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1776 PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES/Terrigenous Sediments
Terrigenous Sediments
S R Hemming, Lamont-Doherty Earth Observatory,
NY, USA
ª
2007 Elsevier B.V. All rights reserved.
Introduction
Terrigenous sediments are the weathering products
of rocks exposed at the Earth’s surface. They are
brought to the ocean by rivers, winds, and ice and
may be redistributed in the ocean by currents.
Accordingly variations in composition, grain size,
and flux of terrigenous marine sediments hold clues
to important paleoclimate variables such as wind
speed and direction, aridity, glacial activity, and
ocean currents. This article gives an overview that
outlines the potential of terrigenous sediments in
paleoceanography applications, specifically concerning their provenance (sources), weathering history,
sorting (grain size distribution), and flux (see
Paleoceanography).
Away from continental slopes, clay minerals are
the most widely distributed terrigenous components
in pelagic marine sediments, and their occurrence
and distribution is well reviewed by Kennett (1982)
(see Eolian Records, Deep-Sea Sediments). Some
clays are formed as weathering or alteration products of primary silicate minerals, and others are
recycled from sedimentary sources. All clay minerals are very fine-grained due to their crystal structure. Four types of clays are dominantly found in
marine sediments: chlorite, illite, kaolinite, and
smectite. In most cases chlorite and illite are primary minerals from the source while kaolinite and
smectite are newly formed alteration products.
Mineral composition of the clays is related to a
number of factors, including source rock type and
weathering. Weathering in turn is controlled by
relief, temperature, precipitation, and vegetation,
and consequently the different kinds of clay reflect
the major soil types in the source areas (Gibbs,
1967). Most clays are carried to the ocean and
deposited without being changed, and thus marine
sediments contain clays that reflect the weathering
process that produced them (e.g., Biscaye (1965)).
Primary minerals such as chlorite and illite are
relatively more abundant in colder climates, while
kaolinite is characteristic of tropical zones (Fig. 1).
Smectite tends to be associated with distribution of
volcanic sources. Although substantially more sediment is transported to the ocean by rivers than
winds, most is retained near margins. Thus, most
of the clays deposited in areas far away from the
continents are delivered by winds (Kennett (1982),
Rea and Hovan (1995), see also review in Kohfeld
and Harrison (2001)), particularly in the Pacific
Ocean. Additionally, clay distribution is also
related to ocean current distribution, both surface
and deep (e.g., Kennett (1982), Petschick et al.
(1996), Franzese et al. (submitted)).
Although clay minerals are the dominant terrigenous component of the abyssal plains, in polar and
subpolar regions melting icebergs may contribute a
significant amount of coarse sediment, called icerafted detritus (IRD) (see Glacimarine Sediments
and Ice-Rafted Debris). IRD is composed of a large
spectrum of grain sizes, ranging from clay to sand
and gravel, but its presence is identified in marine
sediments by the coarse fraction because rafting is the
only means of delivering coarse sediment to these
regions (e.g., Ruddiman (1977)). The recognition of
IRD allows paleoceanographers to study ice- sheet
dynamics in the past, and its distribution indicates
the flow of surface currents that carried the icebergs
(Fig. 2).
Provenance and Process and the
Application of Terrigenous Sediments
in Paleoceanography Research
To identify approaches for characterizing sediment
provenance, it is important to consider which parameters are more prone to preserving signals of sedimentary processes such as weathering and sediment
transport versus those that are likely to look beyond
sedimentary processes to yield insights into the crystalline source composition. For the purpose of using
the terrigenous fraction of marine sediments for
paleoceanography/paleoclimate applications, weathering is an important parameter to constrain.
Additionally, it should be noted that weathering and
sorting history are distinguished as processes in this
treatment because they are part of the sedimentary
processes, but the entire compositional constraints
include provenance characteristics, and likewise all
the compositional variations are controlled by processes: igneous, metamorphic, and sedimentary. In
the following, some tools to constrain sedimentary
provenance and process are reviewed, followed by a
brief review of sedimentary processes, and finally, an
overview of terrane types or provenance components.
