Geochemistry of DSDP Hole 83-504B minerals

Leg 83 of the Deep Sea Drilling Project has deepened Hole 504B to over 1 km into basement, 1350 m below the seafloor (BSF). The hole previously extended through 274.5 m of sediment and 561.5 m of pillow basalts altered at low temperature (< 100°C), to 836 m BSF. Leg 83 drilling penetrated an additional 10 m of pillows, a 209-m transition zone, and 295 m into a sheeted dike complex. Leg 83 basalts (836-1350 m BSF) generally contain superimposed greenschist and zeolite-facies mineral parageneses. Alteration of pillows and dikes from 836 to 898 m BSF occurred under reducing conditions at low water/rock ratios, and at temperatures probably greater than 100°C. Evolution of fluid composition resulted in the formation of (1) clay minerals, followed by (2) zeolites, anhydrite, and calcite. Alteration of basalts in the transition zone and dike sections (898-1350 m BSF) occurred in three basic stages, defined by the opening of fractures and the formation of characteristic secondary minerals. (1) Chlorite, actinolite, pyrite, albite, sphene, and minor quartz formed in veins and host basalts from partially reacted seawater (Mg-bearing, locally metal-and Si-enriched) at temperatures of at least 200-250°C. (2) Quartz, epidote, and sulfides formed in veins at temperatures of up to 380°C, from more evolved (Mg-depleted, metal-, Si-, and 18O-enriched) fluids. (3) The last stage is characterized by zeolite formation: (a) analcite and stilbite formed locally, possibly at temperatures less than 200°C followed by (b) formation of laumontite, heulàndite, scolecite, calcite, and prehnite from solutions depleted in Mg and enriched in Ca and 18O, at temperatures of up to 250°C. The presence of small amounts of anhydrite locally may be due to ingress of relatively unaltered seawater into the system during Stage 3. Alteration was controlled by the permeability of the crust and is characterized by generally incomplete recrystallization and replacement reactions among secondary minerals. Secondary mineralogy in the host basalts is strongly controlled by primary mineralogy. The alteration of Leg 83 basalts can be interpreted in terms of an evolving hydrothermal system, with (a) changes in solution composition because of reaction of seawater fluids with basalts at high temperatures; (b) variations in permeability caused by several stages of sealing and reopening of cracks; and (c) a general cooling of the system, caused either by the cooling of a magma chamber beneath the spreading center and/or the movement of the crust away from the heat source. The relationship of the high-temperature alteration in the transition zone and dike sections to the low-temperature alteration in the overlying pillow section remains uncertain.

Water chemistry and heat budget of the Churchill River estuary region

A conceptual scheme for the transition from winter to spring is developed for a small Arctic estuary (Churchill River, Hudson Bay) using hydrological, meteorological and oceanographic data together with models of the landfast ice. Observations within the Churchill River estuary and away from the direct influence of the river plume (Button Bay), between March and May 2005, show that both sea ice (production and melt) and river water influence the region's freshwater budget. In Button Bay, ice production in the flaw lead or polynya of NW Hudson Bay result in salinization through winter until the end of March, followed by a gradual freshening of the water column through April-May. In the Churchill Estuary, conditions varied abruptly throughout winter-spring depending on the physical interaction among river discharge, the seasonal landfast ice, and the rubble zone along the seaward margin of the landfast ice. Until late May, the rubble zone partially impounded river discharge, influencing the surface salinity, stratification, flushing time, and distribution and abundance of nutrients in the estuary. The river discharge, in turn, advanced and enhanced sea ice ablation in the estuary by delivering sensible heat. Weak stratification, the supply of riverine nitrogen and silicate, and a relatively long flushing time (~6 days) in the period preceding melt may have briefly favoured phytoplankton production in the estuary when conditions were still poor in the surrounding coastal environment. However, in late May, the peak flow and breakdown of the ice-rubble zone around the estuary brought abrupt changes, including increased stratification and turbidity, reduced marine and freshwater nutrient supply, a shorter flushing time, and the release of the freshwater pool into the interior ocean. These conditions suppressed phytoplankton productivity while enhancing the inventory of particulate organic matter delivered by the river. The physical and biological changes observed in this study highlight the variability and instability of small frozen estuaries during winter-spring transition, which implies sensitivity to climate change.

