Palaeoenvironmental and diagenetic reconstruction of a closed‐lacustrine carbonate system – the challenging marginal setting of the Miocene Ries Crater Lake (Germany)

Abstract

Chemostratigraphic studies on lacustrine sedimentary sequences provide essential insights on past cyclic climatic events, on their repetition and prediction through time. Diagenetic overprint of primary features often hinders the use of such studies for palaeoenvironmental reconstruction. Here the potential of integrated geochemical and petrographic methods is evaluated to record freshwater to saline oscillations within the ancient marginal lacustrine carbonates of the Miocene Ries Crater Lake (Germany). This area is critical because it represents the transition from shoreline to proximal domains of a hydrologically closed system, affected by recurrent emergent events, representing the boundaries of successive sedimentary cycles. Chemostratigraphy targets shifts related to subaerial exposure and/or climatic fluctuations. Methods combine facies changes with d 13 C-d 18 O chemostratigraphy from matrix carbonates across five closely spaced, temporally equivalent stratigraphic sections. Isotope composition of ostracod shells, gastropods and cements is provided for comparison. Cathodoluminescence and back-scatter electron microscopy were performed to discriminate primary (syn-)depositional, from secondary diagenetic features. Meteoric diagenesis is expressed by substantial early dissolution and dark blue luminescent sparry cements carrying negative d 13 C and d 18 O. Sedimentary cycles are not correlated by isotope chemostratigraphy. Both matrix d 13 C and d 18 O range from ca À7Á5 to +4Á0& and show clear positive covariance (R = 0Á97) whose nature differs from that of previous basin-oriented studies on the lake: negative values are here unconnected to original freshwater lacustrine conditions but reflect extensive meteoric diagenesis, while positive values probably represent primary saline lake water chemistry. Noisy geochemical curves relate to heterogeneities in (primary) porosity, resulting in selective carbonate diagenesis. This study exemplifies that ancient lacustrine carbonates, despite extensive meteoric weathering, are able to retain key information for both palaeoenvironmental reconstruction and the understanding of diagenetic processes in relation to those primary conditions. Also, it emphasizes the limitation of chemostratigraphy in fossil carbonates, and specifically in settings that are sensitive for the preservation of primary environmental signals, such as lake margins prone to meteoric diagenesis. 235

Evolution of organic matter quantity and quality in a warm, hypersaline, alkaline lake: The example of the Miocene Nördlinger Ries impact crater, Germany

“…As one of the best-preserved and best-known impact structures on Earth (Hörz et al, 1983;Stöffler et al, 2013), NR is characterized by a pronounced structural rim around the crater (Rullkötter et al, 1990; Figure 1). A hydrologically closed lacustrine depositional system (Rothe and Hoefs, 1977;Arp et al, 2014;Christ et al, 2018) formed after the impact, exhibiting an alkaline depositional water body for most of the time and silting up during the late Miocene (Arp et al, 2017;Stüeken et al, 2020). With the rising interests in paleo-environmental archives and post-impact sedimentation processes in general, not only the impact process itself but also the post-impact sequence in the NR became a hot spot for scientific studies lately (e.g., Stöffler et al, 2013;Arp et al, 2014;Sturm et al, 2015;Arp et al, 2017;Christ et al, 2018;Arp et al, 2019;Stüeken et al, 2020;Arp et al, 2021;Zeng et al, 2021).…”

The Limagne Basin: a journey through modern and fossil microbial deposits

“…Similar environmental conditions have been proposed in the northern part of the Limagne Basin (Wattinne et al, 2003) and the Reis impact crater (Germany; e.g. Reis, 1921;Riding, 1979;Christ et al, 2018). The maximum water level elevation is estimated from the maximum vertical development of the microbialites without interruption during their growth, until the formation of an erosion or emersion surface, i.e.…”

Petrography and Provenance of the Sub-Himalayan Kuldana Formation: Implications for Tectonic Setting and Palaeoclimatic Conditions

“…The association of gastropods and bivalves suggests brackish, lagoonal facies. One of the most distinguishing features of the lagoon is that almost all gastropod shells are filled with thin cement [80]. The abundance of nummulites and associated foraminifera indicated deposition in an open sea or inner-ramp environment [81].…”