Calibrating Greenland coralline algal proxies with high-resolution in situ water temperature

Geochemical paleoproxies from marine archives are typically calibrated with temperatures from satellite- or ship-based observations that are spatially averaged over large areas, or with temperature data measured at a distance to the sampling site. These observations do not necessarily reflect local conditions and averaging effects potentially reduce variability. However, this is often the only viable option as no other direct observations are available in most settings. In subarctic and arctic regions in situ observations are even rarer and the availability of subannual-resolution surface ocean climate archives is very limited as well, especially when compared to the tropics. Encrusting coralline algae are important shallow-marine calcifiers that provide high-resolution archives of past ocean and environmental variability, but only few calibration studies of algal proxies exist from the climatically important high-latitude regions. At the same time these regions have experienced the most drastic changes in recent decades, e.g. Arctic surface temperature warming is more than twice as large as the global average.

Here, we present results from calibrating subannual-resolution coralline algal proxies with four years of continuous in situ measured temperature in Disko Bay, Greenland. Sensors were deployed in summer 2019 at sites of sample collection, recording water temperature in hourly resolution, providing data from the same water depth where the corallines grow. Coralline algal samples (Clathromorphum compactum) were retrieved together with sensors in summer of 2023. Temperature cycles are matched to coralline growth increments and geochemical data analyzed by Laser Ablation ICP-MS. The four year overlap makes our record the longest continuous calibration interval for coralline algae from high-latitudes. We align element/Ca-ratios with sclerochronological results and compare temperature measurements and proxies to high-resolution satellite observations and reanalysis data. Pairing of sub-seasonal resolution element/Ca-ratio time series measured in the uppermost years of algal growth with high-resolution in situ temperature allows us to directly connect individual short-time variability and warming/cooling recorded in the algal record to real-time observations. Water temperature plays a major role in controlling underlying processes in recently observed regional and large-scale Arctic and Greenland Ice Sheet (GIS) change. Coralline algal samples resolve ultra-high-resolution Disko Bay environmental variability in close proximity to Jakobshavn Glavier, one of the largest GIS glaciers, which delivers a significant amount of freshwater to the coastal West-Greenland surface ocean. Our study is testing proxy-temperature relationships on multiple algal samples, facilitating intra- and intersample comparisons, and thus helps to improve calibration of long-term algal proxy records used for paleo-reconstructions.