Thermal reaction norms of growth in Atlantic and Pacific silverside fishes

How organisms may adapt to rising global temperatures is uncertain, but concepts can emerge from studying adaptive physiological trait variations across existing spatial climate gradients. Many ectotherms, particularly fish, have evolved increasing genetic growth capacities with latitude (i.e. countergradient variation (CnGV) in growth), which are thought to be an adaptation primarily to strong gradients in seasonality. In contrast, evolutionary responses to gradients in mean temperature are often assumed to involve an alternative mode, 'thermal adaptation'. We measured thermal growth reaction norms in Pacific silverside populations (Atherinops affinis) occurring across a weak latitudinal temperature gradient with invariant seasonality along the North American Pacific coast. Instead of thermal adaptation, we found novel evidence for CnGV in growth, suggesting that CnGV is a ubiquitous mode of reaction-norm evolution in ectotherms even in response to weak spatial and, by inference, temporal climate gradients. A novel, large-scale comparison between ecologically equivalent Pacific versus Atlantic silversides (Menidia menidia) revealed how closely growth CnGV patterns reflect their respective climate gradients. While steep growth reaction norms and increasing growth plasticity with latitude in M. menidia mimicked the strong, highly seasonal Atlantic coastal gradient, shallow reaction norms and much smaller, latitude-independent growth plasticity in A. affinis resembled the weak Pacific latitudinal temperature gradient.

(Table 3) Geochemistry of borehole fluids of ODP Hole 137-504B

Circulation of seawater through basaltic basement for several million years after crustal emplacement has been inferred from studies of surface heat flow, and may play a significant role in the exchange of elements between the oceanic crust and seawater. Without direct observation of the fluid chemistry, interpretations regarding the extent and timing of this exchange must be based on the integrated signal of alteration found in sampled basalts. Much interest has thus been expressed in obtaining and analyzing fluids directly from basaltic formations. It has been proposed that open oceanic boreholes can be used as oceanic groundwater wells to obtain fluids that are circulating within the formation. Water samples were collected from the open borehole in Hole 504B prior to drilling operations on Leg 137, with the original intention of collecting formation fluids from the surrounding basaltic rocks. Past results have yielded ambiguous conclusions as to the origin of the fluids recovered-specifically, whether or not the fluids were true formation fluids or merely the result of reaction of seawater in the borehole environment. The chemistry of eight borehole fluid samples collected during Leg 137 is discussed in this paper. Large changes in major, minor, and isotopic compositions relative to unaltered seawater were observed in the borehole fluids. Compositional changes increased with depth in the borehole. The samples exhibit the effect of simple mixing of seawater, throughout the borehole, with a single reacted fluid component. Analysis and interpretation of the results from Leg 137 in light of past results suggest that the chemical signals observed may originate predominantly from reaction with basaltic rubble residing at the bottom of the hole during the interim between drilling legs. Although this endeavor apparently did not recover formation waters, information on the nature of reaction between seawater and basalt at the prevalent temperatures in Hole 504B (>160°C) has been gained that can be related to reconstruction of the alteration history of the oceanic crust. Isotopic analyses allow calculation of element-specific water/rock mass ratios (Li and Sr) and are related to the extent of chemical exchange between the borehole fluids and basalt.

Sulphur contents and partition of iron in ODP Leg 112 sediments

Porewaters in site 680 Peru Margin sediments contain dissolved sulfide over a depth of approximately 70 m which, at a sedimentation rate of 0.005 cm/yr, gives a sediment exposure time to dissolved sulfide of about 1.4 Myr. Reactions with dissolved sulfide cause the site 680 sediments to show a progressive decrease in a poorly-reactive silicate iron fraction, defined as the difference between iron extracted by dithionite (FeD) at room temperature and that extracted by boiling concentrated HCl (FeH), normalised to the total iron content (FeT). Straight line plots are obtained for ln[(FeH - FeD)/FeT] against time of burial, from which a first order rate constant of 0.29 1/Myr (equivalent to a half-life of 2.4 Myr) can be derived for the sulfidation of this silicate iron. Comparable half-lives are also found for the same poorly-reactive iron fraction in the nearby site 681 and 684 sediments. This silicate Fe fraction comprises 0.8-1.0% Fe, only 30-60% of which reacts even with 1.5-3 million years exposure to dissolved sulfide. Diagenetic models based on porewater concentrations of sulfate and sulfide, and solid phase iron contents, at site 680 are consistent in indicating that this poorly-reactive iron fraction is only sulfidized on a million year time scale. Silicate iron not extracted by HCl can be regarded as unreactive towards dissolved sulfide on the time scales encountered in marine sediments.

