Nd- and Pb-isotope time series from Atlantic ferromanganese crusts: implications for changes in provenance and paleocirculation over the last 8 Myr

Abstract

High-resolution Nd- and Pb-isotope time series for the last 8 Myr are reported for four Atlantic ferromanganese crusts, dated by 10Be/ 9Be chronology. These are compared to new high-resolution and high-precision Pb-isotope time series and recently published Nd-isotope time series for two previously studied crusts from the NW Atlantic Ocean. These records allow a more detailed examination of Atlantic deepwater variability over the time period of intensification of Northern Hemisphere Glaciation (NHG). Changes in the Pb-isotope time series started after 3 Ma, but were most dramatic over the last 1.8 Myr, coinciding with changes of Pb-isotopes in the Arctic Ocean. This latter change post-dates the intensification of NHG at 3.1 to 2.5 Ma and reflects an increase in the input of material eroded from the Archean Shield of Canada and Greenland. Shifts in Nd- and Pb-isotope compositions in a crust from the Blake Plateau occur before ∼5 Ma and most prominent at ∼8 Ma suggest that water masses from either the Pacific or Southern Ocean influenced the isotope composition of this crust. The relatively high ε Nd values around 8 Ma recorded by the Blake Plateau crust are explained by a contribution of eastward flowing Pacific water through the Panama Gateway into the Caribbean Sea. This high ε Nd signal decreased between 8 and 5 Ma suggesting that the supply of Pacific water into the Caribbean became restricted. This is earlier than the Caribbean seawater salinity increase at 4.2 Ma deduced from δ 18O data, and may indicate that there was only a surface water connection between the Caribbean and Pacific between ∼5 and 4.2 Ma. The closure of the Panama Gateway to intermediate and deep water exchange (>200 m depth) apparently occurred much earlier than the intensification of Northern Hemisphere Glaciation at 3.1–2.5 Ma, and cannot therefore have been a direct cause of this climatic change, but may, as recently argued, only have been a necessary precondition.