Tools
Mineralogy
Mineralogy by X-ray diffraction (XRD) provides
important information on and weathering history of
PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES/Terrigenous Sediments
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Figure 1 Map of kaolinite/chlorite distribution of the <2 mm terrigenous fraction of sediments from the Atlantic Ocean. Modified from
Biscaye PE (1965) Mineralogy and sedimentation of recent deep-sea clay in Atlantic Ocean and adjacent seas and oceans. Geological
Society of America Bulletin 76: 803.
marine sediments (e.g., Biscaye (1965)). For example,
the kaolinite/chlorite of the <2 mm fraction of surface sediments from the Atlantic Ocean gives an
important first-order view of the sources and processes responsible for distributing sediments in the
ocean (Fig. 1). Mineralogy by XRD has become a
widely used tool in paleoceanography to constrain
weathering intensity as well as sources and transportation pathways.
Geographic variations in mineralogy can help to
constrain sediment transportation processes. For
example, Petschick et al. (1996) made a more
detailed map of the clay mineral distribution in the
South Atlantic Ocean than the one from Biscaye
(1965). The summary map from Petschick et al.
(1996) is shown in Figure 3A, along with surface
contributions and deep current systems. An example
of inherited ‘tropical’ signal is seen in the kaoliniterich zone near the Filchner-Ronne Ice shelf. Although
deep currents represent an important means of transporting sediment, for example, the tongue of chloriterich sediment in the western South Atlantic along the
South American margin, comparison of the clay province boundaries with the distribution of surface currents in the South Atlantic (Fig. 3B), the bathymetry
of the South Atlantic (Fig. 3C), and the July and
January average winds (Figs. 3D and E), it becomes
clear that surface currents (and/or the winds that drive
them) are also responsible for the first-order distribution of terrigenous sediments in the South Atlantic.
Further support for the importance of surface currents
for distributing sediments from long-distance
1778 PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES/Terrigenous Sediments
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X-ray fluorescence scanners are part of emerging technology that allows rapid semiquantitative determination of elemental concentrations in marine sediments at
resolutions down to 100 mm. This has important implications for high-resolution surveying of marine sediment cores. An important example is the recognition
of greatly increased flux of terrigenous sediments based
on the Fe/Ca ratio of sediments from the Brazil margin,
at the approximate times of Heinrich Events (massive
iceberg events in the North Atlantic), that correlated to
rapid climate variability in many parts of the world
(Arz et al., 1998; Hemming, 2004). Another fundamental example is the correlation of Ti concentrations,
as a proxy for rainfall in the Cariaco Basin, with collapse of the Mayan civilization (Haug et al., 2003) used
terrigenous sediment compositions to evaluate variations in rainfall in the source areas to the Cariaco
Basin, South America. In this case, the bulk titanium
content of undisturbed sediment reflects variations in
riverine input and the hydrological cycle over northern
tropical South America. This is significant because it
ties drought to the collapse of the Mayan civilization.
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(C)
Phanerozoic (<543 Ma)
Mixed Proterozoic and Archean
Late Proterozoic
Early Proterozoic
Middle Proterozoic
Archean
Iceland
Figure 2 Maps of IRD flux in the North Atlantic. Units are mg/cm2/
kyr of terrigenous silicates in the >63 mm fractions. (A) 125–115 ka,
(B) 40–25 ka, (C) 25–13 ka (Last Glacial Maximum). The zone with
Last Glacial Maximum flux of >250 mg/cm2/kyr is shown on each
panel for comparison purposes. Modified from Ruddiman WF (1977)
Late Quaternary deposition of ice-rafted sand in the subpolar North
Atlantic (lat 40 to 65 N). Geological Society of America Bulletin 88:
1813–1827 and Hemming SR (2004) Heinrich events: Massive late
pleistocene detritus layers of the North Atlantic and their global
climate imprint. Reviews of Geophysics 42: Art. No. RC1005.
transport comes from the emerging work of Franzese
et al. (submitted), that makes a compelling case for the
importance of the Agulhas Current as a key pathway
for sediment transport around the tip of Africa.
