Climate Proxy

Climate observations prior to the instrumental era are necessarily indirect. These observations are made on climate proxies in various geological (e.g. lake or marine sediments, living or fossil coral reefs, cave deposits), glaciological (ice cores or snow pits) or biological (trees) archives. Many types of data can often be collected from each archives, each sensing a different aspect of the environment (sometimes, several aspects at once). A paleoclimate dataset is almost always a time series of observations made on a proxy system.

Evans et al. (2013) define a proxy system as comprised of three components:

• The sensor comprises physical, chemical and/or biological components that react to environmental conditions. The term "proxy" used ubiquitously, and often ambiguously, throughout the paleoclimate literature, is most commonly equivalent to sensor. Sensor is the more explicit, and thus, preferred term, however "proxy" can be treated as a synonym of "sensor". Sensors often respond to more than one environmental variable, and may have complex responses to the environment they sense, including thresholds (record only part of the range of environmental conditions), seasonal biases (record environmental conditions over a few months of the year), and/or nonlinear responses. For instance, Mg/Ca in foraminifera is an often used sensor for sea-surface temperature but depends also on sea-surface salinity and deep-ocean carbonate saturation . Furthermore, its temperature dependence is exponential . It is thus a multivariate and nonlinear sensor.

• The archive is the medium in which the response of a sensor to environmental forcing is recorded (Fig. 1). Marine sediments are a type of archive, on which many sensors may be measured (e.g. Mg/Ca, δ¹⁸O, $U_{37}^{k'}$, TEX86, δD)

• observations are made on archives, and involve several processes:
  • sampling is the process where the subsets of the archive are extracted, usually via coring or drilling.
  • often, the observations are made on a purified, chemically transformed (.e.g leached) form of the sampled material
  • what else?

Picking foraminifera of a given species to conduct the measurements is part of the observation process, though it does affect the sensor definition : the habitat of these forams determines with environmental variable (e.g. surface, sub-surface, or thermocline temperature) they are most sensitive to.

These three major components may be individually modeled, and linked together within a Proxy System Model . Some sensors are common to multiple archives (e.g. δ¹⁸O), and all archives support more than one possible sensor.

ASK VARUN TO GENERATE A TABLE OF EXISTING ARCHIVES AND SENSORS HERE

--Julien (talk) 16:47, 13 January 2016 (PST)