Holocene Sapropel Deposition in The Gulf of Kuşadası: Evidence from Sediment Cores Multi-Proxy Data

Abstract

Holocene sedimentary records were obtained by three sediment cores retrieved from Gulf of Kuşadası (Aydın, Turkey), located in the SW part of the Aegean Sea. The sediment cores taking several depths were examined by using multi-proxy analyses to investigate paleoclimatic and palaeoceanographic changes, took place during the last 12 ka before present (BP). The µ-XRF data together with Total Organic Carbon (TOC) contents of the sediment cores reveals the deposition of two discrete sapropels in the gulf that timely coincide with the middle (SMH) and early Holocene (S1) sapropel layers previously described elsewhere from the Aegean and Mediterranean seas. The deposition of those sapropels is associated with high climate oscillations that gave rise to different palaeoceanographic conditions in the gulf as inferred from the multi-proxy dataset. The beginning of the Holocene is represented by elevated lithogenous elements (e.g. K and Ti in µ-XRF) values whereas gradual increases in biogenic calcite production together with partly higher TOC content imply marine organic productivity that was possibly provided by warm and wet climate conditions until the deposition of early Holocene sapropel (S1). The remarkable high value in µ-XRF Sr could be attributed to elevated salinity during the post-glacial warming that in turn may have given rise to increasing marine organic productivity. The climatic deterioration to a cooler and drier phase during the early to middle Holocene is assigned to the deposition of the S1 sapropel based on the TOC content of the core that is subdivided into two halves, S1a and S1b, respectively. The prominent increase in µ-XRF Fe/Mn in the initial half of the S1 implies that deposition of the S1a took place under a relatively poorly-oxygenated deep water condition in the gulf whereas it becomes relatively sub-oxic during the latter half of the sapropel deposition (S1b). The mid-to-late Holocene is represented by the deposition of the younger sapropel layer (SMH) in the gulf which is represented by a lower TOC content in comparison to the older sapropel layer (S1). Its initial deposition in the gulf is associated with more oxygenated deep water conditions under warmer and drier climates as depicted by higher µ-XRF Sr and lower Ca element values. After a short-term interruption, the deposition of the latter half of the SMH is represented by a lesser TOC content and increasing lithogenous elements (e.g., µ-XRF K and Ti) as a result of a relatively wetter climate during the latest Holocene. In contrast, the lower µ-XRF Ca and Ca/Ti values strongly suggest decreased marine organic productivity, thus, the main source of the organic matter was possibly derived from the detrital supply in contrast to the older sapropel (S1). The SMH sapropel deposition in the gulf was followed by increasing humidity with warmer climatic conditions, even though oxygenated deep-water conditions could not have allowed for better preservation of marine organic matter.