| This research begins with a paleoceanographic reconstruction of the eastern tropical Pacific (ETP) and subtropical Northwest Atlantic (NWA) during the early stages of uplift of the Central American Isthmus and associated shoaling of the Central American Seaway (CAS). In the subtropical NWA (DSDP 103 and ODP 1006), the 5.2 to 5.1 Ma interval is characterized by an increase in SST and sea surface salinity, indicating a strengthening of the Florida Current (FC) and Gulf Stream (GS). Sea surface temperature in the ETP Warm Pool (DSDP Site 84) remained relatively stable between 6.9 and 5.1 Ma, during which El Nino-like conditions persisted. A slight cooling is observed after this interval (with synchronous warming in the NWA), followed by the onset of major cooling at ∼3.2 Ma, both of which are preceded by a shallowing of the thermocline. Stepwise cooling is attributed to enhanced Atlantic meridional overturn circulation (AMOC), which caused a shoaling of the main tropical thermocline, thereby strengthening the Walker Circulation and weakening the Pacific North Equatorial Counter Current. During the mid-Piacenzian warm period (MPWP; ∼3.3--3.0 Ma), SST in the Panama Basin was ∼0.8°C cooler than today, while the subtropical NWA was only ∼1.1°C warmer. This corroborates evidence for reduced meridional SST gradients during the mid-Pliocene as well as the hypothesis that more vigorous ocean circulation---particularly in the NWA---was critical during this period. The timing of SST changes in the ETP and NWA (∼5.1 Ma) suggest that the termination of permanent El Nino and enhanced AMOC did not contribute significantly to the onset of major Northern Hemisphere glaciation (NHG), as both of these events occur well before the beginning of the glacial cycles.;During the MPWP, mean global surface temperatures were similar to those predicted for the next century (2--3° C warmer) while atmospheric CO2 concentrations, paleogeography, and paleobiology were similar to today. As such, the MPWP has been studied in detail as a potential (albeit imperfect) analog for future climate change and has provided a natural and unique test-bed for the integration of proxy data and general circulation models. Central to this research effort is the Pliocene Research, Interpretation, and Synoptic Mapping (PRISM) project, an iterative paleoenvironmental reconstruction of the MP focused on increasing our understanding of warm-period climate forcings, dynamics, and feedbacks by providing three-dimensional data sets for general circulation models. A mainstay of the PRISM project has been the development of a global sea surface temperature (SST) data set based primarily upon quantitative analyses of planktic foraminifer assemblages, supplemented with geochemical SST estimates wherever possible.;Assemblages from each core provide evidence for a temperature- and circulation-driven 5--10° northward displacement of MP faunal provinces, as well as regional shifts in planktic foraminifer populations linked to species ecology and interactions. General biogeographic trends also indicate that, relative to modern conditions, gyre circulation was stronger (particularly the Gulf Stream, North Atlantic Current, and North Equatorial Current) and meridionally broader. Overall, SST estimates suggest that surface waters in this region were not significantly warmer (1--2° C) than today and that mean annual SSTs along LL western boundary currents were indistinguishable from modern. Multiproxy SST data also provide evidence for enhanced northward transport of warm, salty, oligotrophic surface waters via a vigorous western boundary current system with warmer (cooler) cold-season (warm-season) temperatures.;A strengthening of the AMOC under warmer-than-modern conditions has significant implications for future climate change. The current generation of climate models suggests that HL warming and associated ice-sheet melting will induce a freshening of the North Atlantic and thus, to a reduction in the strength of the AMOC, thereby buffering surface temperature increases in the Northern Hemisphere. However, if after this transient period of climate system adjustment, Earth returns to a more Pliocene-like climate state the AMOC system may strengthen, thereby exacerbating the HL warmth caused by elevated atmospheric CO 2 concentrations. Thus, through the reconstruction of warm-period SST and ocean circulation, this research provides insight into the potential operation of the LL North Atlantic and its associated impact on broad-scale Northern Hemisphere climate. (Abstract shortened by UMI.)... |
