On the interaction between tropical and high latitude climate over the past 5 million years.

摘要

The tropical oceans and atmosphere are a major source of energy for the climate system. Changes in tropical climate can have large impacts at higher latitudes. This thesis is composed of three studies investigating the relationship between high and low latitude climates over the past 5 m.y.; First, I test the hypothesis that the timing of sub-Milankovitch variations in sub-polar Atlantic climate indices is paced by variations in tropical insolation or climate. I constructed high-resolution records of North Atlantic sea ice (ice rafted debris counts from Deep Sea Drilling Program [DSDP] Site 609), sea surface temperature (foraminiferal faunal censes from DSDP Site 609), and deep water formation rate (benthic foraminiferal carbon isotope ratios from DSDP Site 607) spanning the period 0.225–0.970 Ma. Time series relationships between these records, insolation, and climate records from the tropical Atlantic show that the timing of rapid variations in these high latitude climate indices is controlled by nonlinearities in high latitude climate rather than low latitude climate.; Second, I present the first evidence that the boron isotope composition (δ11B) of foraminifera is controlled by environmental variables in addition to pH. 5 m.y. records of planktonic foraminiferal δ 11B, boron/calcium and magnesium/Ca ratios, and size normalized shell mass from the western equatorial Pacific Ocean (Ocean Drilling Program [ODP] Site 806) are presented. These records indicate that foraminiferal δ 11B, heretofore interpreted as a paleo-pH proxy, may also be controlled by calcification temperature and/or partial dissolution.; Third, I test the hypothesis that changes in the tropical Pacific climate regime over the past 5 m.y. are related to changes in continental ice sheets. I present two 5 m.y. records of sea surface temperature (SST) based upon Mg/Ca ratios in planktonic foraminifera from sites in the eastern and western equatorial Pacific Ocean (ODP Site 847 and 806, respectively). These records show that the zonal SST gradient currently observed in the equatorial Pacific first emerged at 1.7 Ma. The difference in timing between the onset of significant northern hemisphere glaciation at ∼3 Ma and the tropical Pacific reorganization at 1.7 Ma indicates that the two may be unrelated.