Inconsistencies between 14C and short-lived radionuclides-based sediment accumulation rates: Effects of long-term remineralization.

14C is the most widely utilized geochronometer to investigate geological, geochemical and geophysical problems over the past 5 decades. Establishment of precise sedimentation rates is crucial for the reconstruction of paleo-climate, -ecological and - environmental studies when extrapolation of sedimentation rates is utilized for time scales beyond the dating range. However, agreement between short-term and long-term sedimentation rates in anthropogenically unperturbed sediment cores has not been shown. Here we show that the AMS 14C-based long-term mass accumulation rate (MAR) of an organic-rich (>70%) sediment core from Mud Lake, Florida to be ∼5 times lower than the short-term MAR obtained using 239,240Pu, 137Cs and excess 210Pb (210Pbxs). The measured sediment inventories of 210Pbxs, 137Cs and 239,240Pu are comparable to the atmospheric fallout for the sampling site, indicating very little accelerated sediment erosion over the past several decades. Presence of sharp fallout peaks of 239,240Pu indicates very little sediment mixing. The penetration depths of 137Cs and 239,240Pu were found to be much deeper than expected and this is attributed to their post-depositional mobility. MAR calculated using 14C-ages in successive layers also indicated decreasing MARs with depth, and was reflective of progressive remineralization. Using first-order kinetics, the sediment remineralization rate was found to be 4.4 × 10-4 y-1 and propose that over the long-term, remineralization of organic-rich sediment affected the long-term MAR, but not the ratio of 14C/12C. Thus, the MAR and linear sedimentation rate obtained using 14C (and other isotope-based methods) could be erroneous, although 14C ages may not be affected by such remineralization. Long-term remineralization rates of organic matter has a direct bearing on the biogeochemical cycling of elements in aqueous systems and mass balance of elements needs to be taken into consideration.

Nitrogen cycling by wood decomposing soft-rot fungi in the "King Midas tomb," Gordion, Turkey.

Archaeological wood in ancient tombs is found usually with extensive degradation, limiting what can be learned about the diet, environment, health, and cultural practices of the tomb builders and occupants. Within Tumulus Midas Mound at Gordion, Turkey, thought to be the tomb of the Phrygian King Midas of the 8th century B.C., we applied a stable nitrogen isotope test to infer the paleodiet of the king and determine the nitrogen sources for the fungal community that decomposed the wooden tomb, cultural objects, and human remains. Here we show through analysis of the coffin, furniture, and wooden tomb structure that the principal degrader, a soft-rot fungus, mobilized the king's highly (15)N-enriched nutrients, values indicative of a diet rich in meat, to decay wood throughout the tomb. It is also evident from the delta(15)N values of the degraded wood that the nitrogen needed for the decay of many of the artifacts in the tomb came from multiple sources, mobilized at potentially different episodes of decay. The redistribution of nutrients by the fungus was restricted by constraints imposed by the cellular structure of the different wood materials that apparently were used intentionally in the construction to minimize decay.

Selective adsorption of L- and D-amino acids on calcite: Implications for biochemical homochirality.

The emergence of biochemical homochirality was a key step in the origin of life, yet prebiotic mechanisms for chiral separation are not well constrained. Here we demonstrate a geochemically plausible scenario for chiral separation of amino acids by adsorption on mineral surfaces. Crystals of the common rock-forming mineral calcite (CaCO(3)), when immersed in a racemic aspartic acid solution, display significant adsorption and chiral selectivity of d- and l-enantiomers on pairs of mirror-related crystal-growth surfaces. This selective adsorption is greater on crystals with terraced surface textures, which indicates that d- and l-aspartic acid concentrate along step-like linear growth features. Thus, selective adsorption of linear arrays of d- and l-amino acids on calcite, with subsequent condensation polymerization, represents a plausible geochemical mechanism for the production of homochiral polypeptides on the prebiotic Earth.

Primordial carbonylated iron-sulfur compounds and the synthesis of pyruvate.

Experiments exploring the potential catalytic role of iron sulfide at 250 degrees C and elevated pressures (50, 100, and 200 megapascals) revealed a facile, pressure-enhanced synthesis of organometallic phases formed through the reaction of alkyl thiols and carbon monoxide with iron sulfide. A suite of organometallic compounds were characterized with ultraviolet-visible and Raman spectroscopy. The natural synthesis of such compounds is anticipated in present-day and ancient environments wherever reduced hydrothermal fluids pass through iron sulfide-containing crust. Here, pyruvic acid was synthesized in the presence of such organometallic phases. These compounds could have provided the prebiotic Earth with critical biochemical functionality.