An ideal climate environmental archive will:
- be of high temporal resolution, preferably at annual or seasonal scale,
- be responsive to small changes in one or a few climate variables
- be a closed system so as to retain information
- record a continuous time series of variation
- be global in distribution, and
- be accurately and precisely datable.
The current suite of natural archives includes corals, ice cores, tree rings, varved sediments and speleothems, but none meet all of the ideal criteria. Because of this it becomes necessary to cross correlate both archives/records and proxies, and this increases uncertainty in interpretation. Corals contain seasonal or sub-seasonal growth rings that incorporate a chemical record of variables such as sea surface temperature and salinity in their skeletons, but are generally short in their time span with individual colonies living no more than about 100-150 years. Ice cores and tree rings have annual or sub-annual “growth” patterns, and can persist continuously for up to several hundred thousand years in the case of the former. But both of these are restricted in their geographic range, as are corals. More recently the terrestrial palaeoclimate record has been enhanced by the addition of data from cave deposits which for the most part show excellent incremental growth patterns and proxy records, but suffer from problems of being difficult to date accurately. Terrestrial sedimentary deposits, particularly those from lakes, can contain high-resolution layering, but generally have many hiatuses or other breaks in the record. Marine sediments generally accumulate at rates too slow to give high temporal resolution records, but in a few places, particularly where upwelling results in high productivity, or where terrestrial and marine sediments interfinger, this limitation may be overcome.