Microbially mediated carbonate precipitation in a hypersaline lake,Big Pond (Eleuthera,Bahamas)

Microbial metabolism impacts the degree of carbonate saturation by changing the total alkalinity and calcium availability; this can result in the precipitation of carbonate minerals and thus the formation of microbialites. Here, the microbial metabolic activity, the characteristics and turnover of the extracellular polymeric substances and the physicochemical conditions in the water column and sediments of a hypersaline lake, Big Pond, Bahamas, were determined to identify the driving forces in microbialite formation. A conceptual model for organomineralization within the active part of the microbial mats that cover the lake sediments is presented. Geochemical modelling indicated an oversaturation with respect to carbonates (including calcite, aragonite and dolomite), but these minerals were never observed to precipitate at the mat–water interface. This failure is attributed to the capacity of the water column and upper layers of the microbial mat to bind calcium. A layer of high Mg‐calcite was present 4 to 6 mm below the surface of the mat, just beneath the horizons of maximum photosynthesis and aerobic respiration. This carbonate layer was associated with the zone of maximum sulphate reduction. It is postulated that extracellular polymeric substances and low molecular weight organic carbon produced at the surface (i.e. the cyanobacterial layer) of the mat bind calcium. Both aerobic and anaerobic heterotrophic microbes consume extracellular polymeric substances (each process accounting for approximately half of the total consumption) and low molecular weight organic carbon, liberating calcium and producing inorganic carbon. The combination of these geochemical changes can increase the carbonate saturation index, which may result in carbonate precipitation. In conclusion, the formation and degradation of extracellular polymeric substances, as well as sulphate reduction, may play a pivotal role in the formation of microbialites both in marine and hypersaline environments.     

Isotopic Evolution of the Tonga Arc during Lau Basin Rifting; Evidence from the Volcaniclastic Record

New Pb, Sr and Nd isotope data from volcaniclastic sedimentsrecovered from the Tonga forearc and Lau Basin permit the isotopicevolution of a section of this arc system, next to the modernisland of Ata, to be traced through the backarc basin riftingprocess from 7.0 Ma to the present. The new data suggest thatthe isotopic character of the mantle wedge remains constant,and of Pacific mid-ocean ridge basalt (MORB) character, duringthe early rifting phase. The isotopic evidence supports traceelement data in showing an increase in the sediment contributionto arc petrogenesis about 2–3 m.y. after the start ofLau Basin rifting. Since 0.45 Ma the sediment contribution decreasedto pre-rift values with the initiation of spreading in the adjacentbackarc basin, where the high sediment influence is not seenin the isotopes. The isotopes show a relative increase in thevolcaniclastic compared with pelagic sediment involvement duringrifting. The inferred peak in sediment subduction is probablythe result of a decoupling of the two plates owing to roll-backof the Pacific lithosphere at the time of arc rifting. KEY WORDS: isotopes; Pacific; rifting; subduction; volcanism ^*Corresponding author. Telephone: (508) 289 3437. Fax: (508) 457 2187. e-mail: pclift{at}whoi.edu