Black Sea “Lake” reservoir age evolution since the Last Glacial — Hydrologic and climatic implications

Abstract

Chronologies of sediments that document the last glacial history of the Black Sea "Lake" are hampered by issues relating to reservoir age. Regulated by basin hydrology, reservoir ages represent a tool that could potentially be used to better understand the response of Black Sea "Lake" hydrology to climate change. Therefore, deciphering reservoir age evolution is crucial both for better constraining the basin chronological framework and for providing new insights into our understanding of Black Sea "Lake" hydrology. By tuning a meaningful new high-resolution geochemical dataset (obtained from core MD04-2790) to a climate reference record, here, we propose a reliable chronology spanning the last 32 kyr BP. The chronology is compared to a large AMS radiocarbon dataset (n = 51). Pairs of calendar and radiocarbon ages allowed us to compute reservoir ages, and to, then, reconstruct a high-resolution quantitative reservoir age record for the last glacial history of the Black Sea "Lake". The main factor controlling reservoir ages in lakes is the Hard Water Effect (HWE), which is regulated by basin hydrology. Therefore, changes in the reconstructed reservoir age record have been qualitatively interpreted in terms of the hydrologic responses of the Black Sea "Lake" to climate change. Our results allowed us to determine periods of complete isolation or outflow for the Black Sea "Lake". During Heinrich Event 2 (HE2) and during the Last Glacial Maximum (LGM) the basin was strictly isolated, whereas prior to HE2 and during HE1 it outflowed into the Marmara Sea. Following the onset of the Bølling-Allerød, factors other than the HWE are thought to have influenced the reservoir age, preventing conclusive interpretations. We also determined an undocumented, to date, phase of Black Sea "Lake" stratification during the full glacial (HE2 and LGM). Our results indicate that reservoir age is a powerful tool for investigating and better understanding past hydrologic changes in lakes and inland seas. Research highlights ► Reservoir ages record hydrologic responses to climate change. ► By tuning, we provide a calendar chronology for Black Sea "Lake" sediments. ► Then, using a 14 C age dataset, we provide a highresolution reservoir age record. ► The Black Sea "Lake" outflowed into the Marmara Sea prior to HE2 and during HE1. ► The basin was strictly isolated during the LGM, with water column stratified.

Abrupt drainage cycles of the Fennoscandian Ice Sheet

“…Indeed, the early stage of Baltic Sea filling occurred at ca. 16-15 ka (11,23), which lends support to the concomitant disappearance of water pulses observed in the BS (15.7 ± 0.3 ka) (16).…”

“…Additionally, to the south, the hydrology of the BS was also strongly affected by the cyclic drainage of Lake Disna. The increase in runoff due to repeated seasonal floods over four successive periods of ∼220 y raised the BS water level by around 100 m, until the sill of the Bosphorus Strait was reached (12,13,16,44,45), leading to the flooding of the vast northwestern shelf of the BS basin and the attendant drastic retreat of the coastline (SI Text). The resulting outflow of BS waters into the northeastern Mediterranean basins altered the hydrology of both the Marmara Sea (45) and the Aegean Sea (46,47).…”

“…1A) was recently suggested by the discovery of peculiar sedimentary structures known as the Red Layers (RLs). RLs are a series of four individualized intervals of reddishbrown clays and are thought to represent the sedimentary imprints of the meltwater inputs (12)(13)(14)(15) that occurred during Heinrich Stadial 1 (HS1) (16). To date, however, the genesis and origin of the RLs remain enigmatic, even if three main hypotheses have been proposed to account for their occurrence in BS sediment sequences.…”

“…Lake Disna initially increased in size over ca. 3,000 y between ∼20 ka (10) and ∼17 ka, at which time the first BSWP occurred (16). Lake level rise caused the formation of a calving wall at the ice front.…”

“…To constrain the geographical origin of the RLs and to ascertain their meltwater-induced deposition, we measured inorganic tracers for sediment provenance [X-ray fluorescence (XRF) Ti/Ca ratio and e Nd signatures] and biomarkers as proxies for terrigeneous supply to the BS [branched and isoprenoid tetraether (BIT) index and C 25 -alkane/total organic carbon (TOC)] in the well-dated core MD04-2790 (16,26) collected in the near vicinity of the mouths of the Danube and Dniepr Rivers (Fig. 1A).…”