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OCR for page 3380
Proc. Natl. Acad. Sci. USA
Vol. 96, pp. 3380-3387, March 1999
Colloquium Paper
This paper was presented at the National Academy of Sciences colloquium "Geology, Mineralogy, and Human Welfare,"
held November 8-9, 1998 at the Arnold and Mabel Beckman Center in Irvine, CA.
Oceanic minerals: Their origin, nature of their environment,
and significance
MIRIAM KASTNER*
Scripps Institution of Oceanography, University of California-San Diego, La Jolla, CA 92093-0212
ABSTRACT The chemical and isotopic compositions of
oceanic biogenic and authigenic minerals contain invaluable
information on the evolution of seawater, hence on the history of
interaction between tectonics, climate, ocean circulation, and the
evolution of life. Important advances and greater understanding
of (a) key minor and trace element cycles with various residence
times, (b) isotopic sources and sinks and fractionation behaviors,
and (c) potential diagenetic problems, as well as developments in
high-precision instrumentation, recently have been achieved.
These advances provided new compelling evidence that neither
gradualism nor uniformitarianism can explain many of the new
important discoveries obtained from the chemistry and isotopic
compositions of oceanic minerals. Presently, the best-developed
geochemical proxies in biogenic carbonates are ~8o/~60 and
Sr/Ca ratios (possibly Mg/Ca) for temperature; 87Sr/86Sr for
input sources, Cd/Ca and Ba/Ca ratios for phosphate and
alkalinity concentrations, respectively, thus also for ocean cir-
culation; ~3C/~2C for ocean productivity; B isotopes for seawater
pH;, U. Th isotopes, and ~4C for dating; and Sr and Mn
concentrations for diagenesis. The oceanic authigenic minerals
most widely used for chemical paleoceanography are barite,
evaporite sulfates, and hydrogenous ferromanganese nodules.
Marine barite is an effective alternative monitor of seawater
87Sr/86Sr, especially where carbonates are diagenetically altered
or absent. It also provides a high-resolution record of seawater
sulfate S isotopes, (evaporite sulfates only carry an episodic
record), with new insights on factors affecting the S and C cycles
and atmospheric oxygen. High-resolution studies of Sr, Nd, and
Pb isotopes of well-dated ferromanganese nodules contain invalu-
able records on climate driven changes in oceanic circulation.
-
Before the first global oceanographic HMS Challenger expedi-
tion (1873-1876) and the publication by Murray and Renard (1)
on deep-sea sediments that was based on the expedition, little was
known about oceanic minerals, their origin, distribution, or
relevance. The first recovery of long cores on the Albatross
Swedish expedition (1947-1949) revolutionized our understand-
ing of the Pleistocene-Holocene ice ages recorded in oceanic
minerals. The enormous potential of these minerals, especially of
their chemical and isotopic tracers, containing information on the
chemical history of seawater, thus on the record of interaction
between tectonics, climate, ocean circulation, and evolution of life
was realized. Since, great advances have been made in our
understanding of the chemical and isotopic signatures of marine
minerals, and therefore, of their applicability for unraveling Earth
system's evolution and operation. The history of the rates and
nature of interactions of physical, chemical, and biological pro-
cesses in the ocean, which reflect the interplay between Earth's
internal and external environmental processes, is recorded only in
. .
Oceanlc mmera. .s.
~_
PNAS is available online at www.pnas.org.
Although detrital minerals also are discussed, first-order in-
valuable information on Earth system processes is recorded and
stored in oceanic minerals; hence, in this synthesis the emphasis
is on biogenic and authigenic minerals. Ocean-atmosphere cli-
mate models depend on such data. In addition, the role of biology
in oceanic authigenic mineral formation is briefly evaluated. The
oceanic minerals francolite (carbonate-F-apatite), hydrothermal
Fe-Cu-Zn sulfides, and possibly Mn-Fe oxyhydroxides also may
have economic significance. Characterizing active processes of
formation and understanding modes of occurrence of these
minerals in the modern ocean are necessary for assessing their
potential economic importance and for guiding exploration for
similar deposits in older geologic terrains. The potential influence
of the large amount of clathrate hydrate of methane in continen-
tal margins on global warming and on its status as a future energy
resource is discussed by Kvenvolden (124) in this issue of the
Proceedings.