Petrography studies allow identification of major
lithological components in the sand fraction, as well
as potentially diagnostic minerals and/or rock types.
This approach has been particularly useful for identifying sources of IRD in the ocean. For example, it was
the first line of evidence used to infer the importance
of Hudson Strait sources to the Heinrich layers, layers
rich in IRD in the North Atlantic associated with
major abrupt climate change (Bond et al. (1992),
reviewed in Hemming (2004)). It was also used by
Bond and Lotti (1995), as well as others afterwards,
to examine the relative timing of different iceberg
contributors in the North Atlantic. Petrographic studies of IRD can sometimes be used to identify IRD
from specific ice sheets and thus be used to make
inferences about the behavior of specific ice sheets
during times of climate change, and can also be used
to trace the average path of the main surface currents
in regions where icebergs and sea ice are produced.
Geochemical Approaches to Terrigenous
Sediment Provenance
Geochemical approaches, including isotopes, complement information inferred from mineralogical investigations. Although the composition of organic matter
from terrigenous sources is acknowledged as a powerful provenance tracer, only the silicate portion of these
sediments is reviewed here. Geochemical approaches to
the silicate portion of terrigenous sediments allow us to
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Figure 3 Maps of clay mineral distribution in the South Atlantic, along with regional deep currents and local, near-continent surface
currents (A) from Petschick R, Kuhn G, and Gingele F (1996) Clay mineral distribution in surface sediments of the South Atlantic: Sources,
transport, and relation to oceanography. Marine Geology 130: 203–229), surface currents in the South Atlantic (B) from Open University
Oceanography Course Team (1989) Ocean Circulation, 238 p. Oxford/ New York: Pergamon Press; Milton Keynes, England: The Open
University), bathymetry of the South Atlantic (C), January winds (D), and July winds (E) from Open University Oceanography Course Team
(1989) Ocean Circulation, 238 p. Oxford/ New York: Pergamon Press; Milton Keynes, England: The Open University. The distribution of
clay provinces in the South Atlantic appears to be controlled by a combination of surface and deep currents, and possibly surface winds.
constrain provenance age and geochemical history of
the source as well as the weathering history. As
described by McLennan et al. (1993), among the
important geochemical characteristics that define
fundamental provenance types are radiogenic isotope
compositions (reflecting average provenance age),
large-ion lithophile element enrichments, and other
trace element characteristics such as Europium
anomalies (reflecting provenance compositions),
alkali and alkaline earth element depletions and aluminum enrichment (reflecting weathering and alteration), Zr and Hf enrichments (reflecting increased
concentration of zircon by sedimentary sorting), and
high Cr abundances (indicating ultramafic sources).
1780 PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES/Terrigenous Sediments
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Figure 3 (Continued)
Radiogenic Isotope Provenance Tracers
The commonality in these is that they are the radioactive decay products of parents with long half-lives.
They have different sensitivities depending on the
different geochemical behaviors of the parents and
daughters, and on the rock types and geological history of the sources. Thus, they provide provenance
constraints that are related to both the age and timeaveraged geochemical characteristics of sedimentary
sources. The most widely used for constraining the
provenance of terrigenous marine sediments are the
Sm–Nd and Rb–Srsystems (e.g., Dasch (1969),
Goldstein et al. (1984), Grousset et al (1988), Jones
et al. (1994), Grousset and Biscaye (2005), and
Franzese et al. (submitted)). Although less widely
used to date, the U–Th–Pb, Lu–Hf, and K–Ar systems
have also been shown to provide powerful provenance constraints. The Sm–Nd isotope system provides an average age of crust formation of the
sediment’s sources (Goldstein et al., 1984). As
reviewed by Taylor and McLennan (1985), this is
because Sm and Nd are rare earth elements with
similar radii, and thus are generally not separated
by most sedimentary processes (although there are
exceptions). A compelling example of the utility of
Nd isotopes for tracing sediment sources in the North
Pacific is shown in Figure 4, where the eolian contribution from China dominates the sediments’ provenance in the center of the North Pacific.
