ALBERT

Description: Site 716 is a continuous sequence (upper Miocene to Holocene) of periplatform oozes and chalks from the Maldives Ridge, Indian Ocean. Mineralogical and geochemical studies of these carbonate sediments indicate that submarine burial diagenesis has played an important role in the induration of sediments at this site. Metastable carbonates, high-magnesium calcite (HMC) and aragonite, convert to low-magnesium calcite (LMC) rapidly, within 1.1 and 6.0 Ma, respectively. Strontium concentrations in carbonate decrease with depth as the result of the burial diagenesis of calcium car- bonate, primarily aragonite, with excess strontium being expelled into pore waters. The formation of celestite at depth indicates that sufficient diagenesis of carbonate sediments has occurred to saturate or supersaturate pore waters with re- spect to this authigenic mineral. Sodium also decreases monotonically with depth as a result of the burial diagenesis of calcium carbonate. Magnesium and carbon and oxygen isotopic curves are remarkably similar. Carbon isotopic compositions record inputs of 13C-enriched components from shallow carbonate banks. Magnesium concentrations vary widely, recording enhanced episodes of cementation by LMC with slightly elevated magnesium contents. Positive shifts in oxygen isotopic composition also record episodes of cementation during burial diagenesis. Intervals with increased accumulation rates of metastable components have undergone more rapid diagenesis than intervals with predominately pelagic deposition.

Description: The strontium isotope ratios of authigenic carbonates from Indian Ocean sea-floor basalts have been used to determine the timing of carbonate mineral precipitation and fluid flow. The samples include calcites from 57.2 Ma crust from Ocean Drilling Project (ODP) Site 715, and calcites, aragonites, and siderites from 63.7 Ma crust from ODP Site 707. At Site 715, calcite precipitation may have begun at any time after the basalts cooled, and it continued until approximately 31 Ma, or 26 m.y. after basalt eruption. At Site 707, aragonite and siderite did not begin to precipitate until about 36 Ma, almost 30 m.y. after basalt eruption, and continued to precipitate until at least 30 and 28 Ma, respectively. Calcite precipitation began at approximately 32 Ma and continued until 22 Ma. These ages suggest that vein mineral deposition and low-temperature fluid circulation in the ocean crust may continue for much longer periods of time than previously observed.

Description: The Late Miocene-Early Pliocene Biogenic Bloom (~9–3.5 Ma) was a paleoceanographic phenomenon defined by anomalously high accumulations of biological components at multiple open ocean sites, especially in certain regions of the Indian, and Pacific oceans. Its temporal and spatial extent with available information leaves fundamental questions about driving forces and responses unanswered. In this work, we focus on the middle part of the Biogenic Bloom (7.4–4.5 Ma) at International Ocean Discovery Program (IODP) Site 371-U1506 in the Tasman Sea, where we provide an integrated age model based on orbital tuning of the Natural Gamma Radiation (NGR), benthic foraminiferal oxygen isotopes, and calcareous nannofossil biostratigraphy. Benthic foraminiferal assemblages suggest changes in deep water oxygen concentration and seafloor nutrient supply during generally high export productivity conditions. From 7.4 Ma to 6.7 Ma, seafloor conditions were characterized by episodic nutrient supply, perhaps related to seasonal phytoplankton blooms. From 6.7 Ma to 4.5 Ma, the regime shifted to a more stable interval characterized by eutrophic and dysoxic conditions. Combined with seismic data, a regional change in paleoceanography is inferred at around 6.7 Ma, from stronger and well-oxygenated bottom currents to weaker, oxygen-depleted bottom currents. Our results support the hypothesis that the Biogenic Bloom was a complex, multiphase phenomenon driven by changes in ocean currents, rather than a single uniform period of sustained sea surface water productivity. Highly resolved studies are thus fundamental to its understanding and the disentanglement of local, regional, and global imprints.