Paleogene reconstruction of southern Pacific water mass composition using Nd isotopes

The early Paleogene was a period of extreme global warmth characterized by diminished equator to pole thermal gradients, as well as between oceanic surface- and deep-water temperatures. This different thermal structure, combined with differences in ocean basin geometry, complicates our understanding of early Paleogene deep-water circulation. A growing body of Nd isotope data is beginning to constrain the distribution of different water masses, however little is still known about circulation patterns particularly in the Pacific basin. Existing fossil fish debris and Fe-Mn crust Nd isotope data from the Pacific spanning the interval ~65 to ~35 Ma indicate water mass compositions with relatively radiogenic values, typically ranging between -3 to -6 epsilon units. Records from ODP Legs 198 and 199 suggest the possibility that the deep tropical Pacific was a mixing location of water masses that sank in both the Southern Ocean and North Pacific from ~65 to ~45 Ma. The fundamental challenge in testing this mode of circulation is the lack of data from high latitude locations, as most of the existing Pacific data are from the low latitudes. Here we present new fossil fish debris Nd isotope data from the southern portion of the Pacific from Ocean Drilling Program (ODP) and Deep Sea Drilling Project (DSDP) Sites; 463, 596, and 323 at paleolatitude locations 0°, 44°S, 65°S, and water depths 2000m, 5000m, and 4000m, respectively. The data indicate water mass compositions that were generally more unradiogenic than the northern Pacific, albeit with some similarities to sites further north. The one analysis generated to date from DSDP Site 323 at paleolatitude 65°S yielded an ɛNd(t) value of -5.6, a more radiogenic value than expected given its high latitude. However, further analyses will provide better constraints on the range of values of the water mass compositions for this Site. DSDP Site 596 at 44°S had values ranging from -5.4 to -9.3 in which values decrease from -5.5 to -7.2 over the interval ~82.2 to ~63.1 Ma then increase to -5.4 at ~53 Ma and then decrease again to -9.3 at ~51.0 Ma. DSDP Site 463, located near the paleo-equator, yielded values ranging from -3.8 to -6.4 with values increasing from -4.9 to -3.8 over the interval ~67.1 to ~58.0 Ma, followed by a decrease to -6.4 at approximately 31.0 Ma. The relatively unradiogenic values recorded by the more southern Pacific sites are interpreted to reflect a greater contribution of deep waters sourced in the Southern Ocean. Waters derived from the Southern Ocean are expected to be unradiogenic, and become more radiogenic as they circulate northward and are influenced by North Pacific weathering products. Thus sites located further north should exhibit more radiogenic deep-water Nd isotope compositions. The new Nd isotope data generally follow this trend. DSDP Site 463 ɛNd (t) values are comparable to those recorded at ODP Sites 1209 and 1211, situated at a similar latitude, but DSDP Site 596, located further to the south, indicates a stronger influence from a southern water mass source.