Severe late Miocene droughts affected western Eurasia

https://doi.org/10.1016/j.gloplacha.2021.103644Get rights and content

Highlights

  • Three droughts peaking at 9.65, 9.4 and 7.9 Ma disturbed circum-Paratethys ecosystem.

  • Carbon isotopes indicate increase in C4 plant contribution during these droughts.

  • Increased fire activity during droughts parallel changes towards open vegetation.

  • Alleged connection to a proximal basin may trigger the alkenone producers advent.

  • Decoupling of Paratethys from the global climate and hydrology hints its importance.

Abstract

A large and highly dynamic aquatic system called Paratethys governed important elements of the middle and late Miocene (15.97–5.33 Ma) hydrology in western Eurasia. So far, the impact of the vast Paratethys water body on the Eurasian climate, however, is not yet understood. Here we apply biomarker analyses coupled to compound-specific hydrogen and carbon isotope data to track changes in sea surface temperature, mean annual air temperature, hydrological budget and vegetation changes to reconstruct long-term western Eurasian climate conditions between 12.7 and 7.65 Ma in the Black Sea region. Biomarker data from Panagia (Russia) indicate the presence of three exceptionally evaporative intervals peaking at 9.65, 9.4 and 7.9 Ma. These peaks in evaporation relate to aridity, parallel increasing fire activity and are associated with changes in vegetation. Carbon isotope and pollen data support the evidence of an increase in C4 plants associated with these dry intervals. At 9.66 Ma, alkenone producing algae appear in the basin and thrive for the subsequent two million years. Cumulative fluctuations in both hydrology and surface temperature of Paratethys might have enhanced rainfall seasonality in western Eurasia as a response to changes in evaporation over the Paratethys basin. Our combined data suggest a strong regional imprint on overall climate patterns, dominated by basin dynamics causing Paratethys volume and surface reduction. Collectively, the presented biomarker results provide evidence of severe droughts affecting the late Miocene circum-Paratethys region, leading to a direct impact on the evolution of biota in the basin and its surroundings.

Introduction

The interplay between tectonics, landscape and climate change influenced the middle and late Miocene development of the Eurasian continental interior (Fig. 1). As a result, the Paratethys, a large epicontinental water body, became fragmented into smaller basins during the late Miocene (11.6–5.3 Ma), with temporary or no connection to the global oceans (Popov et al., 2006; Fig. 1). Such semi-isolated water bodies are highly sensitive to changes in their hydrological balance, which is directly reflected in water temperature, salinity, circulation, and therefore oxygenation of the water column. A general lack of age-diagnostic marine biota characterizes the Paratethyan sub-basins, causing difficulties for reliable biostratigraphic correlation to the marine realm. Furthermore, commonly used geochemical methods in paleoceanography (e.g. stable oxygen and carbon isotopes on foraminifera) are of limited use as the high degree of endemism led to the absence of widespread marine (e.g. planktonic) foraminifera species. As Paratethys water bodies evolved (quasi)disconnected from the global oceans they acquired particular environmental characteristics which can be untangled only through multi-proxy approaches.

Recently established magneto-biostratigraphy provides a reliable geochronological framework for the middle to late Miocene Panagia section of the Eastern Paratethys (Popov et al., 2016; Palcu et al., 2021), a section exposed on the Taman Peninsula (northern Black Sea, Russia; Fig. 1). Here, we investigate the biomarker record of the Panagia section to reconstruct the climatic conditions between 12.7 and 7.65 Ma (Volhynian to Khersonian local stages), with a special focus on the 9.75 to 7.65 Ma (latest Bessarabian to Khersonian) time interval. We use biomarker analyses coupled to compound-specific hydrogen (δ2H) and carbon (δ13C) isotopes to track a complex array of environmental changes. We reconstruct: 1) sea surface temperatures (SSTs) using isoprenoidal glycerol dialkyl tetraethers (isoGDGTs), biomarkers synthesized by Archaea group that reflect the near-surface conditions within the water column; 2) mean annual air temperatures (MAT) based on branched (brGDGTs), biomarkers produced primarily by soil bacteria in the circum-Paratethys region. We further analyze changes in the hydrological budget of the Paratethys basin through: 3) δ2H values from alkenones (produced by coccolithophoriid algae within the uppermost Paratethys water column) and 4) δ2H measured on long chain n-alkanes (produced by higher terrestrial plants recording precipitation changes in the basin catchment). We monitor: 5) changes in basin productivity through δ13C values of alkenones and reconstruct 6) changes in vegetation surrounding the basin using δ13C values of n-alkanes. Biomarker data are finally supplemented by 7) charcoal analysis and coupled to existing palynological data (Razumkova, 2012) to identify changes in paleo-fire activity and paleo-vegetation.

