Middle to Late Holocene Variations in Salinity and Primary Productivity in the Central Baltic Sea: A Multiproxy Study From the Landsort Deep

Abstract

Anthropogenic forcing has led to an increased extent of hypoxic bottom areas in the Baltic Sea during recent decades. The Baltic Sea ecosystem is naturally prone to the development of hypoxic conditions due to its geographical, hydrographical, geological, and climate features. Besides the current spreading of hypoxia, the Baltic Sea has experienced two extensive periods of hypoxic conditions during the Holocene, caused by changing climate conditions during the Holocene Thermal Maximum (HTM; 8-4.8 cal ka BP) and the Medieval Climate Anomaly (MCA; 1-0.7 cal ka BP). We studied the variations in surface and bottom water salinity and primary productivity and their relative importance for the development and termination of hypoxia by using microfossil and geochemical data from a sediment core retrieved from the Landsort Deep during IODP Expedition 347 (Site M0063). Our findings demonstrate that increased salinity was of major importance for the development of hypoxic conditions during the HTM. In contrast, we could not clearly relate the termination of this hypoxic period to salinity changes. The reconstructed high primary productivity associated with the hypoxic period during the MCA is not accompanied by considerable increases in salinity. Our proxies for salinity show a decreasing trend before, during and after the MCA. Therefore, we suggest that this period of hypoxia is primarily driven by increasing temperatures due to the warmer climate. These results highlight the importance of natural climate driven changes in salinity and primary productivity for the development of hypoxia during a warming climate.

Holocene Spatiotemporal Redox Variations in the Southern Baltic Sea

“…A secondary peak in TOC observed at Lille Belt (Figure 2B) and Landsort Deep (Figure 4B; Dijkstra et al, 2016Dijkstra et al, , 2018aHardisty et al, 2016) is contemporaneous with the MCA (1.2-0.8 ka) and has been observed previously in both Fårö and Gotland Deeps (Jilbert and Slomp, 2013). Enhanced productivity and organic carbon burial during this interval occurred with warmer sea surface temperatures (Kabel et al, 2012;van Wirdum et al, 2019), all of which likely contributed to de-oxygenation during the MCA (Papadomanolaki et al, 2018).…”

“…The lowermost peak can be linked to the HTM (Figures 2B, 3B, 4B), lasting from 8 to 4 ka (Jilbert and Slomp, 2013;Dijkstra et al, 2016;Hardisty et al, 2016;van Helmond et al, 2017). The HTM was marked by relatively higher temperatures and higher sea level and reduced freshwater input relative to today, likely representing the most saline period in the Baltic Sea since the LGM (Gustafsson and Westman, 2002;van Wirdum et al, 2019;Ni et al, 2020;Warnock et al, 2020). These factors likely contributed to the onset of anoxia observed during this interval (Papadomanolaki et al, 2018).…”

“…These intervals and horizons are markers that can be used to correlate syndepositional periods and events among the sub-basins. The Lille Belt, Bornholm Basin, and Landsort Deep show a systematic shift in diatom assemblage marking the transition from lacustrine to brackish/marine water column conditions-that is, the Ancylus Lake to Littorina Sea transition (Figures 2A, 3A, 4A; Sohlenius et al, 2001;van Wirdum et al, 2019;Warnock et al, 2020). The Initial Littorina Stage, the transition period between the lacustrine Ancylus Lake and brackish Littorina Sea, lasted from ∼9.8 to 8.5 ka and is…”

“…Sulfide build up, in turn, was related to input of saline water and associated sulfate into the basin during the Initial Littorina Sea. The overlying interval where salinity-tolerant diatoms become relatively more abundant marks the onset of the Littorina Sea and the brackish conditions that still exist today (Sohlenius et al, 2001;van Wirdum et al, 2019;Warnock et al, 2020). This transition occurred 7500-7150 BP at Lille Belt (Kotthoff et al, 2017;Warnock et al, 2020).…”

Holocene Hydrographic Variations From the Baltic‐North Sea Transitional Area (IODP Site M0059)

“…Hypoxia is defined as <1.4 ml/L dissolved oxygen concentration ([O 2 ]) in the water. It has occurred intermittently, in particular in the deepest basins, in the Baltic Sea throughout the Holocene (Jilbert & Slomp, ; Kabel et al, ; van Wirdum et al, ), and the area of hypoxia has expanded sixfold from 1950 to 2000 CE (Carstensen et al, ). Baltic Sea hypoxic events have been steadily increasing with time and affect nutrient biogeochemical processes, ecosystem services, coastal habitats (Conley et al, , ; Ning et al, ), and fisheries (Breitburg, ).…”