Sediment Types, Sources, and Distribution in the Ocean
lithe sediments in the ocean, which consist of three major com-
ponents of detrital, biogenic, and authigenic origins, contain
direct and/or indirect evidence of chemical and material inputs
to the ocean and recycling within it. Even though each of the
detrital minerals in the marine sedimentary record contains
significant environmental information (Table 1), the focus of this
paper is on the oceanic minerals. Because oceanic minerals
record the history of the ocean-atmosphere system and hydro-
thermal mineralization strongly impacts seawater composition,
the formation and occurrence of hydrothermal sulfide minerals is
summarized as well.
Detrital Minerals. Aluminosilicate minerals, ultimately de-
rived from the weathering and erosion of rocks on land, comprise
the bulk of detrital sediments. Detrital sediments are transported
by water, wind, or ice. Their distribution in the ocean thus
depends on climate and weathering at the source, the geographic
disposition of rivers, existence of glaciers, and prevailing winds.
They are most abundant in continental margins but occur over the
entire seafloor. Detrital minerals and their spatial and temporal
distribution contain extremely important information on climate
and tectonics (refs. 2-6 and references therein) such as the
changing patterns and intensities of winds (refs. 7-9 and refer-
ences therein), the nature and intensity of weathering and ero-
sion, and detritus transport by rivers and ice. A prime example are
pulses of massive detritus transport by icebergs from northeast
Canada into the Labrador Sea and North Atlantic Ocean during
glacial retreat. These episodes are known as Heinrichs events (i.e.,
ref. 10~. Fresh water from the melting of these icebergs disrupted
the oceanic conveyor circulation and Heinrichs events corre-
spond in time with important climate changes; for example, one
makes the end of the last glacial cycle.
Abbreviation: SST, sea surface temperature.
*To whom reprint requests should be addressed. e-mail: mkastner@
ucsd.edu.
3380
OCR for page 3381
Colloquium Paper: Kastner
Table 1. Oceanic minerals, sources, transport, occurrence, and significance
Proc. Natl. Acad. Sci. USA 96 /61999) 3381
Mineral type Sources and transport Occurrence and significance
Detrital
Terrigenous, mostly clay minerals,
feldspars, and heavy minerals
Volcanic glass (ash) and minerals
Cosmogenic, magnetic metallic
spherules and silicates
Biogenic
Calcite
Aragonite
Opal-A
Celestite
Apatite
Magnetite
Organic matter
Authigenic
Sulfates and phosphate
Barite
Evaporites
Francolite
Mn-Fe oxyphydroxides
Silicates
sio2 phases
Clay minerals
Zeolites
Carbonates
Calciate
Dolomite
Rhodochrosite
Siderite
Sulfides
~ .
Pyrite
Greigite
Hydrothermal
Fe, Cu. Zn sulfides
Weathering and erosion on land, transported as suspended
matter of water, ice, and wind
Subaerial or submarine, volcanic ejecta transported by wind
and water
Extraterrestrial interplanetary dust, meteoritic and cometary
material; gravitational input
Utilizing dissolved chemical species from weathering and
erosion on land, hydrothermal submarine plumes, and
elemental recycling in the ocean
Formation in situ from dissolved matter from weathering
and erosion on land and hydrothermal submarine
plumes, and by diagenesis of detrital and biogenic
particular matter.
.
7'
Formation in situ from discharging hydrothermal
fluids-highly enriched in dissolved metals from
seawater-oceanic basement interaction.
Highest concentrations in continental margins but spread over all ocean floor.
Indicative of source material, type and intensity of weathering and erosion,
glaciation, wind patterns and intensities.
Occur throughout the ocean. Some concentrated at both divergent and convergent
plate boundaries. Indicate history of volcanism and links to climate, valuable for
dating.