Chemical composition of interstitial solutions from sediments of DSDP Legs 6-8, 11, 12, 14, 15, and 22

Through the Deep Sea Drilling Project samples of interstitial solutions of deeply buried marine sediments throughout the World Ocean have been obtained and analyzed. The studies have shown that in all but the most slowly deposited sediments pore fluids exhibit changes in composition upon burial. These changes can be grouped into a few consistent patterns that facilitate identification of the diagenetic reactions occurring in the sediments. Pelagic clays and slowly deposited (<1 cm/1000 yr) biogenic sediments are the only types that exhibit little evidence of reaction in the pore waters. In most biogenic sediments sea water undergoes considerable alteration. In sediments deposited at rates up to a few cm/1000 yr the changes chiefly involve gains of Ca(2+) and Sr(2+) and losses of Mg(2+) which balance the Ca(2+) enrichment. The Ca-Mg substitution may often reach 30 mM/kg while Sr(2+) may be enriched 15-fold over sea water. These changes reflect recrystallization of biogenic calcite and the substitution of Mg(2+) for Ca(2+) during this reaction. The Ca-Mg-carbonate formed is most likely a dolomitic phase. A related but more complex pattern is found in carbonate sediments deposited at somewhat greater rates. Ca(2+) and Sr(2+) enrichment is again characteristic, but Mg(2+) losses exceed Ca(2+) gains with the excess being balanced by SO4(post staggered 2-) losses. The data indicate that the reactions are similar to those noted above, except that the Ca(2+) released is not kept in solution but is precipitated by the HCO3(post staggered -) produced in SO4(post staggered 2-) reduction. In both these types of pore waters Na(+) is usually conservative, but K(+) depletions are frequent. In several partly consolidated sediment sections approaching igneous basement contact, very marked interstitial calcium enrichment has been found (to 5.5 g/kg). These phenomena are marked by pronounced depletion in Na(+), Si and CO2, and slight enhancement in Cl(-). The changes are attributed to exchange of Na(+) for Ca(2+) in silicate minerals forming from submarine weathering of igneous rocks such as basalts. Water is also consumed in these reactions, accounting for minor increases in total interstitial salinity. Terrigenous, organic-rich sediments deposited rapidly along continental margins also exhibit significant evidences of alteration. Microbial reactions involving organic matter lead to complete removal of SO4(post staggered 2-), strong HCO3(post staggered -) enrichment, formation of NH4(post staggered +), and methane synthesis from H2 and CO2 once SO4(post staggered 2-) is eliminated. K+ and often Na+ (slightly) are depleted in the interstitial waters. Ca(2+) depletion may occur owing to precipitation of CaCO3. In most cases interstitial Cl- remains relatively constant, but increases are noted over evaporitic strata, and decreases in interstitial Cl- are observed in some sediments adjacent to continents.

(Table 1) Pore-water composition of ODP Leg 126 sites

The Ocean Drilling Program Leg 126 sites may be classified into two categories depending on the presence (Group I: Sites 787, 792, and 793) or absence (Group II: Sites 788, 790, and 791) of steep concentration gradients. Shipboard X-ray diffraction analyses of bulk sediments from Group I sites revealed the presence of a number of diagenetic minerals (some of which are incompatible), but no systematic diagenetic zonation. The results of the chemical analyses of the pore waters from Group I have been used to estimate the activities of dissolved species. Thermodynamic analyses of the composition of the pore waters and the stability of authigenic minerals (gypsum, zeolites, feldspars, smectites, chlorites, and micas) show that the pore waters are close to equilibrium with most of the observed phases. Thus, only a small perturbation of the system (substitution in minerals and fluctuations in pore-water composition, in particular, in pH and SiO2 activity) will cause any of these phases to precipitate. Therefore, one would not expect mineralogical observations to show systematic vertical zonations at these sites. It is suggested that chlorites and high-temperature zeolites are not diagenetic sensu stricto, but were eroded from volcaniclastic highs. The absence of concentration gradients at the Group II sites has been analyzed in terms of reaction kinetics, hydrothermal advection, and temperature distribution. The absence of diagenetic imprints on the pore-water concentration profiles at these sites is probably caused by the slow nucleation of silica phases.

Amino acid racemization for several DSDP samples (Table 1)

Kinetic parameters for the epimerization of isoleucine in multispecific foraminiferal asemblages were used to establish the effects of burial depth and the geothermal gradient on the extent of reaction. It was observed that with a little as thirty meters of burial in a normal thermal regime there were differences between the extent of epimerization measured and that which would have been predicted for thermal equilibrium with bottom water temperatures. As would be expected, these differences are greatest when the heat flow (the geothermal gradient) and/or the sedimentation rates are highest. These effects were observed in most of the DSDP samples studied, and have been used to estimate the average heat flux since the time of sample deposition. Occasional anomalous effects were observed which could not be related to past or present heat flux. These were determined to be due to such geologic occurrences as slumping and reworking or to recent sample contamination. Other problems emerged related to bottom water temperatures including changes over geologic time which are unknown and could not be deduced. Thus, the presence of epimerization anomalies in DSDP cores as noted above limits the effectiveness of amino acid geochronology in such cores, unless these anomalies can be recognized as ab initio.