Ejecta erosion, hydrological, and chemical lake evolution in the Ries impact basin (Miocene, Germany)
by Lingqi Zeng
Date of Examination:2022-03-11
Date of issue:2022-04-08
Advisor:Prof. Dr. Gernot Arp
Referee:Prof. Dr. Andreas Pack
Referee:Prof. Dr. Volker Thiel
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EnglishTo inspect past environmental changes and potential habitability, the growing interests for the ancient Martian crater lakes call for better understandings of palaeolimnological evolution of impact crater lakes. Particularly, their hydrological and chemical lake histories are poorly understood because the impact processes partly recreate and complicate the limnological settings. On the other hand, lacustrine deposits are known as sensitive archives of climate changes, superimposed on the limnological processes of the impact crater catchment. The Nördlinger Ries impact structure (Miocene, Germany) hence provides unique and handy insights for similar palaeolimnological processes of the mid-sized impact craters not only on Earth, as well as on the remote, hardly ventured Martian surface. The 14.9 Ma Ries impact event produced an eventually 24 km-wide crater by penetrating a water-saturated, bedded sedimentary cover above the crystalline basement. Well-preserved impact ejecta formations, comprising the suevite (impact melt-bearing breccia), the Bunte Breccia (primary ejecta blanket) and parautochthonous megablocks were present in the catchment and subject to erosion. Lacustrine sedimentation started on top of the crater suevite and over 300-meter-thick post-impact, mixed siliciclastic-carbonate sediments were deposited within ca. 1-2 Ma immediately after the impact. Previous investigations, largely benefited from Sr, C and O isotope of the carbonates, have suggested that the lacustrine succession in the Ries crater may record the successive weathering in the crater lake catchment under hydrologically closed conditions. The two ejecta layers with an inverted stratigraphy (the suevite on top of the Bunte Breccia) lead to chemically distinct weathering products, resulting in different influxes to the lake water and discernible chemical lake stages. However, the throughout and general chemical lake evolution is still poorly constrained. Based on this assumption, it also remains unknown whether the biogeochemical processes, specifically microbial respirations, follow and correspond to the evolving chemical lake stages. Beyond that, due to the difficulty in sedimentary architecture by post-depositional erosion, the hydrological balance between surface runoffs and groundwater along the lake history remains unknown. The Ries lacustrine succession might have also intercepted a climate change event from the warm Miocene Climatic Optimum (MCO) to the long-term cooling during mid-Miocene Climate Transition. Thereby the Ries lacustrine deposits are of great potential to examine how the hydrology and hydrochemistry of an impact crater lake react to possible climate changes. This thesis contains a series of sedimentological, mineralogical, geochemical and isotopic investigations, with most of the focus on a drill core from the central Ries crater (NR1003), aiming at i) reconstructing hydrological, chemical and biogeochemical lake histories of the Ries crater; ii) clarifying the importance of extrinsic changes (i.e. climate change) versus intrinsic change (catchment change and internal tectonics) on the hydrological and chemical crater lake evolutions. The first study shows a systematic trend in biogeochemical processes following the successive erosion of the ejecta formations and hydrological changes, using stable oxygen and carbon isotopes in conjunction with biomarkers. An early sulfidic lake stage with extensive sulfate reduction is characterized by abundant thiophenes as well as biomarkers for sulfate reducing bacteria. The next stage is poor in evidence of sulfate reduction whereas pronouncedly 13C-enriched dolomites are associated with an archaea-derived lipid biomarker, archaeol, demonstrating a methanogenic lake bottom. A subsequent decline in both δ13Ccarb and archaeol follows a decreasing lake level with signatures of subaerial exposure events, supporting aerobic methanotrophy as indicated by the observation of 3-methyl-hopanoids. The second study describes the discovery of a volcanic ash layer throughout the crater basin. The observed deeply bowl-shaped geometry of this layer cannot be reconciled by calculations of sediment compaction based on sediment load and burial alone. Additional sagging would have been attributed to the subsidence of the crater floor substrate and closure of the dilatant macro-porosity, closely related to the deep, in-situ impact fractures. In the third study, to simulate the ejecta erosion processes, a series of leaching experiments were conducted to the pre-impact target formations as well as the ejecta formations. The process from ejecta erosion to lacustrine carbonate precipitation were traced by Sr isotope to inspect the detailed hydrological and chemical lake evolutions. In conjunction with sedimentological evidence and hydrochemical modelling tests, different mixing scenarios of fluid endmember, from leachate or field waters, were tested and reconciled for each lake stage. A systematic provenance change is evident from crystalline basement and suevite, to Bunte Breccia and finally Lower/Middle Jurassic formations. This trend suggests an expanse of the catchment from central crater towards outside the crater, similar to the pre-impact hydrological condition. In addition, the sedimentary structures of the lacustrine deposits reflect a cooling and stabilizing crater floor due to long-term sagging and sediment loading, significantly changed the groundwater discharge balance via the closure of the mega-porosity. Therefore, the unique self-readjusted hydrological and hydrochemical evolutions are independent from climate change but rather controlled by the ejecta erosion and porosity change, both representing the intrinsic limnological processes of an impact crater lake. In the fourth study, as a side topic, the deposits from a supposed impact ejecta-dammed lake, known as “Rezat-Altmühl-Lake”, were investigated because it might be the only known case on Earth. However, no pebbles from impact ejecta have been detected as well as the key criteria for lacustrine lithofacies. Instead, the succession is characterized by less diagnostic floodplain fines with palaeosols, palustrine limestones with root voids and intercalated thin sandstone beds. Low, invariant δ18Ocarb reflects a short water residence time and highly variable δ13Ccarb indicates a variable degree of pedogenesis, substantiating the interpretation of a fluvial setting. The trend in 87Sr/86Sr in the carbonate and bulk carbonate content argue for increasing weathering products from the topographically low Triassics, to Jurassics at higher level.
Keywords: Ries Crater; lacustrine carbonate; carbon and oxygen isotope; strontium isotope; biogeochemistry; hydrology