Occur over all of the ocean floor. Indicate history of input and implications for
climate, and indicate bolide-related catastrophies.
Calcite and opal-A as oozes that transform diagenetically to chalk, limestone,
porcellanite, and chert. Widespread in ocean and concentrated below high
productivity zones. Corals consist of aragonite, most grow in tropical seas at the
sea-surface, and some in the deep geochemistry and biologic evolution of the
calcite and opal-A biogenic oozes and on coral aragonite. The ultramorphology of
diatom opal-A is valuable for filtration. Apatite occurs primarily as fish teeth is
used for stratigraphy in red-clay deep sea regions devoid of biogenic calcite and
opal-A. Its geochemistry is highly informative of chemical paleoceanography.
Magnetotactic bacteria, that form single domain magnetite crystals, are useful for
magnetic stratigraphy. Organic matter is concentrated beneath high productivity
regions; highly enriched strata may evolve into prolific hydrocarbon producers.
Barite concentrates beneath high productivity regions and its geochemistry
provides critical information on the history of seawater chemistry,
paleoceanography, especially on paleo-productivity. Evaporites for special
tectonic-climate events and desiccation in semi-isolated basins and seas, i.e., the
Mediterranean Sea.
Francolite is an indicator of high productivity zones at continental margins with
medium sedimentation rates. Forms phosphorites, the most important natural
fertilizer, by enrichment through winnowing and redeposition with fluctuating
sea levels, and by replacement of carbonate on some oceanic islands rich in
guano.
As nodules, crusts, and coatings. Scavenge metals from seawater that are extremely
important for chemical paleoceanography.
SiO2 as opal-CT porcellanite or quartz chert mostly from diagenesis of biogenic
opal-A. Some opal forms at hydrothermal vents from elevated silica
concentrations and hydrothermal quartz cements sulfide stockworks. By reacting
with Fe opal-A may transform to nontronite. The smectites nontronite and
saponite are the most important authigenic clay minerals in slow-sedimentation
deep-sea environments, and form by diagenetic alteration of volcanic matter
and oceanic basement. The zeolites phillipsite and analcime mostly reflect on
diagenesis of volcanic matter; clinoptilote and heulandite occur in Si-rich
diagenetic environments.
Diagenetic calcite forms from biogenic calcite recrystallization beneath equatorial
high productivity regions. The other carbonates indicate diagenetic formation in
modibled seawater by primary precipitation or replacement of calcite and
aragonite; mostly in both active and passive continental margins where pore
waters become suboxic to anoxic and Fe-Mn are mobil~zed. Dolomite also
forms in evaporite settings.
Present mostly as pyrite, but also greigite that occurs in excess-Fe or S-deficient
anoxic pore waters. Pyrite is widespread in continental margin sediments with
medium to high sedimentation rates and detritus input that supplies the Fe.
Greigite may effect the magnetic properties of sediments.
In association with submarine hydrothermal systems at spreading centers, including
back arcs, with and without sediment cover, some may be of economic
significance; provide new insights on the origin and evolution of sulfide deposits
that should aid mineral exploration.
Volcanic ejecta comprise another type of detrital sediment
carried by water and wind into the ocean. Volcanic sediments
provide important information on periods of intense island arc
and submarine volcanic activities, which can be compared with
records of submarine tectonics and with climate and biological
productivity records (11, 12~.
About 4-6 x 104 tons/year of cosmogonic particles survive the
Earth's atmosphere and enter the sedimentary record. They
include recognizable magnetic spherules (13), which have distinct
geochemical signatures, in particular, extreme 3He concentra-
tions and 3He/4He ratios (14-16), and high Ir and Os concen-
trations and distinct Os isotope ratios (17-19~. They also provide
crucial information on meteorite impacts and impact catastro-
phes (i.e., refs. 18 and 20) and on the history of cosmogonic
bombardment, which may be one of the controls on climatic
fluctuations (21~.
