New Paper: Arctic Sea ‘Ice-Free’ During Early Holocene
Written by Kenneth Richard
Biomarker evidence for Arctic-region sea ice coverage in the northern Barents Sea indicates the most extensive sea ice conditions of the last 9,500 years occurred during the 20th century (0 cal yr BP).
In contrast, this region was ice-free with open water conditions during the Early Holocene (9,500-5,800 years ago).
Atlantic water inflow & sea ice distribution in the northern Barents Sea: A Holocene palaeoceanographic evolution
The early Holocene (ca. 9500 – 5800 cal yr BP) … Relatively low IP25 concentrations [a proxy for sea ice presence] with increased brassicasterol abundances indicate reduced seasonal (spring) sea ice cover and longer (warmer) summers with open water conditions suitable for phytoplankton production.
The occurrence of reduced sea ice cover and longer summers is consistent with increased planktic foraminiferal concentrations (reported here and Carstens et al., 1997) and with longer ice-free seasons and a retreated ice margin in the northern Barents Sea (Duplessy et al., 2001) as well as increased phytoplankton production in the northern Fram Strait (Müller et al., 2009).
Reduced spring sea ice cover also indicates the HTM recorded at the sea surface between ca. 9300 and 6500 cal yr BP, which probably results from maximum summer insolation at 78° N.”
Our proposed sea ice scenario suggests that water masses south of the study area were ice free, which agrees with open water conditions observed in the western Barents Sea (Berben et al., 2014) and the West Svalbard margin (Müller et al., 2012) during the early Holocene.
For the West Svalbard margin, Werner et al. (2013) associated high planktic foraminiferal fluxes ca. 8000 cal yr BP to ice-free or seasonally fluctuating sea ice margin conditions.
The PBIP25 index shows the lowest values of the record (0.16 – 0.40) suggesting a period characterized by low or variable seasonal sea ice cover and influenced substantially by open water conditions (Müller et al., 2011).
The late Holocene (ca. 2200 – 0 cal yr BP) is characterized by the highest abundances of IP25 (0.35 µg/g OC)and relatively low (but stable) brassicasterol (12.5 µg/g OC) (Figure 7A-B).).
Consistent with the opposing trends in the IP25 and brassicasterol records, the PBIP25 values reach their highest value (0.87) of the record at ca. 0 cal yr BP. An increase in PBIP25 suggests a further extension in sea ice cover, reflecting Arctic Front conditions (Müller et al., 2011), most similar to modern conditions.
The Early Holocene was about 6-7°C warmer than today in this region (NW Barents Sea).
Image Source: Tarasov et al., 2018
Another recent reconstruction for this region also indicated the Early Holocene was sea ice free and that modern sea ice conditions are among the most extensive of the last 9,500 years.
Image Source: Köseoğlu et al., 2018
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