Although there is a gross negative correlation
between Nd and Sr isotopes in terrigenous sediments
(Fig. 5), the Rb–Sr isotope system may be disturbed
by many geological processes (Dasch, 1969;
Goldstein and Jacobsen, 1988) and thus provides a
wide range of compositions for a given Sm–Nd model
age or "Nd. This is because of the large geochemical
difference between Rb (alkali metal) and Sr (alkaline
earth element) as well as their mineralogical hosts. In
addition to the sedimentary alteration of the Rb–Sr
system described below, metamorphism tends to
break down and reform the mineral hosts of these
elements, and thus metamorphic resetting of the Rb–
Sr isotope system is common. Accordingly, the Rb–Sr
isotope system provides a large signal compared to
our ability to measure the Sr isotope composition.
Constant Flux Proxies
These provide important constraints on sedimentary
redistribution in the ocean. Examples are 230Thxs
(derived from radioactive decay of 234U in the water
column, e.g., Bacon (1984), Henderson and Anderson
(2003)) and 3He (derived from interplanetary dust
particles, e.g., Ozima et al. (1984)). 230Th is a constant
flux proxy because thorium is highly particle reactive,
and thus it is quickly removed from the water column
after forming by radioactive decay of 234U which uniformly produces 230Th in the water column at a
known and constant rate. The 3He content of marine
sediments is a constant flux proxy because interplanetary dust particles rain to the earth at approximately
constant rates and they have extremely high 3He contents. Thus they act like an isotope spike. The
PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES/Terrigenous Sediments
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Figure 4 Nd isotope variations in the North Pacific, shown as "Nd which is the deviation in parts per 104 of a sample from chondritic
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Nd/144Nd. Note the homogeneous distribution of "Nd of –10 in the central North Pacific and the contrast between values in the center
and near the margins where volcanic contributions are important. The composition of terrigneous sediment in the central North Pacific
matches Chinese loess, and thus these data corroborate inferences that wind is an important sediment contributor here. From Jones CE,
Halliday AN, Rea DK, and Owen RM (1994) Neodymium isotopic variations in North Pacific modern silicate sediment and the
insignificance of detrital REE contributions to seawater. Earth and Planetary Science Letters 127: 55–66.
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Figure 5 Nd (reported as "Nd) versus Sr isotope variations in
river sediments (Goldstein and Jacobsen, 1988) and deep-sea
turbidites (McLennan et al., 1990; Hemming and McLennan,
2001). Although there is generally a negative correlation between
these two isotopes, the range in 87Sr/86Sr for a given provenance is
great, particularly for sediments of ancient (low "Nd) provenance.
enhancement of sedimentation rate by advection is
well-known on drift deposits, for example, Bermuda
Rise. In fact, they are favored sites for paleoceanography research due to the high accumulation rates that
provide great temporal resolution. This process is
termed ‘focusing,’ and the ‘focusing factor’ is the
ratio of accumulation rate from chronology to accumulation rate that would hold if only sediment settling
vertically through the water column above the deposition site were deposited. However, geochemical
proxies for constant flux have identified additional
sites where the sediment morphology does not clearly
indicate drift deposits, yet the vertical accumulation of
sediments based on chronology of the sediments is
greater than that estimated from the concentrations
of constant flux proxies in the sediment. Reconciling
diverse interpretations of paleoproductivity changes
and terrigenous contributions to iron-fertilization, for
example, in the eastern equatorial Pacific (e.g., Murray
et al. (1995), Paytan et al. (1996), Marcantonio et al.
(2001), Loubere et al. (2004), Paytan et al. (2004),
Francois et al. (2004), Anderson and Winckler
(2005), Lyle et al. (2005), Anderson et al. (2006)), is
expected to yield important insights into marine sedimentary processes.
Grain Size Distributions
These can provide compelling information about the
transporting agent (wind, water, ice) as well as the
velocity of wind and water. For example, wind tends
to carry fine-grained sediments, with average grain size
decreasing with distance from source (e.g., Rea
(1994)). Bottom currents act to move sediments
their paths along shaped by bathymetric boundaries.
