Instability in tropical Pacific sea surface temperatures during the Early Aptian

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 ighresolution 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 0C and include two prominent cooling episodes of ~4 0C. 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

Isoprenoid thiophenes: Novel products of sediment diagenesis?

Sulphur is a significant component of the organic matter in recent and ancient sediments and in petroleums, yet the precise nature of its association and incorporation is poorly understood. Various sulphur-containing compounds have been recognized in petroleums, but little is known about their origins and mode of generation during sediment burial, and for only a few organo-sulphur compounds with >15 carbon atoms have the structures been determined. Here we identify one of the alkyl thiophenes which occur widely in both recent and ancient deep-sea sediments as 3-methyl-2-(3, 7, 11-trimethyldodecyl)-thiophene, occurring as a limited number of the possible stereoisomers. This compound is presumed to originate from the incorporation of sulphur into chlorophyll-derived phytol, or archaebacterial phytenes or their diagenetic products. Its recognition suggests a novel diagenetic pathway for acyclic isoprenoids involving the introduction of sulphur into specific lipid moieties. Similar, but intermolecular, sulphur incorporation might give rise to sulphurlinked macromolecular materials and thereby contribute significantly to the formation of kerogens

The identification of mono-, di- and trimethyl 2-methyl-2-(4,8,12-trimethyltridecyl) chromans and their occurrence in the geosphere

A series of novel mono-, di- and trimethyl 2-methyl-2-(4,8,12-trimethyltridecyl) chromans (MTTC) have been identified in a number of sediment extracts and oils ranging from Pleistocene to Permian

Sulphur-containing compounds in sulphur-rich crude oils from hypersaline lake sediments and their geochemical implications

Three sulphur-rich commercial crude oils have been studied, which contain sulphur as high as up to 4 —12 %. These samples were collected from Tertiary hypersaline lake sediments of the Jianghan Basin, Hubei Province at different depths, but above the oil generation threshold (2200m). FPD-GC and GC-MS data show that aromatic fractions of the crude oils are composed of different homologues of sulphurcontaining compounds, including long-chain normal alkyl-thiophenes and-thiolanes, long-chain isoprenoid-thiophenes and -thiolanes, and benzothiophenes. It is worth noting that the distribution patterns of long-chain alkyl-thiophenes and -thiolanes from two shallow-seated crude oils are quite similar to those of normal alkanes showing marked even-odd predominance. lt seems that the even-odd predominance of sulphur-containing compounds decreases with increasing burial depth of the crude oils. The major component of aliphatic fraction is phytane, and similarly the major peaks of aromatic fractions also represent C₂₀ isoprenoid thiophenes. Some preliminary conclusions have been drawn from the above discussion: (1) Abundant sulphurcontaining compounds may be used as an indicator of low mature or immature crude oils produced from bypersaline lake sediments; (2) Sulphur-containing compounds are considered to be early diagenetic products of reactions between elemental sulphur or sulfides and alkanes or their precursors (phytols, fatty acids, alcohols, etc. ), or of bacterial activities, but not direct inputs of organisms

Recognition of alkenones in a lower Aptian porcellanite from the west-central Pacific

ODP Leg 198 drilling on Shatsky Rise recovered a lower Aptian porcellanite ( 3c120.5 Ma) deposited during oceanic anoxic event (OAE) 1a that contains C36-C39 alkadienones: C37:2 and C 39:2 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

Cretaceous sea-surface temperature evolution: Constraints from TEX₈₆ and planktonic foraminiferal oxygen isotopes

It is well established that greenhouse conditions prevailed during the Cretaceous Period (~ 145–66 Ma). Determining the exact nature of the greenhouse-gas forcing, climatic warming and climate sensitivity remains, however, an active topic of research. Quantitative and qualitative geochemical and palaeontological proxies provide valuable observational constraints on Cretaceous climate. In particular, reconstructions of Cretaceous sea-surface temperatures (SSTs) have been revolutionised firstly by the recognition that clay-rich sequences can host exceptionally preserved planktonic foraminifera allowing for reliable oxygen-isotope analyses and, secondly by the development of the organic palaeothermometer TEX86, based on the distribution of marine archaeal membrane lipids. Here we provide a new compilation and synthesis of available planktonic foraminiferal δ18O (δ18Opl) and TEX86-SST proxy data for almost the entire Cretaceous Period. The compilation uses SSTs recalculated from published raw data, allowing examination of the sensitivity of each proxy to the calculation method (e.g., choice of calibration) and places all data on a common timescale. Overall, the compilation shows many similarities with trends present in individual records of Cretaceous climate change. For example, both SST proxies and benthic foraminiferal δ18O records indicate maximum warmth in the Cenomanian–Turonian interval. Our reconstruction of the evolution of latitudinal temperature gradients (low, ±48°, palaeolatitudes) reveals temporal changes. In the Valanginian–Aptian, the low-to-higher mid-latitudinal temperature gradient was weak (decreasing from ~ 10–17 °C in the Valanginian, to ~ 3–5 °C in the Aptian, based on TEX86-SSTs). In the Cenomanian–Santonian, reconstructed latitudinal temperature contrasts are also small relative to modern (< 14 °C, based on low-latitude TEX86 and δ18Opl SSTs minus higher latitude δ18Opl SSTs, compared with ~ 20 °C for the modern). In the mid-Campanian to end-Maastrichtian, latitudinal temperature gradients strengthened (~ 19–21 °C, based on low-latitude TEX86 and δ18Opl SSTs minus higher latitude δ18Opl SSTs), with cooling occurring at low-, middle- and higher palaeolatitude sites, implying global surface-ocean cooling and/or changes in ocean heat transport in the Late Cretaceous. These reconstructed long-term trends are resilient, regardless of the choice of proxy (TEX86 or δ18Opl) or calibration. This new Cretaceous SST synthesis provides an up-to-date target for modelling studies investigating the mechanics of extreme climates