Late Glacial to Holocene benthic foraminifera in the Marmara Sea: implications for Black Sea–Mediterranean Sea connections following the last deglaciation

Abstract

Benthic foraminifera were studied from four gravity cores that penetrated Holocene marine sediments in the Marmara Sea. Morphogroup and assemblage analyses reveal that the Holocene sea-level rise did not result in a catastrophic flooding event as proposed by W.B.F. Ryan and others, whereby well-oxygenated, saline Mediterranean waters rapidly inundated a low-lying low salinity ‘Black Sea Lake’ at ∼7.15 ka (popularly known as the ‘Noah’s Flood Hypothesis’). Rather, the benthic foraminiferal data confirm the hypothesis that the Dardanelles sill was breached by the Mediterranean at ∼12 ka, allowing saline waters to penetrate the Marmara Sea. These saline waters reached the level of the Bosphorus sill at ∼9.5 ka, but were unable to penetrate into the Black Sea until after ∼9.1 ka because of the persistent strong outflow of brackish to fresh water from the Black Sea. The initial colonisation of the Marmara Sea by benthic foraminifera is essentially synchronous with the re-establishment of marine connections through the Dardanelles Strait at ∼12 ka. By ∼10 ka, Ammonia-dominated faunas developed on the strait-exit delta (Δ1) at the southern end of the Bosphorus, and at ∼9.1 ka the appearance of fully marine species documents the establishment of a more stratified water column over Δ1. Finally, the increase in abundance of planktonic foraminifera at the southern exit of the Bosphorus after ∼6.1 ka reflects a decreased volume of outflow water from the Black Sea. Quantitative analysis of benthic foraminiferal morphogroups reveals that the oxygen content of sub-halocline water was low (below ∼1.5 ml/l) throughout the Holocene, and the occurrence of sapropel sediment in the deeper part of the basin suggests bottom waters may have been anoxic at times. After ∼4.5 ka, an increase in benthic foraminiferal oxic morphotypes suggests a reduction in Black Sea outflow and weakening of the halocline. The strong and persistent stratification of the water column in the Marmara Sea throughout the Holocene is entirely incompatible with the ‘Noah’s Flood Hypothesis’.