Late Quaternary Paleoclimatology and Paleoceanography of the Amazon Continental Margin ...

219 Pages·2012·11.07 MB·English

Late Quaternary Paleoclimatology and Paleoceanography of the Amazon
Continental Margin, Brazil
by
Trevor Eaton Nace
Earth and Ocean Sciences
Duke University

Date:_______________________
Approved:

___________________________
Paul A. Baker, Supervisor

___________________________
Gary S. Dwyer

___________________________
Bruce H. Corliss

___________________________
Wenhong Li

___________________________
Cleverson G. Silva

Dissertation submitted in partial fulfillment of
the requirements for the degree of Doctor of Philosophy
in Earth and Ocean Sciences
in the Graduate School
of Duke University

2012
ABSTRACT
Late Quaternary Paleoclimatology and Paleoceanography of the Amazon
Continental Margin, Brazil
by
Trevor Eaton Nace
Earth and Ocean Sciences
Duke University

Date:_______________________
Approved:

___________________________
Paul A. Baker, Supervisor

___________________________
Gary S. Dwyer

___________________________
Bruce H. Corliss

___________________________
Wenhong Li

___________________________
Cleverson G. Silva

An abstract of a dissertation submitted in partial fulfillment of
the requirements for the degree of Doctor of Philosophy
in Earth and Ocean Sciences
in the Graduate School
of Duke University

2012
Copyright by
Trevor Eaton Nace
2012
Abstract
The tropics are a significant source of heat and moisture export, which drive
global circulation patterns, thus it is vital to systematically understand the land, ocean and
sedimentological interactions within the tropics. The Brazilian continental margin is an
ideal region to characterize the tropics due to its unique local oceanography and
proximity to the atmospheric engine that is the Amazon Basin. A combination of: 1)
terrestrial organics and hydrology; 2) oceanographic temperature, isotopic composition,
and salinity and 3) early diagenesis and geochemistry of sedimentary interstitial water
and methane hydrate, provide a detailed understanding of the primary constituents that
influence the South American tropics.
Sedimentological, organic and paleoceanographic reconstructions of the Amazon
Basin, Brazilian Nordeste, and western equatorial Atlantic have been undertaken on two
sediment cores located on the Brazilian continental slope representing 30 and 110 ka,
respectively. High-resolution XRF analyses of Fe, Ti, K and Ca are used to define the
sedimentological history of the Amazon Basin and northern Nordeste. Here I present
elemental ratios of Ti/Ca and Fe/K, in addition to magnetic susceptibility, to determine
variability in Amazon Basin and Nordeste hydrology. Bulk organic proxies ∂13C and
∂15N of sedimentary organic carbon are used to define the organic history of the Amazon
Basin. Peaks in Ti/Ca and Fe/K ratios largely correlate in both the Amazon Basin and in
the Nordeste Record. These excursions correlate with commonly modeled global
slowdown of the Atlantic meridional overturning circulation (AMOC) during increased

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northern hemisphere glaciation. Differences in the Fe/K and Ti/Ca records suggest
periods of increased chemical weathering independent of precipitation driven sediment
discharge in the Brazilian Nordeste. Bulk organic geochemistry indicates the vegetative
history of the Amazon Basin has been relatively stable during the late Quaternary.
High-resolution stable oxygen isotopic analysis and Mg/Ca paleothermometry
undertaken on the near-surface-dwelling planktic foraminiferal species Globierinoides
ruber provide a picture of paleoceanographic forcings in the western equatorial Atlantic.
The Nordeste core exhibits a rapid warming of ~3.5ºC between the last glacial maximum
and the early Holocene. Furthermore, in almost all cases during the last glacial stage,
there was a 0.5 to 2ºC warming of the western equatorial Atlantic during the periods of
high Ti/Ca ratios that correlate with slowdown of AMOC. Thus, as observed in some
previous studies, the western equatorial Atlantic was warm and the adjacent southern
tropical continent was wet coincident with increased glaciation in the high latitude
northern hemisphere.
Interstitial pore waters were analyzed from the Amazon Fan and Brazilian
continental slope to determine early diagenesis, methane hydrate potential and its
geographic variability. Interstitial waters were measured for total Mg, Ca, SO 2-,
4
alkalinity, Cl, and ∂18O, combined with seismic bottom simulating reflectors (BSRs) to
investigate early diagenesis. Interpolated maps of sulfate reduction, sedimentation rate
and maximum alkalinity were produced to examine geographic variability in early
diagenesis and methane hydrates. Inorganic precipitation of calcium and magnesium,

