Paleoclimatic Investigation of a Late Pleistocene Caribbean Deep-Sea Core: Comparison of Isotopic and Faunal Methods

Abstract

A simplified and more accurate version of the quantitative paleoenvironmental method proposed by Imbrie and Kipp (1971) is described, which is based on untransformed rather than transformed species per cent data. The method yields faunal indices (T_s, T_w, S), useful both as objective measures of paleontological properties and as estimates of Pleistocene sea-surface summer and winter temperatures (T_s, T_w) and salinity (S). Similarly, the oxygen-isotope method yields objective measurements of δ O¹⁸, useful stratigraphically and as indications of past changes in isotopic water composition and temperature. Laboratory errors of the two methods have about the same magnitude relative to ranges observed in V12-122. Accuracy of faunal indices as estimates of oceanic conditions is evaluated by study of modern oceanographic data and sea-bed samples. Under favorable conditions, accuracy is apparently limited primarily by the degree of ecological control exercised by the estimated parameter. Accuracy of the isotopic paleotemperature estimates is limited primarily by uncertainty as to the magnitude of the water-correction term in the isotope equation, a value which combines global ice-volume and local evaporation-precipitation effects. Curves of δ O¹³ and S in V12-122 record all or part of seven major climatic cycles, and display a fundamental periodicity of about 85,000 years. T_s and T_w curves show small but significant differences: two phase shifts in estimated temperature minima, and a long-term increasing trend. Amplitude-frequency histograms of T_w and δ O¹⁸ indicate that only two percent of the time during the past 450,000 years have Caribbean temperatures been as warm and isotopic ratios as low as they are today. A comparison of the magnitude of δ O¹⁸ change (2.2%.) during the shift from late-glacial to post-glacial times, with that of the faunally estimated change in average temperature (2.2°C), provides a basis for estimating the associated change in isotopic water composition (1.8%.) by back-calculation in the isotope equation. At least 0.4%. of this change in A is attributed to an evaporation-precipitation effect, and the balance (≤1.4%.) to an icevolume effect. Isotopic and faunal methods monitor different responses to global climatic change. Used in conjunction, they provide deeper insights into the past than either could achieve alone.