Drift deposits accumulate in association with the movement of these deep water boundary currents. An example in which mineralogy elucidates the movement of
sediments
along
the
Western
Boundary
Undercurrent in the North Atlantic comes from the
eastern Labrador Sea (e.g., Fagel et al. (2001)). These
1782 PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES/Terrigenous Sediments
scientists have shown the dispersion of smectite from
volcanic sources associated with the Icelandic hot spot
into the Labrador Sea, with maximum concentration at
the depth of the Western Boundary Undercurrent.
Bottom currents tend to enrich drift deposits in sortable
silt, grain sizes between 10 and 63 mm (e.g., Bianchi and
McCave (2000)), with the mean sortable silt grain size
being proportional to current velocity and thus an
important measure of bottom currents’ relationship to
climate variability (e.g., Hall et al. (1998), Bianchi and
McCave (1999)). Weltje and Prinz (2003) have showed
that a combined approach of grain size and compositional variability with inverse, end-member modeling
allows de-convolving source and process controls in
terrigenous marine sediments.
A
Smectite
Weathering
Predictable bulk chemical changes occur during
weathering, and the Chemical Index of Alteration
(CIA) has been established as a general guide to the
degree of weathering (Nesbitt and Young, 1982).
Using molecular proportions (and only considering
CaO residing in silicate minerals rather than in phosphate and carbonate):
CIA ¼ 100 * Al2 O3 =½Al2 O3 þ K2 O þ Na2 O þ CaO
For marine sediments it is also necessary to make a
correction on Na2O for seawater salt (e.g., McLennan
et al. (1990)). On the CIA scale, unweathered igneous
rocks have values of 50 or slightly below, while the
most extreme residual clays have values of almost 100.
Typical shales average about 70–75 (Fig. 6). The bulkrock chemistry is dominated by the conversion of
feldspars or glass to clay minerals during weathering.
The contents of some trace elements in sediments
are different from those of their sources due to sedimentary processes of weathering and sorting, and
measurement of these may provide clues about the
processes and pathways of sediment distribution. For
example, during weathering, there is a substantial
increase in the Rb/Sr ratio of most solid residues
(e.g., Dasch (1969)). Rb tends to follow K and is
enriched in K-feldspar and micas, whereas Sr tends
to follow Ca and is enriched in plagioclase and pyroxene. Plagioclase and pyroxene are generally susceptible to chemical attack during weathering, and Sr is
removed to the aqueous system. Rubidium’s hosts
have greater resistance to chemical weathering and
additionally, Rb released during weathering tends to
follow Al and thus be retained in the solid system. This
is because Rb þ , a large alkali trace element, is more
Illite
Muscovite
Shale
Biotite
Plagioclase
Granodiorite
Andesite
Basalt
K-feldspar
Hornblende
Clinopyroxene
CN
Sedimentary Processes
Kaolinite, gibbsite, chlorite
Natural waters
K
Figure 6 Ternary plot of Al2O3, CaO þ Na2O, and K2O of the
terrigenous silicate fraction of sediments. Chemical index of
alteration (CIA) is a measure of the degree of chemical weathering. The terminology comes from Nesbit and Young (1982) and
is reviewed in McLennan et al. (1990, 1993).
readily retained on exchange sites of clays that the
smaller alkaline earth element, Sr2 þ . Thus, terrigenous sediments that are derived from highly weathered terrains and/or sedimentary sources will tend to
have high Rb/Sr ratios. Recent enrichments in Rb
relative to Sr will result in lower 87Sr/86Sr and thus
younger Rb–Sr model ages, so in favorable cases it is
possible to distinguish recent weathering from sedimentary sources in terrigenous marine sediments.
The K2O/Na2O and K2O/CaO of solid residues of
weathering are also increased. Additionally, the K/Ar
system may be almost entirely reset during extreme
weathering. This is because K þ is retained on
exchange sites of clays, analogous to Rb þ , and Ar is
a noble gas that is released to the atmosphere during
the breakdown of feldspar. Accordingly, the CAI, Rb–
Sr and K–Ar systems are complementary approaches
to study time-integrated weathering effects on terrigenous sediments. The fact that much illite is brought to
the ocean as a primary mineral is supported by the
survey of K/Ar ages by Hurley et al. (1963).
