The impact of alkenone degradation on UK'37 paleothermometry: a model-derived assessment

The Uk’ 37 proxy for past sea surface temperature (SST) is based on the unsaturation ratio of C37 alkenones. It is considered a diagenetically robust proxy, but biases have been invoked because the index can be altered by preferential degradation of the C37:3 alkenone, resulting in higher reconstructed SST. However, alkenone degradation rate constants are poorly constrained, making it difficult to evaluate the plausibility of such a bias. Therefore, we quantitatively assessed the effect of (1) different alkenone degradation rate constants; (2) differential degradation factors between diunsaturated and triunsaturated C37 alkenones; (3) and initial Uk’ 37 values on the Uk’ 37 paleothermometer for two depositional environments (shelf and upper slope), by means of a reaction-transport model (RTM). RTM results reveal that preferential degradation of C37:3 can potentially alter the original signal of the Uk’ 37 paleothermometer, but SST biases (ΔSST) are largely within Uk’ 37 calibration error (ΔSST 1.5°C are associated with marked downcore decreases in alkenone concentration. Consequently, we caution against the interpretation of Uk’ 37 indices when extensive degradation results in very low alkenone concentrations (<5 ng g−1).SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Nutrients as the dominant control on the spread of anoxia and euxinia across the Cenomanian-Turonian oceanic anoxic event (OAE2): Model-data comparison

International audienc

Transition from a warm and dry to a cold and wet climate in NE China across the Holocene

Northeast (NE) China lies in the northernmost part of the East Asian Summer monsoon (EASM) region. Although a series of Holocene climatic records have been obtained from lakes and peats in this region, the Holocene hydrological history and its controls remain unclear. More specifically, it is currently debated whether NE China experienced a dry or wet climate during the early Holocene. Here we reconstruct changes in mean annual air temperature and peat soil moisture across the last similar to 13,000 year BP using samples from the Gushantun and Hani peat, located in NE China. Our approach is based on the distribution of bacterial branched glycerol dialkyl glycerol tetraethers (brGDGTs) and the abundance of the archaeal isoprenoidal (iso)GDGT crenarchaeol. Using the recently developed peat-specific MAAT(p)(eat) temperature calibration we find that NE China experienced a relatively warm early Holocene (similar to 5-7 degrees C warmer than today), followed by a cooling trend towards modern-day values during the mid- and late Holocene. Moreover, crenarchaeol concentrations, brGDGT-based pH values, and the distribution of 6-methyl brGDGTs, all indicate an increase in soil moisture content from the early to late Holocene in both peats, which is largely consistent with other data from NE China. This trend towards increasing soil moisture/wetter conditions across the Holocene in NE China records contrasts with the trends observed in other parts of the EASM region, which exhibit an early and/or mid-Holocene moisture/precipitation maximum. However, the Holocene soil moisture variations and temperature-moisture relationships (warm-dry and cold-wet) observed in NE China are similar to those observed in the core area of arid central Asia which is dominated by the westerlies. We therefore propose that an increase in the intensity of the westerlies across the Holocene, driven by increasing winter insolation, expanding Arctic sea ice extent and the enhanced Okhotsk High, caused an increase in moisture during the late Holocene in NE China. (C) 2018 Elsevier B.V. All rights reserved