Relationships Between Ice-Rafted Detritus, Geochemical Trends, and Biogenic Components of

84 Pages·2014·6.32 MB·English

by  Jason Lewis Gross

Wesleyan University The Honors College

Relationships Between Ice-Rafted Detritus, Geochemical
Trends, and Biogenic Components of Early Pliocene
Ocean Sediment in Weddell Sea, Antarctica

by

Jason Lewis Gross
Class of 2014

A thesis submitted to the
faculty of Wesleyan University
in partial fulfillment of the requirements for the
Degree of Bachelor of Arts
with Departmental Honors in Earth and Environmental Sciences

Middletown, Connecticut April, 2014

i
Acknowledgements

This thesis is:

For Suzanne “Master Fence Climber/Snack Maker/Greatest Thesis Advisor of the
Land” O’Connell, for Joel “The Man” LaBella, for Joop “Prosciutto Sandwich”
Varekamp, for Ginny “Heart of Gold” Harris, for Dana “Vertisol” Royer, for Marty
“Bacardi” Gilmore, for Peter “100-Year Flood” Patton, for Tim “Geochem Crash
Course” Ku, for Phil “Contact” Resor, for Kate “Wet-Siever Extraordinaire” Enright.
For Danielle, for Shamar, for family, for Fauverville 107, for a Rinehart, for Throw
Culture, for Bridie, for OTP, for Wesleyan, for South Pasadena, for the people who
complimented me on my mustache, for science believers and deniers, for Antarctica,
for the Sun, for the dreamers, for the reader. For Larry David, Scott McMicken, and
Toby Leaman because I think you guys are really cool and I hope you read this
someday.
Much love and enjoy the dirt.

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Table of Contents
ACKNOWLEDGEMENTS
  I

LIST
 OF
 FIGURES
 AND
 TABLES
  III

ABSTRACT
  1

INTRODUCTION
  2

1.1
 ICE-­‐RAFTED
 DETRITUS
  4

1.2
 ANTARCTIC
 GLACIAL
 HISTORY
  6

1.3
 ANTARCTIC
 GEOLOGY
  8

1.4
 THE
 PLIOCENE
 (5.3-­‐2.6
 MA)
  9

1.5
 OCEAN
 DRILLING
 PROGRAM
  10

1.6
 WEDDELL
 SEA,
 ANTARCTICA
  11

1.7
 ODP
 SITE
 693
 AND
 CORE
 8R
  12

1.8
 CURRENT
 CONDITIONS
  14

METHODS
  29

2.1
 X-­‐RAY
 FLUORESCENCE
 CORE
 SCANNING
  29

2.2
 PRINCIPAL
 COMPONENTS
 ANALYSIS
  30

2.3
 PARTICLE
 SIZE
 ANALYSIS
  31

2.4
 CHARACTERIZING
 IRD
  32

RESULTS
  35

3.1
 XRF
 CORE
 SCANNING
  35

3.2
 PRINCIPAL
 COMPONENTS
 ANALYSIS
  36

3.3
 GRAIN
 SIZE
 DISTRIBUTION
  38

3.4
 IRD
 GRAIN
 COUNTS
  39

DISCUSSION
  54

4.1
 PCA
  54

4.2
 IRD,
 BIOSILICA,
 AND
 XRF
 ELEMENTAL
 DATA
  55

4.3
 BIOSILICA
 AND
 FE
 FERTILIZATION
  58

CONCLUSION
  64

FUTURE
 WORK
  65

APPENDIX
 A

 PCA/XRF
 DATA

 66

APPENDIX
 B

 GRAIN
 SIZE
 DATA

 71

REFERENCES
 CITED
  75

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List of Figures and Tables
Page

Figure 1.1 Global water budget 15
Figure 1.2 Map of Antarctica 16
Figure 1.3 Continental shelf environment 17
Figure 1.4 Heinrich Events observed in North Atlantic cores 18
Figure 1.5 Zachos Curve 19
Figure 1.6 East Antarctic Ice Sheet growth model 20
Figure 1.7 Antarctic cross section and topography 21
Figure 1.8 Tectonic domains of Antarctica 22
Figure 1.9 CO ppm of past 5 million years 23
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Figure 1.10 Global ocean circulation patterns 23
Figure 1.11 Creation of Antarctic Bottom Water 24
Figure 1.12 Location of ODP Site 693 26
Figure 1.13 Lithostratigraphic summary of Site 693 27
Figure 1.14 Present day Antarctic Ice sheet mass balance 28
Figure 2.1 X-ray Fluorescence diagram 33
Figure 2.2 X-ray core scanner diagram 33
Figure 2.3 Core 113-693-A-8R photographs 34
Figure 3.1 XRF core scan data 43
Figure 3.2 PCA biplot 44
Figure 3.3 Grain size distribution 45
Figure 3.4 Grain size distribution with XRF data 48
Figure 3.5 Grain count sample selection 49
Figure 3.6 Total lithics/biosilica ratio 50
Figure 3.7 Total lithic counts per gram 50
Figure 3.8 Biosilica counts per gram 51
Figure 3.9 Percentage quartz 51
Table 3.1 Description of lithologic units 52
Table 3.2 Importance of principal components 53
Figure 4.1 PCA clusters plotted on elemental data 60
Figure 4.2 IRD-isotope correlation 61
Figure 4.3 Iron-biosilica relationship 62
Table 4.1 IRD hypotheses 63

