Earth Sciences Department - Geological Institute

In accretionary margins, the estimation of peak heating through the thermal maturity of organic material reveals differentiated deformational pathways in the adjoining sections of the wedge. As the thermal maturity of the deformed structures is a function of both the geothermal gradient and the duration of time it has been exposed to different gradients, the distribution of vitrinite reflectance (the measure of thermal maturity) in the wedge does not necessarily reveal the temporal deformational pathway of the individual pockets. As a result, the different portions of the accretionary wedge exhibit different correlations between the thermal maturity and a unique deformational pathway. These different correlations can be explored through the means of geodynamic modelling which can then be used in the field to ascertain a more holistic picture of deformation in an accretionary wedge. Therefore, in this study, we use a 2D plate subduction thermomechanical model, to simulate formation o...

15 Thermal maturity assessments of hydrocarbon-generation potential and thermal history rarely consider how 16 upper-plate structures developing during subduction influence the trajectories of accreted sediments. Our 17 thermomechanical models of subduction support that thrusts evolving under variable sedimentation rates 18 and décollement strengths fundamentally influence the trajectory, temperature, and thermal maturity of 19 accreting sediments. This is notably true for the frontal thrust, which pervasively partitions sediments along 20 a low and a high maturity path. Our findings imply that interpretations of the distribution of thermal maturity 21 cannot be detached from accounts of the length and frequency of thrusts and their controlling factors. 22 Taking these factors into consideration, our approach reduces former inconsistencies between predicted and 23 factual thermal maturity distributions in accretionary wedges and provides a first-order predictive indicator 24 for the...

Syntectonic sedimentation history is a potential cause of differentiated accretionary wedge structures along the subduction margin. Recent efforts to model the role of sedimentation on wedge evolution have highlighted the importance of spatiotemporal history of sedimentation on the evolution of the wedge. Moreover, reconstruction of deformation history of the accretionary wedges using reflection seismic and borehole data has further substantiated the impact of sedimentation on wedge evolution. We conduct several numerical experiments using a high‐resolution dynamic 2‐D thermomechanical plate subduction model to systematically investigate and quantify different effects of sedimentation on accretionary wedge evolution. Models with sedimentation suggest migration of deformation to parts of the wedge lying outside the sedimentation zone leading to emergence/reactivation of out‐of‐sequence thrusts (OOSTs). Frequency and length of new thrust sheets are correlated with sedimentation in the trench. Models undergo a transition period of ~1.5 Myr following the onset of sedimentation, after which they continue to grow under a new steady state. Stabilization of the wedge and increased load on the oceanic plate due to sedimentation create conditions in which smaller wedge‐top basins combine to form a large and flat forearc basin. Last but not the least, emergence of OOST in models of accretionary wedges undergoing sedimentation provides important insights in to evolution of potentially tsunamigenic OOSTs like the Megasplay Fault seaward of the Kumano forearc basin.

Our understanding of past continental climate in East Asia is mainly based on loess-paleosol sequences and summer monsoon precipitation reconstructions based on oxygen isotopes (δ18O) of stalagmites from several Chinese caves. They show that the intensity of East Asian Summer Monsoon (EASM) precipitation generally follows Northern Hemisphere (NH) summer insolation. However, not much is known about the magnitude and timing of deglacial warming on the East Asian continent. We here present an absolute continental air temperature record for this region, using the novel Methylation of Branched Tetraethers/Cyclisation of Branched Tetraethers (MBT/CBT) proxy [1], based on the distribution of fossil membrane lipids of soil bacteria, in a loess-paleosol sequence from the Mangshan loess plateau. Our continuous, high resolution temperature record covers the last 34,000 years, and shows that air temperatures varied in phase with NH summer insolation. The onset of deglacial warming at ~17.5 kyr BP is parallel in timing with known continental records from e.g. Antarctica, Africa and South America. Strikingly, comparison of the MBT/CBT-derived temperature record with other loess-paleosol proxy records and stalagmite δ18O records shows that the strengthening of EASM precipitation lagged that of deglacial warming by ca. 2 kyrs. Moreover, soil formation in the loess deposits, caused by substantial increases in summer monsoon precipitation, only started around 12 kyr BP (>5 kyrs lag). Our results thus show that the intensification of EASM precipitation unambiguously lagged deglacial warming and NH summer insolation. We are currently analysing n-alkane distributions throughout the same loess-paleosol sequence to see how the vegetation responded on the difference in timing between atmospheric warming and the intensification of the summer monsoon during the last deglaciation. Reference: [1] Weijers et al., 2007, GCA 71, 703-713.

