ALBERT

Description: The occurrence of intervals of enhanced sequestration of organic matter (OM) during the Mid-Cretaceous-collectively termed "Oceanic Anoxic Events" (OAE)-reflect abrupt changes in global carbon cycling. These episodes raise questions about the causes for such perturbations, and their linkages to fluctuations in climate, marine biota, and ocean geochemistry preserved in the sedimentary record. Detailed assessment of the early Aptian event (OAE1a) in the Pacific has been precluded by the limited recovery of this stratigraphic interval, in contrast to the many records from the Tethyan realm. The recovery of pelagic sequences of OM-rich sediments at Site 1207 on Shatsky Rise (ODP Leg 198) corresponding to OAE1a has enabled recognition of highresolution temporal variations in the elemental and isotopic compositions of this event in the Pacific. However, the continuous 45 cm OM-rich interval recovered is likely only the initiation of OAE1a rather than a complete section based on natural gamma and uranium logs, which suggest that the full thickness of this interval is ~1.2 m. Exceedingly high organic carbon (CorgN20%) and total sulfur (TSN5%) contents–among the highest ever recorded for pelagic Cretaceous sequences-attest to the enhanced sequestration of OM during OAE1a. Delta13Corg values are lowest (27.7‰) at the base of the recovered OM-rich interval, followed by a 2.4‰ positive shift. This isotopic profile resembles part of the delta13Ccarb record at Resolution Guyot and is comparable with segments C3 and C4 of the Cismon delta13Corg record. Low delta15N values (〈= 2‰) provide evidence for nitrogen fixation, perhaps coupled with elevated cyanobacterial populations, that suggest that the marine nitrogen cycle in the Cretaceous was different from the modern. Overall, the temporal fluctuations in elemental and isotopic data through OAE1a at Shatsky Rise describe differences in production and preservation of OM that likely reflect variations in ocean conditions associated with both local and global environmental changes.

Description: ODP Leg 198 drilling on Shatsky Rise recovered a lower Aptian porcellanite (~120.5 Ma) deposited during oceanic anoxic event (OAE) 1a that contains C36-C39 alkadienones: C37:2 and C39:2 alkadien-2-ones and C36:2 and C38:2 alkadien-3-ones. This alkenone distribution differs from that typical of contemporary sediments and haptophyte algae, but resembles that of Cretaceous sediments from the Blake-Bahama basin. The discovery of alkenones in the early Aptian extends their sedimentary record by 15 M.y. to 120.5 M.y. and demonstrates the potential for long-term survival of these diagnostic functional lipids under favorable depositional conditions and subsequent shallow burial. It also contributes to the understanding and reconstruction of evolutionary developments in alkenone distributions and biosynthesis over geologic time.

Description: The Cretaceous has long been recognized as a time when greenhouse conditions were fueled by elevated atmospheric CO2 and accompanied by perturbations of the global carbon cycle described as oceanic anoxic events (OAEs). Yet, the magnitude and frequency of temperature change during this interval of warm and equable climate are poorly constrained. Here we present a high-resolution record of sea-surface temperatures (SSTs) reconstructed using the TEX86 paleothermometer for a sequence of early Aptian organic-rich sediments deposited during the first Cretaceous OAE (OAE1a) at Shatsky Rise in the tropical Pacific. SSTs range from ~30 to ~36 °C and include two prominent cooling episodes of ~4 °C. The cooler temperatures reflect significant temperature instability in the tropics likely triggered by changes in carbon cycling induced by enhanced burial of organic matter. SST instability recorded during the early Aptian in the Pacific is comparable to that reported for the late Albian-early Cenomanian in the Atlantic, suggesting that such climate perturbations may have recurred during the Cretaceous with concomitant consequences for biota and the marine environment.

Description: This study investigates organic-rich sedimentary sequences deposited during the early Aptian Oceanic Anoxic Event (OAE1a) at Sites 1207 and 1213 on Shatsky Rise (ODP Leg 198) in the west-central Pacific. Biomarker analyses provide evidence of the algal and bacterial origin of organic matter (OM) in these sediments where the abundance of steroidal components, particularly sterenes and sterones, suggests that the OM includes major contributions from eukaryotic sources in an environment characterized by high phytoplankton productivity. The presence of alkenones at Site 1213B is diagnostic of OM derived from representatives of haptophyte algae among the calcareous nannoplankton and their d13C values (average -31.6 per mil) are consistent with those expected during elevated pCO2. The occurrence and prominence of 2b-methylhopanes and 2b-methylhopanones indicates significant contributions to the OM from cyanobacteria, which are also likely contributors of hopanoids based on their d13C compositions. These biomarker data suggest that oceanic conditions, perhaps nitrate- or iron-limited, were conducive to cyanobacteria production during OAE1a, which appears to distinguish this event from other Cretaceous OAE.