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likely via dolomite and siderite, correlates with a decrease in alkalinity through inorganic
carbonate precipitation and methanogenesis, but alternatively increases through redox
pathways, specifically sulfate reduction.
Through multiple lines of evidence it is likely that there is extensive methane
hydrate occurrence on the Amazon continental shelf. A combination of: 1) A rapid
linear decline in sulfate at an unusually shallow depth; 2) Uncommonly high
sedimentation rates and terrestrial organic carbon input; 3) Significant variability in what
should be conservative chloride concentrations; 4) Large variability in interstitial oxygen
isotopes; 5) Widespread occurrence of BSRs provide substantial support for the presence
of methane hydrates. Sulfate reduction rates are lowest along the main Amazon channel,
with highest values distal of the main channel. Sedimentation rates are relatively low on
the continental shelf and surrounding the main channel and highest toward the distal end
of the main channel. Sulfate reduction rates provide key insights to the potential
geographic variability of methanogenesis and methane hydrate formation. Given the
tremendous influx of sediment from the Amazon River, this region is prone to massive
sediment failures, subsequent release of methane hydrates, as well as significant potential
for natural gas hydrates.

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Dedication
This dissertation is dedicated to my parents, Gregory Alan Nace and Barbara Jane Nace
for instilling in me a passion to explore my surroundings and appreciation of an infinitely
complex world. This dissertation is also dedicated to Haley Davis, for her continuous
and unwavering support.

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Contents
Abstract  ...........................................................................................................................................  iv

List of Tables  ..................................................................................................................................  xi

List of Figures  ................................................................................................................................  xii

Acknowledgements  .......................................................................................................................  xiv

Introduction  ......................................................................................................................................  1

1. Paleohydrology of the Amazon Basin and paleoceanography of the western tropical Atlantic  ..  6

1.1 Introduction  .........................................................................................................................  6

1.2 Study Area  ...........................................................................................................................  9

1.3 Materials and Methods  ......................................................................................................  12

1.4 Results  ...............................................................................................................................  16

1.4.1 Age Determination  .......................................................................................................  16

1.4.2 XRF and Magnetic Susceptibility  ................................................................................  19

1.4.3 Foraminifera ∂18O and Mg/Ca derived paleothermometry  .........................................  22

1.4.4 Organic Matter  .............................................................................................................  23

1.5 Discussion  .........................................................................................................................  25

1.5.1 Continental Hydrology  .................................................................................................  25

1.5.2 Principal Component Analysis  .....................................................................................  31

1.5.3 Oceanographic Conditions  ...........................................................................................  33

1.6 Conclusion  .........................................................................................................................  36

2. Rapid millennial scale changes in the hydrology of the Brazilian Nordeste and oceanography of
the western tropical Atlantic throughout the past 110,000 years  ...................................................  37

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2.1 Introduction  .......................................................................................................................  37

2.2 Study Area  .........................................................................................................................  39

2.3 Materials and Methods  ......................................................................................................  42

2.4 Results  ...............................................................................................................................  46

2.4.1 Age Determination  .......................................................................................................  46

2.4.2 XRF and Magnetic Susceptibility  ................................................................................  48

2.4.3 Foraminifera ∂18O and Mg/Ca derived paleothermometry  ..........................................  51

2.5 Discussion  .........................................................................................................................  54

2.5.1 Continental Hydrology  .................................................................................................  54

2.5.2 Principal Component Analysis  .....................................................................................  57

2.5.3 Oceanographic Conditions  ...........................................................................................  59

2.5.4 Global Record Associations  .........................................................................................  61

2.6 Conclusions  .......................................................................................................................  66

3. Geographic Variability of Interstitial Water Diagenesis in the Amazon Fan and Brazilian
Continental Slope  ...........................................................................................................................  67

3.1 Introduction  .......................................................................................................................  67

3.2 Study Area  .........................................................................................................................  68

3.3 Methods  .............................................................................................................................  70

3.4 Results  ...............................................................................................................................  72

3.4.1 Chemical Profiles  .........................................................................................................  72

3.4.2 Spatial Analysis  ............................................................................................................  75

3.5 Discussion  .........................................................................................................................  78

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3.5.1 Chemical Profiles  .........................................................................................................  78

3.5.2 Spatial Analysis  ............................................................................................................  82

3.6 Conclusions  .......................................................................................................................  83

Conclusions  ....................................................................................................................................  85

Appendix A  ....................................................................................................................................  88

References  ....................................................................................................................................  191

Biography  .....................................................................................................................................  205

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