Eocene-Oligocene Transition Deep Sea Temperature and Saturation State Changes from Benthic Foraminiferal Trace Metal Analysis

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  • Eocene-Oligocene Transition Deep Sea Temperature

    and Saturation State Changes from Benthic

    Foraminiferal Trace Metal Analysis

    John S. Crowe

    Master of Earth Science

    Marine Geoscience (International)

    April 2015

    School of Earth and Ocean Sciences, Cardiff University, Main Building, Park

    Place, Cardiff, UK, CF10 3AT

  • We live on a planet that has a more or less infinite capacity to surprise. What

    reasoning person could possibly want it any other way?

    -Bill Bryson

  • 3

    DECLARATION

    STATEMENT

    This work has not previously been accepted in substance for any degree and is not being

    concurrently submitted in candidature for any degree.

    Signed: ________________________________ (candidate)

    Date: __________________________________

    STATEMENT

    This dissertation is being submitted in partial fulfilment of the requirements for the degree of

    Master of Earth Sciences.

    Signed: ________________________________ (candidate)

    Date: __________________________________

    STATEMENT

    This dissertation is the result of my own independent work, except where otherwise stated.

    Signed: ________________________________ (candidate)

    Date: __________________________________

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    Abstract

    The climate transition that occurs at the Eocene-Oligocene boundary (~33.7 Ma) is marked by the first

    significant Antarctic glaciation of the Cenozoic. Across the Eocene-Oligocene transition, two upward

    shifts in deep sea benthic foraminiferal isotope (18O) values occur, Step 1 and Step 2. These shifts in

    18O reflect a combination of bottom water temperature and 18O of seawater. Published

    paleotemperature records, calculated from foraminiferal Mg/Ca, do not show a cooling across the

    Eocene-Oligocene transition, potentially indicating a saturation state effect on benthic foraminiferal

    Mg/Ca during this period.

    This thesis attempts to ascertain the saturation state history of Ocean Drilling Program Site 757

    through the use of benthic foraminiferal trace metal climate proxies. Inductively coupled plasma mass

    spectrometry was used to analyse the trace metal geochemistry of two benthic foraminifera species,

    Bulimina jarvisi (n=56) and Cibicidoides havanensis (n=62). An infaunal and epifaunal benthic

    foraminifera species were chosen, enabling an assessment to be made of how saturation state change

    effects the different microhabitats inhabited by foraminifera. Quantification of saturation state change

    enabled benthic foraminiferal Mg/Ca values to be corrected for any saturation state effect, allowing

    accurate paleotemperature change to be determined. By calculating accurate changes in past bottom

    water temperature, it was possible to quantify change in 18O of seawater and thus ice growth and sea

    level change.

    Here we present data that shows a clear rise in infaunal (0.12 mmol/mol) and epifaunal (0.06

    mmol/mol) benthic foraminiferal Mg/Ca across the Eocene-Oligocene transition. Further, a clear

    increase in saturation state (18.17 mol kg-1) was shown for the same period. By correcting Mg/Ca

    paleotemperatures for saturation state change, it was found that bottom water temperature cooled

    across the Eocene-Oligocene transition (~1.5 C). The timings of the cooling and warming events

    across the Eocene-Oligocene transition, with regard to the two 18O isotope shifts, do not easily

    correlate with published records. To some degree, both 18O shifts appear to be associated with

    warming and ice growth. Across the Eocene-Oligocene transition, sea level has been shown to fall

    (~50m), however sea level appears to rise as well as fall during this period.

    This study concluded that non saturation state corrected temperatures calculated from infaunal

    foraminifera are not accurate or reliable indicators of changes in bottom water temperature. Despite

    accounting for saturation state change, warming periods across the Eocene-Oligocene climate

    transition were discovered here. These warming periods are associated with the two shifts in 18O

    values, Step 1 and Step 2. Significant periods of ice growth were associated also with the shifts.

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    Table of Contents

    1. INTRODUCTION ................................................................................................................ 8

    1.1. Foraminiferal Stable Isotope Analysis ......................................................................... 9

    1.1.1. Oxygen ................................................................................................................. 9

    1.1.2. Carbon ................................................................................................................ 10

    1.2. Carbonate System ...................................................................................................... 11

    1.3. Foraminiferal Test Mass as a Proxy for Carbonate Saturation State ......................... 13

    1.4. Benthic Foraminiferal Abundance as a Proxy for Productivity ................................. 14

    1.5. Trace Metal/Calcium Proxies Using Benthic Foraminifera....................................... 15

    1.5.1. Mg/Ca ................................................................................................................. 15

    1.5.2. Li/Ca ................................................................................................................... 19

    1.5.3. B/Ca .................................................................................................................... 21

    1.5.4. Sr/Ca ................................................................................................................... 23

    1.5.5. U/Ca .................................................................................................................... 24

    1.6. Eocene-Oligocene Transition .................................................................................... 27

    1.6.1. Oxygen Stable Isotope Records .......................................................................... 27

    1.6.2. Trace Metal Records ........................................................................................... 28

    1.6.3. Proposed Mechanisms for EOT Initiation .......................................................... 29

    1.7. Regional and Geological Setting ............................................................................... 31

    1.8. Benthic Foraminiferal Biostratigraphy of ODP Hole 757B ...................................... 32

    1.9. Pilot Ostracod Study .................................................................................................. 33

    1.10. Motivation .............................................................................................................. 37

    1.10.1. Hypothesis ...................................................................................................... 37

    2. MATERIALS AND METHODS ....................................................................................... 39

    2.1. Sampling .................................................................................................................... 40

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    2.1.1. Species Selection ................................................................................................ 40

    2.2. Microscopy ................................................................................................................ 40

    2.3. Weighing .................................................................................................................... 41

    2.4. Sample Preparation and Chemical Cleaning.............................................................. 41

    2.4.1. Crushing and Pre-Cleaning ................................................................................. 41

    2.4.2. Cleaning Procedure............................................................................................. 41

    2.5. Sample Dissolution and Calcium Concentration Analysis ........................................ 44

    2.6. Trace Metal Analysis ................................................................................................. 44

    2.7. Age Model ................................................................................................................. 46

    2.8. Calculation of Temperature, Saturation State and Ice Volume ................................. 47

    3. RESULTS ........................................................................................................................... 50

    3.1. Microscopy ................................................................................................................ 51

    3.2. Foraminiferal Average Test Masses .......................................................................... 52

    3.3. Species Abundances .................................................................................................. 54

    3.4. Foraminiferal Mg/Ca ................................................................................................. 56

    3.5. Foraminiferal Li/Ca ................................................................................................... 58

    3.6. Foraminiferal B/Ca .................................................................................................... 60

    3.7. Foraminiferal Sr/Ca ................................................................................................... 62

    3.8.