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

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

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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.