Sorting
Sorting also tends to separate denser minerals from the
bulk sediment, and elements like Zr and Hf, that are
enriched in the heavy mineral zircon (ZrSiO4), aid in
identifying extreme sorting effects in sediments (e.g.,
McLennan et al. (1990)). For example, McLennan et
al. (1990, 1993) showed that it is possible to isolate
source compositional variability from sedimentary
sorting by plotting Th/Sc versus Zr/Sc (Fig. 7). Th/Sc
is not influenced appreciably by sorting, but is highly
PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES/Terrigenous Sediments
10
1
Th/Sc
Sediment recycling
(zircon addition)
.1
Compositional
variations
.01
.001
.1
Trailing edge turbidites
Active margin turbides
Loess
1
10
100
1000
Zr/Sc
Figure 7 Th/Sc versus Zr/Sc. Th and Zr are both incompatible in
igneous processes, while Sc is compatible. Accordingly increasing
ratios in source rocks indicate more evolved compositions.
Because sorting does not appreciably separate Th from Sc, but
sorting of zircons makes large differences in the Zr/Sc, increase in
Zr/Sc for relatively constant Th/Sc provides a clear indication of
cases where substantial enrichment of zircon has occurred as a
result of sorting. From McLennan SM, Taylor SR, McCulloch MT,
and Maynard JB (1990) Geochemical and Nd-Sr isotopic composition of deep-sea turbidites – Crustal evolution and plate tectonic
associations. Geochimica et Cosmochimica Acta 54: 2015–2050
and McLennan S M, Hemming SR, McDaniel DK, and Hanson GN
(1993) Geochemical approaches to sedimentation, provenance,
and tectonics. In: Johnsson MJ and Basu A (eds.) Processes
Controlling the Composition of Clastic Sediments, pp.21–40.
Geological Society of America Special Paper 284.
sensitive to compositional variability of the source
because Th is incompatible while Sc is moderately
compatible during melting and crystallization of magmas. Zr and Hf are enriched in the heavy mineral
fraction of sediments, particularly in zircon, which is
among the most robust minerals, surviving multiple
sedimentary cycles, as well as high-grade metamorphism and even melting. Sorting may also separate Rb
from Sr. Clay fractions will generally contain higher
Rb/Sr than the coarse fractions. This can be conceived
simply by considering the main host of Rb and Sr in the
clay fraction to be illite and in the silt and sand fractions to be feldspar. In a first order there is a separation
of these elements according to mineralogy such that
Rb is mostly found in mica and Sr is mostly found in
feldspar. This is the reason there tends to be higher 87
Sr/ 86 Sr in clays compared to sands.
Terrane Types or Provenance
Components
For most practical purposes there are no unique ways of
identifying or ‘fingerprinting’ sediment sources, and it is
1783
useful to group sediment provenance characteristic into
terrane types. McLennan et al. (1990, 1993) have done
this with the goal of expanding our ability to understand tectonic settings from ancient sediment compositions. The groupings used by these authors are also
useful in considering provenance components in modern or young terrigenous marine sediments in the pelagic realm, and they are (1) old upper continental crust,
(2) recycled sedimentary rocks, (3) young undifferentiated arc, (4) young differentiated arc, and (5) exotic
components. A sixth component that may be a significant portion of the terrigenous marine sediments in the
pelagic realm, and away from continental inputs (e.g.,
in the Antarctic Circumpolar current, downstream
from oceanic plateaus and ridges) is (6) oceanic basalt.
Continental crust is distinguished from undifferentiated sources (arc and oceanic basalt) by evolved
major element compositions (e.g., high SiO2/Al2O3
and K2O/Na2O), negative Eu anomalies, and enrichments of normally incompatible over compatible
trace elements. For example, continental crust yields
sediments with light rare earth element enrichment,
high Th/Sc and high La/Sc.
