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Hinrichs Lab - Paleo studies

 Introduction

Biomarkers are frequently used to reconstruct environmental conditions and changes in recent and especially ancient environmental settings. Classical applications include the determination of past sea surface and ocean temperatures by using the so-called paleo-proxies UK’37 and TEX86 that are based on the relative distribution of unsaturation of the C37 alkenones (e.g., Prahl & Wakeham, 1987) or the internal variation of pentacyclic rings of glycerol dialkyl glycerol tetraethers (GDGTs; Schouten et al., 2002), respectively. The robustness of these proxies is tested at the moment in a high resolution study focusing on samples from the Gallipoli shelf lead by the Heisenberg stipend Dr. Gerard Versteegh (MOCCHA project) (Leider et al., 2010).

The abundance of certain biomarker groups such as phytoplankton-derived sterols or alkyl diols, straight-chain wax components and lignin phenols of higher land plants, and glycerol-based alkyl ethers or specific fatty acids of bacterial origin provides insights into the overall community structure at the time of deposition (Schmidt et al., 2010). In addition, the stable carbon isotopic compositions of the biomarkers encodes information on source organisms, biogeochemical processes, and the state and perturbations of the carbon cycle (Elvert and Niemann, 2008; Sepúlveda et al., 2009a, 2009b).

Moreover, climate change has a strong impact on the water cycle and hydrological processes which can be directly monitored by analyzing the hydrogen isotopic composition of the biomarkers.
In the Hinrichs Lab, we are using biomarkers to study ancient life and climate change in marine environments and are currently extending our research interests towards lakes from polar regions. In the latter, we use biomarkers to gain insights into the hydrological evolution and climate history of this environment in the Holocene and to detect variations of methane cycling in permafrost regions. Examples of our research are given below.

Example 1: Structure of planktonic communities and paleonenvironmental conditions during Oceanic Anoxic Event 2 (OAE2)

The greenhouse world of the Mesozoic Era is characterized by the deposition of massive organic-rich black shales in coastal and open ocean areas as well as in epicontinental seas, called Oceanic Anoxic Events (OAEs; Jenkyns, 1980). OAEs represent drastic perturbations of the global carbon cycle and the ecology of primary producers. Different models relate the black shale deposition to increased nutrient supply and/or increased preservation from water column stratification (e.g., Meyers, 2006). However, it is difficult to decipher the interaction between paleoenvironmental conditions, primary production, nutrients and carbon cycling during these events.

We have investigated environmental and ecological changes throughout the Cenomanian-Turonian OAE2 on basis of bulk geochemical properties, distribution of source-specific lipid biomarkers and their molecular isotopic composition from an organic-rich deposit in central Jordan (Sepúlveda et al., 2009a). Beside the occurrence of biomarkers for marine algae and dinoflagellates as the dominant primary producing organisms, we evidenced the contribution of cyanobacteria and green-sulfur bacteria and concluded that changes in plankton communities were associated with sea level changes and water column stratification.

From our biomarker perspective we proposed that OAE2 was characterized by water column stratification, anoxic bottom waters and a deep chemocline. In contrast, biomarkers in the post-OAE deposit indicated a shoaling of the chemocline and the occurrence of photic zone euxinia.

Example 2: Rapid recovery of marine productivity after the Creataceous-Paleogene mass extinction

The Cretaceous-Paleogene extinction event occurred 65 million years ago and belongs to one of the big five mass extinction events in earth history (Raup & Sepkoski, 1982). It is well-known as it marks the extinction of the dinosaurs associated with an asteroid impact and its aftermath effects on the earth’s biosphere (e.g., the disruption of photosynthesis). In contrast, its effect on life in the world oceans and the recovery of marine primary productivity is poorly understood due to the lack of preservable skeletons. A cosmopolitan boundary clay deposit is hypothesized to represent the duration required for the restoration of marine life after the impact.

Our analysis focused on the recovery of the marine ecosystem in the Fish Clay boundary layer at Kulstirenden, Denmark (Sepúlveda et al., 2009b). Bulk geochemical isotopes and the presence of algal steranes and bacterial triterpenoids suggested that photosynthesis was strongly reduced possibly less than a century after the impact followed by a rapid resurgence of carbon fixation and ecological reorganization.