ABSTRACT: Preliminary assessment of Canadian marine reservoir ages.

CGRG Bibliography of Canadian Geomorphology
Search Results

Author :

Date :

Title :

Publication :

Issue :

Page(s) :

Abstract

Quaternary researchers, who need to establish time-series of events, such as sea-level change or deglaciation, require valid estimates of the regional reservoir age to integrate marine and terrestrial chronologies and develop coherent age models. We have attempted to define the "reservoir age" in Canadian waters from the Pacific, through the Arctic, to the Atlantic. The reservoir age is essentially the residence time of CO2, and the bicarbonate formed from it, in the ocean; that is, the length of time spent in transit in the ocean between the time of absorption of the gas from the atmosphere and its ventilation back to the atmosphere at sites of ocean upwelling. During this transit time, the radioactive isotope of carbon (14C) decays, imparting an "age" to the carbon in the water. Marine mollusc shells are the best materials to use in addressing this problem, because shell carbonate is deposited in isotopic equilibrium with the bicarbonate in the surrounding water. The shells therefore capture the reservoir age. Thus the problem of measuring reservoir ages should be a simple one of collecting shells where we need them and radiocarbon dating them to determine their "apparent ages". Unfortunately, nuclear bomb testing has spiked the atmosphere and the oceans with large amounts of artificial 14C. Hence, today the reservoir ages can only be determined on shells collected live prior to bomb testing (ca. 1955). Therefore, only material in museum collections can be used to resolve this problem. We have been fortunate to acquire a large number of shell samples that were collected live prior to bomb testing. We now have a database of about 260 radiocarbon ages on marine molluscs shells which were collected live during the interval 1884 to 1959. About 85% of these dates are new and most (90%) have been dated by the AMS technique. Our purpose was to establish a basis for suggesting regionally specific reservoir corrections for radiocarbon dates on marine shells. Although there have been indications that corrections greater than 400 years might be appropriate for the High Arctic and the Pacific coast, the small body of available measurements precluded a consensus on this point. We now have sufficient data to be able to propose regional reservoir corrections. Our preliminary data suggests the following corrections apply to suspension feeding molluscs: Pacific Coast (800); Arctic Ocean (740); Hudson Bay and Foxe Basin (630); Northwest Atlantic (540); Gulf of St. Lawrence, north shore and estuary (610), and southwest (450). Deposit feeding molluscs require greater corrections where they lived in calcareous mud. Unfortunately, this extra correction varies with carbonate content. It is presently poorly constrained but can exceed 1000 years. Dating of deposit feeders is thus discouraged. These preliminary data are undergoing a more rigorous analysis.

Bibliography of Canadian Geomorphology