Abstract
AbstractDeoxygenation affects many continental shelf seas across the world today and results in increasing areas of hypoxia (dissolved oxygen concentration ([O2]) <1.4 ml/L). The Baltic Sea is increasingly affected by deoxygenation. Deoxygenation correlates with other environmental variables such as changing water temperature and salinity and is directly linked to ongoing global climate change. To place the ongoing environmental changes into a larger context and to further understand the complex Baltic Sea history and its impact on North Atlantic climate, we investigated a high accumulation‐rate brackish‐marine sediment core from the Little Belt (Site M0059), Danish Straits, NW Europe, retrieved during the Integrated Ocean Drilling Program (IODP) Expedition 347. We combined benthic foraminiferal geochemistry, faunal assemblages, and pore water stable isotopes to reconstruct seawater conditions (e.g., oxygenation, temperature, and salinity) over the past 7.7 thousand years (ka). Bottom water salinity in the Little Belt reconstructed from modeled pore water oxygen isotope data increased between 7.7 and 7.5 ka BP as a consequence of the transition from freshwater to brackish‐marine conditions. Salinity decreased gradually (from 30 to 24) from 4.1 to ~2.5 ka BP. By using the trace elemental composition (Mg/Ca, Mn/Ca, and Ba/Ca) and stable carbon and oxygen isotopes of foraminiferal species Elphidium selseyensis and E. clavatum, we identified that generally warming and hypoxia occurred between about 7.5 and 3.3 ka BP, approximately coinciding in time with the Holocene Thermal Maximum (HTM). These changes of bottom water conditions were coupled to the North Atlantic Oscillation (NAO) and relative sea level change.
Highlights
The Baltic Sea is one of the world's largest semi‐enclosed brackish water basins with a catchment area of 2.13 × 106 km2, which is nearly 20% of the European continent (Nilsson, 2006)
By using the trace elemental composition (Mg/Ca, Mn/Ca, and Ba/Ca) and stable carbon and oxygen isotopes of foraminiferal species Elphidium selseyensis and E. clavatum, we identified that generally warming and hypoxia occurred between about 7.5 and 3.3 ka BP, approximately coinciding in time with the Holocene Thermal Maximum (HTM)
We substantially increase the temporal resolution of the data sets and we present new proxy variables (trace element/Ca measured by Laser Ablation Inductively Coupled Mass Spectrometry (LA‐ICP‐MS) and pore water oxygen isotopes (δ18Opw))
Summary
The Baltic Sea is one of the world's largest semi‐enclosed brackish water basins with a catchment area of 2.13 × 106 km, which is nearly 20% of the European continent (Nilsson, 2006). The sea experiences a considerable freshwater input from river runoff from the large surrounding catchment area (16,100 m3 s−1) but only a restricted inflow of saline water from the North Sea through the narrow Danish Straits (the Little Belt, the Great Belt, and the Öresund; Figure 1), making it a prime example of a restricted epi‐continental sea. The restricted water exchange results in a permanent halocline that separates an upper layer of brackish water from more saline bottom waters. The salinity changes in the Baltic depend on the irregular inflow of North Sea water through the Danish Straits and the amount of freshwater runoff, which are in turn controlled by climatic factors in the Atlantic (Hänninen et al, 2000). The Baltic freshwater outflow into the Nordic Seas has a large potential influence on water mass transformation (Lambert et al, 2016, 2018; Winsor et al, 2001), by enhancing Polar Water outflow and suppressing Deep Water outflow (Lambert et al, 2018)
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