrichment in rare earth elements (REE), strontium and high field strength elements (HFSE) zirconium and niobium. From comparison with published chemical data of variably serpentinized and melt-unaffected harzburgites, one possible interpretation is that interaction with the adjacent melt veins caused the enrichment in HFSE, whereas the REE contents might also be enriched due to hydrothermal processes. Enrichment in alumina during serpentinization is corroborated by reaction path models for interaction of seawater with harzburgite-plagiogranite mixtures. These models explain both increased amounts of alumina in the serpentinizing fluid for increasing amounts of plagiogranitic material mixed with harzburgite, and the absence of brucite from the secondary mineralogy due to elevated silica activity. By destabilizing brucite, nearby melt veins might fundamentally influence the low-temperature alteration behaviour of serpentinites. Although observations and model results are in general agreement, due to absence of any unaltered protolith a quantification of element transport during serpentinization is not straightforward.

Oxygen and sulfur isotopes of peridotite and gabbro from the Mid-Atlantic Ridge

Whole rock sulfur and oxygen isotope compositions of altered peridotites and gabbros from near the 15°20'N Fracture Zone on the Mid-Atlantic Ridge were analyzed to investigate hydrothermal alteration processes and test for a subsurface biosphere in oceanic basement. Three processes are identified. (1) High-temperature hydrothermal alteration (~250-350°C) at Sites 1268 and 1271 is characterized by 18O depletion (2.6-4.4 per mil), elevated sulfide-S, and high delta34S (up to ~2 wt% and 4.4-10.8 per mil). Fluids were derived from high-temperature (>350°C) reaction of seawater with gabbro at depth. These cores contain gabbroic rocks, suggesting that associated heat may influence serpentinization. (2) Low-temperature (<150°C) serpentinization at Sites 1272 and 1274 is characterized by elevated delta18O (up to 8.1 per mil), high sulfide-S (up to ~3000 ppm), and negative delta34S (to -32.1 per mil) that reflect microbial reduction of seawater sulfate. These holes penetrate faults at depth, suggesting links between faulting and temperatures of serpentinization. (3) Late low-temperature oxidation of sulfide minerals caused loss of sulfur from rocks close to the seafloor. Sulfate at all sites contains a component of oxidized sulfide minerals. Low delta34S of sulfate may result from kinetic isotope fractionation during oxidation or may indicate readily oxidized low-delta34S sulfide derived from microbial sulfate reduction. Results show that peridotite alteration may be commonly affected by fluids +/- heat derived from mafic intrusions and that microbial sulfate reduction is widespread in mantle exposed at the seafloor.

Magnesium isotope ratios of pore-fluids and dolomites

Magnesium concentrations in deep-sea sediment pore-fluids typically decrease down core due to net precipitation of dolomite or clay minerals in the sediments or underlying crust. To better characterize and differentiate these processes, we have measured magnesium isotopes in pore-fluids and sediment samples from Ocean Drilling Program sites (1082, 1086, 1012, 984, 1219, and 925) that span a range of oceanographic settings. At all sites, magnesium concentrations decrease with depth. At sites where diagenetic reactions are dominated by the respiration of organic carbon, pore-fluid d26Mg values increase with depth by as much as 2 per mil. Because carbonates preferentially incorporate 24Mg (low d26Mg), the increase in pore-fluid d26Mg values at these sites is consistent with the removal of magnesium in Mg-carbonate (dolomite). In contrast, at sites where the respiration of organic carbon is not important and/or weatherable minerals are abundant, pore-fluid d26Mg values decrease with depth by up to 2 per mil. The decline in pore-fluid d26Mg at these sites is consistent with a magnesium sink that is isotopically enriched relative to the pore-fluid. The identity of this enriched magnesium sink is likely clay minerals. Using a simple 1D diffusion-advection-reaction model of pore-fluid magnesium, we estimate rates of net magnesium uptake/removal and associated net magnesium isotope fractionation factors for sources and sinks at all sites. Independent estimates of magnesium isotope fractionation during dolomite precipitation from measured d26Mg values of dolomite samples from sites 1082 and 1012 are very similar to modeled net fractionation factors at these sites, suggesting that local exchange of magnesium between sediment and pore-fluid at these sites can be neglected. Our results indicate that the magnesium incorporated in dolomite is 2.0-2.7 per mil depleted in d26Mg relative to the precipitating fluid. Assuming local exchange of magnesium is minor at the rest of the studied sites, our results suggest that magnesium incorporated into clay minerals is enriched in d26Mg by 0 per mil to +1.25 per mil relative to the precipitating fluid. This work demonstrates the utility of magnesium isotopes as a tracer for magnesium sources/sinks in low-temperature aqueous systems.