Challenges and Opportunities for Developing Robust Annually Resolved Paleoenvironmental Reconstructions from the Oceans

With recent progress in sclerochronology, several multi-century long, annually resolved and carefully crossdated marine records have been developed. Many of these records have shown to be sensitive to environmental conditions, including seawater temperature. Considering that the global ocean covers more than 70% of Earth’s surface, these records are required to develop a more complete understanding of oceanic processes (e.g., circulation dynamics and carbon cycling), detail past marine climate variability and determine how the oceans interact with other components of the climate system. However, robust reconstructions of past seawater temperatures with calibration and verification statistics have been hindered largely in part due to inadequate instrumental records and monitoring programs in the ocean environment. To further exacerbate this problem, there are often large spatial differences in hydrographic conditions both in the vertical and horizontal dimensions, especially in dynamic oceanic regions. This latter problem is somewhat unique to the marine environment as the lower atmosphere is generally more homogenous over fairly large distances compared to coastal ocean environments. Thus, calibration and verification procedures used in dendrochronology are not always feasible in the marine environment, especially at depth where few instrumental series exist. Bootstrapping techniques may provide a reasonable approach to reconstruct paleoenvironmental conditions with relatively short instrumental series. However, rigorous error analysis of these records is difficult without relatively lengthy (> 40 years) and appropriately located instrumental series. As such research efforts move forward, the location of future research sites should be carefully considered not only for the scientific potential of a particular oceanic site but also whether or not long-term instrumental series representing multiple depths in the ocean are present.