Abstract
Abstract. The Baltic Sea experienced changes in marine input throughout the Holocene as substantial regional ice retreat led to isostatic adjustment, eustatic sea level change, and periodic isolation from the North Sea. Here, we determine the distributions and isotopic signatures of organic compounds preserved in a sediment record spanning the last ∼11 kyr to reconstruct environmental change under these dynamic conditions. Carbon and hydrogen isotope ratios of short-, mid-, and long-chain n-alkanes, along with long-chain diol and glycerol dialkyl glycerol tetraether abundances, were analyzed in sediments sampled from the Arkona Basin in the southwestern Baltic Sea. In the earlier part of the record (specifically 10–8.2 ka), hydrogen isotope values of higher plant-derived n-alkanes revealed a change in dominant water source from an ice-melt-derived to a precipitation-dominated hydrological regime. Following this shift in water source, carbon isotope values of n-alkanes suggest diversification of vegetation. Shifts in hydrology and vegetation did not coincide with established phase boundaries but instead occurred mid-phase or spanned phase transitions. For this reason, we suggest describing the Ancylus Lake in two phases: a meltwater phase (10.6 to 9.2 ka) and a precipitation phase (9.2 to 7.7 ka). Our results highlight the utility of using C and H isotope ratios in conjunction with other compound abundance proxies to better understand the complex environmental change recorded in paleoarchives.
Highlights
The Baltic Sea, located in northwestern Europe, is a semienclosed marine basin characterized by restricted circulation, strong terrestrial influence, and relatively low oxygen concentrations that promote enhanced sedimentary organic matter contents and diverse phytoplankton communities (e.g., Wasmund and Uhlig, 2003; Golubkov et al, 2021)
total organic carbon (TOC) and total nitrogen (TN) ranged from 0.5 % to 7.2 % and 0.1 % to 0.8 %, respectively, with higher values in the most recent part of the core (Fig. S1 in the Supplement). δ13Cbulk values varied by ∼ 3 ‰ in the Early Holocene followed by a small shift after the marine transgression (Fig. S1). δ15Nbulk values were elevated at the transition from the Yoldia Sea (YS) to the Ancylus Lake (AL) (5.5 ‰ ) followed by a drop to 2.8 ‰ in the early AL phase
C and N (C/N) ratios are plotted against δ13Cbulk values for all samples; a mix of freshwater, marine, and terrestrial C3 plant-derived values are recorded, which are grouped according to established Baltic Sea phases (Fig. 3)
Summary
The Baltic Sea, located in northwestern Europe, is a semienclosed marine basin characterized by restricted circulation, strong terrestrial influence, and relatively low oxygen concentrations that promote enhanced sedimentary organic matter contents and diverse phytoplankton communities (e.g., Wasmund and Uhlig, 2003; Golubkov et al, 2021). There is an open connection between the North Sea and the Baltic Sea via the Danish Straits in the west; this connection has not been permanent, causing the basin to experience substantial environmental change throughout the Holocene. The complex climate dynamics caused substantial shifts in the salinity of the Baltic Sea during the Holocene, reconstructed from geochemical and micropaleontological data (e.g., Alhonen, 1971; Ning et al, 2017; Ni et al, 2020; Weiss et al, 2020). Salinity changes in the Baltic Sea largely indicate periods of connection or isolation from the North Sea via the Danish Straits. The timing of different Holocene phases of the Baltic Sea is debated and appears to be divergent at different locations in the basin (Gustafsson and Westman, 2002; Moros et al, 2002). The dates used hereafter for Baltic Sea phases are from two previously studied Arkona Basin
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