Sediments derived from old upper continental crust
tend to have a substantial component of recycled
sedimentary rocks in their provenance. In addition
to the characteristics generally noted for continental
crust, sediments derived from old continental crust
will tend to have high Rb/Sr and high Th/U reflecting
weathering and sedimentary recycling. The presence
of old continental sources is most simply identified by
radiogenic isotope analyses.
Recycled sedimentary rocks are a common and
important contributor to terrigenous sediments, and
it is difficult to distinguish ambient weathering from
recycled sedimentary sources. Measurements such as
K/Ar ages, Rb/Sr model ages, and clay mineralogy are
probably the best ways to distinguish these in young
terrigenous sediments.
Young undifferentiated arc sources are recently
derived from the mantle without significant intracrustal differentiation. This provenance component
has many attributes in common with oceanic basalt
sources but may have different implications for sedimentary (and thus paleoclimate/paleoceanography)
applications. In Quaternary applications, the distribution of possible sources is largely still available for
study, and the geographic pattern of variation in
provenance helps to distinguish these contributions.
Compared to continental sources they provide sediments with less evolved major element compositions
(e.g., low but variable SiO2/Al2O3 and K2O/Na2O),
absence of negative Eu anomalies, and little enrichments of normally incompatible over compatible
trace elements, so they have low Th/Sc and La/Sc
1784 PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES/Terrigenous Sediments
ratios. They also tend to be less weathered and have a
smaller recycled sedimentary component, so their
CIA and Rb/Sr are low.
Young differentiated arc sources have very similar
characteristics to continental crust, and can be
thought of as young continental crust. In most cases
these sources also tend to be less weathered than old
continental sources, with a smaller recycled sedimentary component, so their CIA and Rb/Sr are low.
In specific cases, identification of even miniscule
amounts of certain exotic components can have very
important application. McLennan et al. (1993) cite
published examples of their utility for constraining the
timing of approach of allochtonous terranes as well as
identifying ocean crust components in modern forearc
turbidites. If identified, exotic components can provide
near-unique information for interpreting provenance of
terrigenous sediments in Quaternary settings.
See also: Glacial Landforms, Sediments: Glacimarine
Sediments and Ice-Rafted Debris. K/Ar and Ar/Ar
Dating. Paleoceanography. Periglacial Landforms,
Rock Forms: Rock Weathering.
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PALEOCEANOGRAPHY, RECORDS
Contents
Early Pleistocene
Late Pleistocene North Atlantic
Late Pleistocene North Pacific
Late Pleistocene South Atlantic
Postglacial Indian Ocean
Postglacial North Atlantic
Postglacial North Pacific
Postglacial South Pacific
Early Pleistocene
T de Garidel-Thoron, CEREGE-CNRS, Aix-Marseille
Université, France
ª
2007 Elsevier B.V. All rights reserved.
Introduction
During the last 3 Myr, the climate of the Earth has
been marked by a succession of large glacial–
interglacial changes which shaped the relief of the
Earth and eventually led toward modern biodiversity. Oceanic sedimentary sequences provide the
most continuous records of these dramatic variations and offer a complementary view of continental records. Among the oceanic indicators of past
climatic changes, the oxygen isotope composition of
foraminifera in deep-sea cores, as a proxy of past
changes in global ice volume, has been one of the
most insightful tools revealing the history of continental ice sheets. The most striking result of
paleoceanographic studies using the 18O was the
discovery of periodic glacial–interglacial successions, which were paced by the orbital forcing as
theorized by Milankovitch. Whereas the late
Pleistocene spectrum is mainly marked by the
100 kyr cycle, the succession of glacial and interglacial periods during the early Pleistocene – from
1.8 to about 0.8 Ma was marked by a very regular
cycle of 41 kyr. Moreover, oxygen isotope records
showed that within the same interval, the buildup
of ice caps in the high latitudes during ice ages was
considerably smaller during the early Pleistocene
than later. The global distribution of continents
during the early Pleistocene was not significantly
different than during the late Pleistocene, eliminating changing geography as a mechanism for the
Pleistocene climate evolution. The 41 kyr ‘world’
of the early Pleistocene is thus a key period of
Earth’s history when global climate changed toward
modern climate without any strong external force
effecting changes, and the study of this interval