Algal pigments in Arctic lake sediments record biogeochemical changes due to Holocene climate variability and anthropogenic global change

In order to better constrain the limnological impacts from recent climate change relative to those of the Holocene, we developed a high-resolution multi-proxy paleoenvironmental record from a small lake on eastern Baffin Island, Arctic Canada. Carbon and nitrogen elemental and isotopic compositions from sediment organic matter, algal pigments, and diatom assemblages are integrated to provide robust indices of paleoclimatic variability. In particular, the ratio between individual carotenoid pigments (lutein:diatoxanthin) reveals a shift in dominant primary production from 'green' taxa (chlorophytes, higher plants, and bryophytes) during the Holocene Thermal Maximum (HTM) to 'brown' taxa (diatoms and chrysophytes) over the mid- to late Holocene. Green pigment abundance appears most sensitive to mean summer temperatures, and their increased relative abundance in the past serves as an indicator of warm times. Regionally, the HTM occurred shortly after local deglaciation (10 ka), persisting until similar to 7 ka. This timing agrees with that revealed by chironomid assemblages and ice-core records elsewhere in the Canadian Arctic, but is significantly earlier than suggestions from palynology on Baffin Island. This study provides additional evidence that this discrepancy represents the ecesis for higher plant dispersal and colonization on distal, freshly deglaciated landscapes. Pigment and diatom data indicate that mid Holocene cooling began between 7 and 6 ka, intensifying after 3 ka. All proxies show pronounced change after 1.5 ka, with the greatest divergence from average Holocene values occurring during the Little Ice Age (LIA), supporting the growing consensus that the LIA was the coldest multi-centennial interval of the Holocene. In the twentieth century, most proxies, including sedimentary carotenoid ratios, abruptly returned to a similar state as the Holocene Thermal Maximum, while diatom species assemblages present a more muted response. This underscores that anthropogenic alteration of the Earth system has created conditions with no exact analog in the past 10,000 years. Collectively, these results add new information on the dimensions of Arctic lake responses to Holocene climate change, which in turn can be used to reconcile paleoclimate reconstructions from diverse proxies.