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ABSTRACT

With the advent of anthropogenic global climate change and sea level rise, the
stability of high latitude ice sheets is being increasingly scrutinized. An historical
perspective of their stability is possible through analysis of marine sediment cores,
which provide a record of glacial dynamics. More information about the Pliocene,
when atmospheric CO may have been as high as today, can be used to better
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understand the consequences of changes in climate.
This study focuses on the Antarctic continental slope by analyzing deep-sea
cores taken from the margin of the East Antarctica Ice Sheet (EAIS) within the
Weddell Sea at ODP Site 693. The study area is adjacent to a region that modeling
simulations suggest was the first to develop continental ice sheets (Deconto and
Pollard, 2003), but failed to grow until the very late stages of continental glaciation.
The coarse grain fraction of Site 693 sediment samples was interpreted as
material deposited by ice-rafting (IRD). The weight percent of IRD was compared to
XRF core scan data, lithic content, and biosilica abundance to illuminate
paleoenvironmental characteristics of ice-rafting events. Peaks in IRD were correlated
with increases in elemental ratios, showing that ice-rafting events are reflected in the
geochemical record. Layers of high IRD were found to contain both high and low
amounts of biosilica, suggesting that increased ice rafting has little or no effect on sea
surface conditions or nutrient upwelling. Instead, maxima and minima Fe/Ti ratios
occur near depths of biosilica abundance and scarcity, respectively, indicating a
possible fertilization-inducing relationship.

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INTRODUCTION

The Earth’s hydrologic system describes the complex circulation of water as it
changes phase, moving from one reservoir to another. The distribution of water at a
point in time represents a transient condition, as changes in climate impose
limitations to the rates at which water is stored, how it is stored, and where it is
stored.
Of the available 1.39×109 km3 water on Earth today, only 2.5% is considered
to be potable. Approximately 69% of this small freshwater reservoir is held in the
cryosphere, with nearly the entirety of this value composed of water contained in
massive polar ice sheets (Gleick, 1993). Water in these sheets is essentially “trapped,”
only released as a consequence of warming temperatures that induce melting; due to
their size and location, release of land-based ice will discharge directly into the ocean.
This relationship demonstrates that polar ice and sea level rise are intertwined,
highlighting the importance of ice sheet stability during major global climatic
transitions. In contrast, alpine and valley glaciers discharge into lakes and rivers,
providing available water to mountain communities—however this proportion is
insignificant in the total global water budget (Figure 1.1).
The largest ice sheets today that form Earth’s cryospheric network are the
Greenland Ice Sheet (GIS) at 2.96 million km3 and the Antarctic Ice Sheet (AIS) at
26.5 million km3 (excluding ice shelves), together covering about 10% of the Earth’s
land surface (Allison et al., 2009; Fretwell et al., 2013). Large ice masses are
constantly under scrutiny as they pose potential high-risks to the worldwide
population. If entirely melted, it is estimated that the GIS and AIS would directly
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contribute to 7.36 m and 57.7 m to global sea level rise, respectively (Bamber et al.,
2013). Complete melting is impossible within the short time period of a human life,
but partial melting causing several meters of rise would have apocalyptic effects to a
human population concentrated on coastlines.
Substantial sea-level fluctuations have not occurred since the advent of
modern human civilizations, only amounting to +0.28 m since preindustrial levels at a
mean rate of 0.01 mm/yr2 (Jevrejeva et al., 2008), but lately this number is increasing.
Gardner et al. (2013) calculated a 1.51±0.16 mm/yr contribution to sea-level rise
between 2003 and 2009 from ice-sheet and glacial melting alone (unadjusted to the
effects of thermal expansion), accounting for 61±19% of total rise between those
years.
As we travel deeper into the Anthropocene, humans experience a new and
unexplored climatic regime. A testament to uncertainty is recent super-storm Sandy—
a category 3 hurricane that made landfall in Cuba during October 2012 before
impacting the eastern seaboard of the United States—which caused a meter of
temporary, regional rise in sea levels in several locations. The storm caused 97 direct
deaths in the US and the subsequent flooding displacing hundreds of thousands of
residents required months of rebuilding efforts in densely populated places such as
New York City and New Jersey (The New York Times, 2012). The effects of super-
storm Sandy suggest the potential disorder that could ensue from what is considered a
minor, temporary imbalance in the global hydrologic system. With the permanence of
modern human infrastructure, larger, longer-scale changes in sea-level rise in
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response to the wasting of Earth’s geographically concentrated ice reservoirs dwarf
such events.
The purpose of this research is to better understand the glacial history of the
East Antarctic Ice Sheet (EAIS) during the Pliocene epoch, using its past behavior as
an analog for future conditions (Figure 1.2). Acquiring this history can be
accomplished by studying seafloor depositional processes adjacent to the Antarctic
cryospheric system. Ice is popularly used as a direct means for interpreting
paleoclimate through analysis of isotopic ratios and chemistry, but ice core records
only have the capability to chronicle events succeeding ~800 ka (Fischer et al., 2013).
We wish to understand processes on longer time scales and as a result we turn to the
more permanent marine sediment record.
Large (greater than 50,000 km2) ice sheets systematically respond to changes
in climate, with the rate of either ablation or growth correlated with temperature or
moisture increase/decrease (Frezzotti and Orombelli, 2014). The interconnectivity of
ocean sediment and proximal ice illustrates that ocean sediment characteristics will
reflect variations in glacial behavior, bioproductivity, and methods of grain transport
that suggest possible paleoclimatic conditions (Henderson, 2002; Wefer et al., 1999).
This study focuses on changes in elemental composition, mineralogy, grain size, and
microfossil abundance within a 9 m vertical core of ocean sediment.
1.1 Ice-Rafted Detritus

