It is always good to find what we can describe as definitive evidence that shuts down argument. Here we have a handy oyster bed that swung in and out of existence like it was some handy toggle switch.
The Roman and Medieval warm spells are locked in and so is the collapse of the little ice age. They are recent enough to clear in the record. The mid Holocene warm period is clear although the length is not mentioned. Other records show a couple of millennia.
This was followed by cooler climate perhaps similar to present conditions. The 4000 – 5000 BP seems surprising in time and place but may mean that we have understated the actual warmth of the
In fact it is becoming timely to compare several sources to try and assemble a better picture than has been bandied about. Even this item is leaving a lot hanging and reporting on the bit they are sure about.
This item is certainly improving resolution and opening an instructive line of inquiry.
, R.E., Ryan, W.B.F., McHugh, C., Slagle, A., Nitsche, F., Rubenstone, J. 2004. Environmental change and oyster colonization within the Bell Hudson River estuary linked to Holocene climate. Geo-Marine Letters 24: 212-224.
The authors located fossil oyster beds within the Tappan Zee area of the Hudson River estuary, New York, USA (~ 41.13°N, 73.90°W), via chirp sub-bottom and side-scan sonar surveys, after which they retrieved sediment cores from the sites that provided shells for radiocarbon dating. Results of their analyses indicated that "oysters flourished during the mid-Holocene warm period," when "summertime temperatures were 2-4°C warmer than today." Thereafter, the oysters "disappeared with the onset of cooler climate at 4,000-5,000 cal. years BP," but "returned during warmer conditions of the late Holocene," which they specifically identified as the Roman and Medieval Warm Periods as delineated by Keigwin (1996) and McDermott et al. (2001), explicitly stating that "these warmer periods coincide with the return of oysters in the Tappan Zee." They further report that their shell dates suggest a final "major demise at ~500-900 years BP," which timing they describe as being "consistent with the onset of the Little Ice Age." Because the oyster beds of
Keigwin, L.D. 1996. The Little Ice Age and Medieval Warm Period in the
McDermott, F., Mattey, D.P. and Hawkesworth, C. 2001. Centennial-scale Holocene climate variability revealed by a high-resolution speleothem δ18O record from
SW Ireland. Science 294: 1328-1331.