Temperatures, salinity, and stable oxygen isotope values of bottom water and of the mollusc shell Arctica islandica from Nantucket Shoals

The carbonate shell of the bivalve Arctica islandica has been recognized, for more than a decade, as a potentially important marine geochemical biorecorder owing to this species' great longevity (200+ years) and wide geographic distribution throughout the northern North Atlantic Ocean, a region vital to global climate and ocean circulation. However, until now this potential has not been realized owing to the difficulty of precisely sampling the shell of this slow growing species. Using newly available automated microsampling techniques combined with micromass stable isotope mass spectrometry, a stable oxygen isotope record (1956-1957 and 1961-1970) has been obtained from a live-captured, 38-year-old A. islandica specimen collected near the former position of the Nantucket Shoals Lightship (41°N. 69°W). The shell's delta18O signal is compared with an expected signal derived from ambient bottom temperature and salinity data recorded at the lightship for the same period. The results show that A islandica's delta18O record (1) is in phase with its growth banding, confirming the annual periodicity of this species' growth bands, (2) is in oxygen isotopic equilibrium with the ambient seawater, (3) shows a consistent shell growth shutdown temperature of ~6°C. which translates into an ~8-month (May-December) shell growth period at this location, and (4) records the ambient bottom temperature with a precision of ~ +/-1.2°C. These results add important information on the life history of this commercially important shellfish species and demonstrate that A. islandica shells can be used to reconstruct inter- and intra-annual records of the continental shelf bottom temperature.

Tab. 3: Substrate pH values of crypthoendolithic microbial communities

Cryploendolithic microbial communities in the Ross Desert (McMurdo Dry Valleys) are characterized on the basis of photosymlictic microorganisms and fungi. Two eukaryotic communities (the lichen-dominated and Hemichloris communities) and three cyanobacterial communities (the red Gloeocapsa, Hormathonema-Gloeocapsa and Chroococcidiopsis communities) are described. Eleven coccoid. ohne pleurocapsoid, and five filamentous cyanobacteria occurring in these communities are characterized and illustrated. The moisture grade of the rock substrate seems to affect pH. Formation of primary iron stain, and the distribution of microbial communities.

(Table 1) Density values of ODP Leg 126 samples

Sediment dry-bulk density values are essential components of mass accumulation rate calculations. This manuscript presents three equations to calculate dry-bulk density from laboratory measurements of physical properties that have been corrected for the salt content of the pore fluid. In addition, two equations for use with values not corrected for salt content are included. Derivations of the equations from first principles are presented. The second part of the manuscript briefly examines laboratory measurements of the various properties used in the dry-bulk density equations. A discussion of the problems inherent in the density measurements and recommendations are included. This work represents the first comprehensive compilation of equations of dry-bulk density and should prove useful to all scientists who investigate accumulation rates.