Seafloor sediments exist at a relatively permanent resting place after long
journeys of transport. As glaciers advance or retreat across continental bedrock, they
have the ability to entrain rocks, minerals, and weathered sediment within their
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structure (Alley et al., 1997; Warnke, 1970). In some instances these glaciers will
calve off into smaller icebergs as they extend out to sea, eventually melting and
depositing grains on the seafloor (Figure 1.3). Because floating icebergs can survive
for months in the open ocean, they become under the influence of major ocean
currents and allow entrained materials to be found as far as hundreds of kilometers
from coastlines (Diekmann and Kuhn, 1999). When deposited, the materials are
characteristically poorly sorted and geologically display the regions of which the ice
has traversed over (Andrews, 2000; Warnke, 1970). These deposited glacial materials
are referred to as ice-rafted detritus (IRD). IRD is distinguished primarily by
individual grain-size, with any lithic entity greater than a size threshold being
classified as IRD. Determining this threshold is subjective and generally falls between
63 and 250 µm. Complications arise when seafloor processes “rework” sediment e.g.
turbidites and slope failure, yielding grain sizes not indicative of the manner in which
they were transported (Warnke, 1970).
In a highly influential study that assisted in further understanding glacial
response to changes in solar radiation, Heinrich (1988) examined IRD in Pleistocene-
age sediments of the eastern North Atlantic Ocean. The study found distinct sediment
layers containing high abundances of IRD in 13 piston cores taken throughout the
Dreizack Seamounts, using 180 µm-3 mm as its criterion (Figure 1.4). Heinrich
interpreted the presence of a high-IRD layer as the product of an event in which
“armadas” of icebergs drifted across the North Atlantic, depositing their entrained
lithics once melted. Atmospheric conditions needed to satisfy these events are
temperatures conducive to large-scale glacier formation and iceberg production,
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followed by mechanisms allowing their melting. Ambiguity regarding the
characteristics of ice growth/melt cycles has sparked debate whether these processes
are influenced by external climatic forcings or internal physical properties of ice.
The sediment examined for this research has entirely different properties than
that of the North Atlantic. Yet, similar questions posed in Heinrich’s study on IRD
can be applied here to the Antarctic continent as a method of elucidating glacial
behavior. Are layers of proportionally high IRD seen in Weddell Sea sediment? Do
these peaks coincide with other physical/chemical properties within the core that can
provide information about the atmospheric or glacial environment during the time of
deposition?
1.2 Antarctic Glacial History

Cenozoic climate evolution has been characterized by several major
transitions between warm-cold/glacial-interglacial intervals, oftentimes marking the
beginnings of geological epochs (Anderson et al., 2011; Barrett et al., 2006; Flower
and Kennet, 1994; Levitan and Leichenkov, 2011; Naish et al., 2007).
Reconstructions of global paleoclimates have heavily relied on the use of δ18O to
δ16O ratios taken from the calcareous skeletons of benthic and planktonic
foraminifera. Ice uses the lighter δ16O isotope during crystallization, and so increasing
δ18O values found in benthic or planktonic foraminifera indicate depleted δ16O in
deep or surface waters, respectively (Rohling, 2007). This reveals the lighter isotope
is instead being used in ice production, indicating glacial presence and colder
climates (Miller and Thomas, 1985). Evaporation also uses the lighter oxygen
isotope, and so depleted δ16O waters in equatorial regions can suggest increased
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