Group 2i Isochrysidales produce characteristic alkenones reflecting sea ice distribution

Abstract

Alkenones are biomarkers produced solely by algae in the order Isochrysidales that have been used to reconstruct sea surface temperature (SST) since the 1980s. However, alkenone-based SST reconstructions in the northern high latitude oceans show significant bias towards warmer temperatures in core-tops, diverge from other SST proxies in down core records, and are often accompanied by anomalously high relative abundance of the C37 tetra-unsaturated methyl alkenone (%C37:4). Elevated %C37:4 is widely interpreted as an indicator of low sea surface salinity from polar water masses, but its biological source has thus far remained elusive. Here we identify a lineage of Isochrysidales that is responsible for elevated C37:4 methyl alkenone in the northern high latitude oceans through next-generation sequencing and lab-culture experiments. This Isochrysidales lineage co-occurs widely with sea ice in marine environments and is distinct from other known marine alkenone-producers, namely Emiliania huxleyi and Gephyrocapsa oceanica. More importantly, the %C37:4 in seawater filtered particulate organic matter and surface sediments is significantly correlated with annual mean sea ice concentrations. In sediment cores from the Svalbard region, the %C37:4 concentration aligns with the Greenland temperature record and other qualitative regional sea ice records spanning the past 14 kyrs, reflecting sea ice concentrations quantitatively. Our findings imply that %C37:4 is a powerful proxy for reconstructing sea ice conditions in the high latitude oceans on thousand- and, potentially, on million-year timescales.

Rapid Atlantification along the Fram Strait at the beginning of the 20th century

“…A recent publication has shown the presence of C 37 :4 alkenone-producing isochrysidales associated with sea ice that can affect the overall interpretation of alkenone-derived SST ( 77 ). As the concentration of C 37 :4 alkenone in our core is relatively low (14.3 ± 2.9%), we believe that our record of U37K primarily reflects SST dynamics.…”

Rapid northern hemisphere ice sheet melting during the penultimate deglaciation

“…While the amplitude and duration of the eastern North Atlantic δ 18 O anomaly may potentially be enhanced if the release of meltwater shuts down AMOC 11 , we consider it unlikely that a more intense or longer duration of AMOC weakening during TII is the only cause for the long period of extreme eastern North Atlantic δ 18 O fresh anomaly. Although there are no direct quantitative proxies for AMOC intensity in TI or TII 51 , some mid-latitude proxy records are likely to be sensitive to the strong winter season climatic impacts of AMOC slowdown such as the relative abundance of Neogloboquadrina pachyderma sinistral 56 or relative abundance of C 37:4 alkenone 57 – 59 (Fig. 6 ).…”

Orbital and Suborbital‐Scale Variations of Productivity and Sea Surface Conditions in the Gulf of Alaska During the Past 54,000 Years: Impact of Iron Fertilization by Icebergs and Meltwater

“…Quantification of the individual C 37 alkenone concentrations (C 37:2 , C 37:2 , and C 37:4 ) was achieved using nonadecanone as an internal standard. Although C 37:4 has been used as an indicator of freshwater inputs to marine environments (e.g., Sánchez‐Montes et al., 2020), recently it has been suggested that there is a potential sea‐ice contribution to C 37:4 (Wang et al., 2021). To account for this potentially different source to the alkenone concentrations, we present concentrations of C 37:3 + C 37:2 separate from C 37:4, although the overall patterns remain the same.…”