Several proxies based on organic compounds exist of which the Uk'37 and the TEX86 are now well established quantitative temperature proxies applied in the marine realm. Although biomarker proxies exist for the terrestrial realm, none of them can be used to quantitatively estimate continental temperatures. Here we introduce two new proxies based on terrestrial derived branched glycerol dialkyl glycerol tetraether (GDGT) membrane lipids to quantitatively estimate continental palaeo-temperatures and palaeo-soil pH. Branched GDGTs are abundant constituents of soils and peat bogs world wide and presumably of bacterial origin based on their stereo configuration. The distribution of individual branched GDGTs, varying in methyl branching and cyclopentanyl content, differs over the 150 soils analyzed. Principle component analysis showed a significant correlation between the degree of cyclisation of branched GDGTs (CBT) and soil pH. Another significant correlation was evident between the degree of methylation (MBT) and both soil pH and annual mean air temperature. This is explained by the fact that microorganisms adjust the composition of their cell membrane as an adaptation to changing environmental conditions. A focused study on soil transects off hot springs confirmed the temperature and pH dependence of the lipid distribution. Upon soil erosion and fluvial transport, branched GDGTs are deposited in near shore marine sediments. Being part of the marine sedimentary archive, branched GDGTs offer the opportunity to reconstruct long term river basin integrated temperature signals for the adjacent continent. A study of branched GDGT distributions in the Congo deep sea fan revealed a gradual temperature rise of 4°C over the last deglaciation in central Africa. Direct comparison with sea surface temperature estimates from the same core pointed to a dual influence of marine and terrestrial temperature development on the central African hydrological cycle. The presence of branched GDGTs in Palaeocene-Eocene sediments from the Arctic Ocean and in a German lignite of Early Cretaceous age, shows the potential of this new proxy on longer time scales.

Increasing our knowledge on Earth's climate system is achieved by an interplay between climate models and climate reconstructions. Temperatures reconstructed for past climate events are used to verify the outcomes of computer models that simulate the same climate transition. The other way around, climate modeling results could indicate the reliability of proxy data results. Especially for the high latitudes, however, comparing model results and temperature reconstructions based on proxy data often show relative large discrepancies. Besides improving models, it is therefore essential to gather temperature data from high latitude environments using multiple proxies. Due to a lack of widely applicable proxies, however, temperature reconstructions from the continents are scarce, especially those pre-dating the Quaternary. Here we present results from a new proxy (the MBT/CBT proxy) for annual mean air temperature that is based on the distribution of branched glycerol dialkyl glycerol tetraether (GDGT) membrane lipids derived from soil bacteria. This distribution is related to temperature and soil pH due to the fact that the bacteria have to adapt their cell membrane to these ambient conditions in order to function properly. The membrane lipids are transported via rivers and ice rafted debris to the marine realm, and therefore the terrestrial temperature signal is eventually recorded in the marine sedimentary archive. To investigate the suitability of this proxy for high latitude environments we anlysed branched GDGT distributions in soils and a fjord in Svalbard, Norway. These show that the proxy in soils reflects mean annual air temperature but also point to some in-situ production of branched GDGTs in the fjord system that has to be accounted for when reconstructing annual mean air temperatures. We applied this proxy to reconstruct temperatures of Greenland near the Eocene-Oligocene boundary. The distribution of branched GDGTs delivered by ice rafting and preserved in a core from the Greenland Basin, indicate a gradual 3-5°C cooling at the start of the Oligocene, in agreement with pollen assemblages in nearby cores and other mid-latitude climate reconstructions from North America. Finally, an application in a marine sediment core from the Lomonosov Ridge in the Arctic Ocean indicates high air temperatures in the Arctic of up to 25°C during the Paleocene-Eocene thermal maximum (~55.5 Ma). These high temperatures support an earlier reconstruction of sea surface temperatures in the Arctic Ocean from the same core using the TEX86 proxy, which shows temperatures of up to 23°C. These results show great promise for future applications which could aid in resolving the discrepancy between model and proxy temperature data at high latitudes.

The recently developed MBT/CBT (methylation of branched tetraethers/cyclisation of branched tetraethers) proxy is based on the relative abundance of branched glycerol dialkyl tetraether (GDGT) lipids, derived from bacteria that occur ubiquitously in soils and peats. The degree of cyclisation and methylation, expressed in the CBT and MBT indices, show an empirical relation with soil pH, and with both mean annual air temperature and soil pH, respectively. Hence, continental paleotemperatures can be reconstructed based on the combination of these two indices [1]. Straight forward validation of the MBT/CBT proxy by cultivation experiments is not possible, since the exact group of bacteria that produces the branched GDGTs is still unknown. Therefore, we tested the influence of soil pH and temperature on the distribution of branched GDGTs in natural and manipulated environments. Indeed, branched GDGT distributions in geothermally heated soils and along altitude transects vary systematically with temperature. Likewise, branched GDGT distributions in a long term pH-manipulated field experiment track the changes in soil pH. Absolute concentrations of branched GDGTs in the pH-manipulated fields are highest at low pH, and suggest that the branched GDGT-producers may be found in the phylum of Acidobacteria. A large variety of cultures are currently screened for these compounds. Reference: [1] Weijers et al., 2007, GCA 71, 703-713.

Based on a global survey of bacterial derived branched glycerol dialkyl glycerol tetraether (GDGT) membrane lipids in soils, it has been found that the degree of methylation and the degree of cyclisation of branched tetraethers (MBT and CBT, respectively) correlates with annual mean air temperature (MAT) and soil pH. This relation is used in the MBT-CBT proxy to reconstruct past