Similar proxy records previously identified two phases of severe droughts in the Black Sea region (around 8 and 5.8 Ma; Vasiliev et al., 2013, Vasiliev et al., 2015, Vasiliev et al., 2019). The present work significantly extends these records back in time to 12.7 Ma, identifying major environmental changes in the Eastern Paratethys that affect large parts of Eurasia.

Section snippets

Stratigraphy of sampled interval and age model

The Panagia section (45°09′ N, 36°38′ E, Taman Peninsula, Black Sea coast, Russia; Fig. 1) covers a significant part of the late Miocene in the Eastern Paratethys (Popov et al., 2016). Because of protracted endemism in the Paratethys basin, we rely on the regional stratigraphy, with the late Miocene successions divided into the Sarmatian sensu lato (s.l.) stage with Volhynian, Bessarabian and Khersonian substages and the Maeotian and Pontian stages (Fig. 2). The 638 m thick Panagia section

Organic geochemistry, lipid extraction, fractions separation and analyses

Fifty-seven sedimentary rock samples weighing between 14 and 32 g were analyzed. The complete and detailed workflow for the organic geochemistry is presented in the supplementary material online. In short, the fifty-seven selected samples underwent drying, grinding, total lipid extraction (TLE) and removal of elemental sulphur. Afterwards, a fraction of the TLE was archived. The remainder was then separated using Al2O3 column chromatography into apolar, ketone and polar fractions. The apolar

SST estimates based on isoGDGTs

The amplitude of calculated SSTs in the Panagia record varies widely with a temperature variation of 17 °C (regardless of the choice of calibration; Fig. 3A; Supplementary Table 1). For the lower part of the record (0–360 m, 12.7–9.66 Ma) estimated SSTs average 20 °C and vary between 13 and 28 °C (TEX86H calibration of Kim et al., 2010). For a short interval (330 to 360 m) temperatures raise remarkably attaining mean values of 27 °C. At 360 m the TEX86H estimated SSTs drop rapidly and remain

Discussion: paleoenvironmental changes between 12.7 and 7.65 Ma

The absolute age constraints for the Panagia section are based on the magnetostratigraphic pattern comprising nine normal and eight reversed polarity intervals with additional seven short-term polarity fluctuations (Palcu et al., 2021) that were correlated to the geomagnetic polarity time scale (GPTS; Hilgen et al., 2012; Ogg, 2020). The transition between Konkian and Volhynian dated at 12.65 Ma (Palcu et al., 2017) is located at 2 m in the section. The interval between 2 and 120 m has poor

Conclusions

The integrated temperature, δ2H and δ13C isotope compositions of n-alkanes and alkenones combined with charcoal data reveal distinctive environmental conditions during the late Miocene in the Eastern Paratethys of Central Eurasia from the well-dated Panagia section (Taman Peninsula, Russia). Based on the multiproxy approach, we observe a series of important environmental events:

  • 1)

    Between 9.68 and 9.66 Ma a short event generated much warmer and most probably more saline waters in the Eastern

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by Netherlands Organization for Scientific Research (NWO) [grant 865.10.011] of W.K., by German Science Foundation (DFG) [grant VA 1221/2-1] of I.V. and Senckenberg Gesellschaft für Naturforschung. D.P. acknowledges the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) for financial support [grant 2018/20733-6]. I.V. and W.K. thank the sampling team (A. Iosifidi, V. Popov, S. Popov and M. Stoica) during 2005–2006 campaigns. G.B. and I.V. thank

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