POTENTIAL DIAGENESIS OF AUTHIGENIC PHOSPHATE MINERALS OVER GEOLOGIC TIMESCALES: IMPLICATIONS FOR INTERPRETATIONS OF THE MARINE PHOSPHORUS CYCLE IN THE GEOLOGIC PAST

Recent studies of the factors that lead to anomalous organic carbon enrichment in ancient marine sediments have increasingly focused on the role of the marine phosphorus (P) cycle. The belief that P acts as the ultimate biolimiting nutrient on geologic timescales has provided a major impetus for these studies. Newly developed sequential extraction procedures, such as the SEDEX method, have recently been applied to samples of ancient marine sediment in an effort to identify the main sources and sinks of P in the geologic past. An important hypothesis to emerge from these studies states that authigenic P minerals (Pauth), which form diagenetically from P released during organic matter degradation, may undergo further diagenetic alteration over geologic time. The proposed mechanism for alteration is a decarboxylation reaction that involves loss of the carbonate ion from Pauth minerals and conversion of these phases to a “purer”, more insoluble apatite mineral resembling detrital P (Pdet) minerals derived from igneous and metamorphic rocks. Because these phases would be misrepresented as non-reactive Pdet rather than a component of the reactive P pool, their presence could have serious implications for interpretations of Pauth vs. Pdet inventories. The alteration hypothesis may be falsifiable through analysis of patterns of accumulation of the operationally defined P phases identified by the SEDEX sequential extraction technique. Two study sites spanning the Late Cretaceous Oceanic Anoxic Event (OAE) II will be examined in this study. The first site is represented by a suite of core samples from the North American Cretaceous Western Interior Seaway (KWIS) and represents deposition across a proximal – distal transect in a shallow marine epicontinental basin. The second data set is from two ODP sites (1258 and 1261) cored during Leg 207 and it represents tropical marine sediments deposited in upper bathyal to middle shelf water depths. Preliminary evidence suggests there is no diagenetic alteration of Pauth at either of the study sites and therefore the alteration hypothesis is not supported.