PB ISOTOPIC COMPOSITION OF LATE CRETACEOUS AND EARLY PALEOGENE PACIFIC WATER MASSES

The meridional overturning circulation (MOC) is an important component of modern global heat transport. Thus past changes in the mode of MOC are widely assumed to have influenced ancient climate. In order to understand the effect of changes in the MOC played on past climate we need to determine where convection occurred. The Late Cretaceous and Early Paleogene interval (~75 to 45 Ma) was characterized by low meridional temperature gradients. Nd isotopes indicate a high-latitude convection in the Pacific Ocean during this interval, with deep waters mixing in the tropical Pacific. Here we investigate the evolution of the Pb isotopic composition of water masses in the Pacific to constrain variability in weathering inputs into the inferred convection regions from ~75 to 45 Ma.

We generated Pb isotope records for North Pacific DSDP and ODP Sites 192, 464, and 883, and South Pacific DSDP Sites 323 and 596. The North Pacific ²⁰⁶Pb/²⁰⁴Pb values range from 18.3 to 18.8, with the exception of one highly radiogenic value (19.1, at ~65 Ma) recorded at Site 464. Site 883 records a trend toward more radiogenic values from 62 to 40 Ma but preliminary data from Sites 192 and 464 indicate no significant trend. The North Pacific values are consistent with end-member values expected in deep Pacific waters from weathering of exposed arc volcanics (~18.5 to 18.8). The high value recorded at site 464 may result from a change in eolian inputs. South Pacific sites record ²⁰⁶Pb/²⁰⁴Pb values between 18.7 and 19.2. Preliminary data from Site 323 indicates no significant variability or trend, and is consistent with the general values expected at this site due to its proximity to the Antarctic Peninsula arc volcanics. Site 596 values vary throughout the record with a maximum value of 19.2 at ~66 Ma. This radiogenic value coincides approximately with the radiogenic value recorded at Site 464. Comparison of preliminary Pb isotope data to existing Nd data indicates that the two tracers appear to be coupled. Sites furthest north and south generally follow the same trends suggesting that deep water composition exerts the strongest influence on the Pb isotopic composition close to the regions of deep-water convection. The higher and more variable Pb values of the subtropical sites 464 and 596 potentially reflect Pb from dust dissolution.