Biogenic Minerals. Dissolved weathering products constitute
most of the salt in the sea. The major- and minor-element
chemistry of biogenically produced minerals, most importantly
calcite, aragonite, and opal-A (amorphous silica) reflect the
geologic processes and rates that control the chemistry of sea-
water, ocean circulation, and biologic evolution. The less reactive,
long residence time elements such as Cl, Na, K, Mg, Ca, and Sr
do not vary throughout the ocean. Others vary with depth and
between oceans because of biological cycling and scavenging by
particles (22~. Plants and organisms preferentially extract some
elements, primarily C, N, P, Ca, and Si, from seawater to form soft
tissues and minerals. Some of these elements are consumed by
other organisms or redissolve, thus they are internally recycled.
The rest reach the seafloor mostly as calcite and opal-A. At the
seafloor, the rather segregated distribution of calcite, formed by
coccolithophores (phytoplankton) and foraminifera (zooplank-
ton), and of opal-A formed by diatoms (phytoplankton) and
radiolaria (zooplankton), are important indicators of the history
of productivity and ocean circulation. The trace element chem-
istry and isotopic compositions of these skeletal components are
the most powerful tools available to unravel the effects of the
interplay between tectonic and surficial processes on the chemical
history of seawater (i.e., refs. 23-32 and references therein).
OCR for page 3382
3382 Colloquium Paper: Kastner
In addition to calcite and opal-A, aragonite is an important
indicator of chemical paleoceanography. Corals form aragonitic
skeletons. Because of their seasonal growth bands and formation
near the sea surface, the isotopic and trace element compositions
of coral heads are important as recorders at high resolution
(decadal or longer) of late Quaternary sea level fluctuations, sea
surface paleo-temperatures, and rainfall data (i.e., refs. 33-36~.
Another important mineral is celestite, which forms biogenically
by acantharians (zooplankton); however, its high solubility pre-
vents its preservation as fossils. Despite the absence of celestite
from the geologic record, its importance lies in the influence on
recycling of Sr and Ba in the uppermost km of the ocean, with
important implications for chemical paleoceanographic interpre-
tations of Sr/Ca and Ba/Ca ratios in corals and planktonic
foraminifera.
Fish-teeth apatite is highly useful for both stratigraphy and
chemical paleoceanography, especially in red clay deep-sea sed-
iments where biogenic calcite and opal-A are absent because of
dissolution (37, 38~.
Magnetite biomineralization first was identified in chiton teeth
(39), as it hardens their surface. The geologically important
magnetite consists of the morphologically distinct single domain
crystals formed by magnetotactic bacteria (39, 40~. This magne-
tite is responsible for much of the stable magnetic remanence in
many marine sediments, which is valuable in paleomagnetic
studies.
Organic matter-rich sediments signify periods of higher pro-
ductivity and/or higher organic C preservation in low-oxygen
waters, either in a more intense and expanded oxygen minimum
zone or in low-oxygen bottom waters (i.e., ref. 41~. The assumed
link between organic C preservation and water oxygen content
has been challenged (i.e., ref. 42) and presently is being tested.
Nevertheless, the stable carbon isotopic composition of organic
matter in marine sediments helps to identify periods of high
productivity or of high terrestrial organic matter input from
enhanced continental weathering. This information is essential
for modeling the oceanic C cycle and atmospheric CO2 and O2
fluctuations (i.e., refs. 22, 43, and 44 and references therein).
Prime examples of periods with widely distributed organic C-rich
sediments are (a) the mid-Cretaceous abnormally organic C-rich
"black" sediments in all major ocean basins, concentrated pri-
marily in Aptian (115-110 Ma) and Cenomanian/Turonian (~90
Ma) (i.e., refs. 45 and 46 and references therein), known as the
Crateceous Anoxic Events (CAE), and (b) the late Neogene
Mediterranean sapropels (i.e., ref. 47 and references therein).
The mid-Cretaceous organic C burial during the CAE has
significant implications for the ocean-atmosphere system C im-
balance, as seen in the C isotopic composition of marine carbon-
ates (48), hence on atmospheric O2 content. Such "black" shales
are potential reservoir rocks. Worldwide, Cretaceous strata are
known for being prolific hydrocarbon producers (i.e., refs. 49-51~.
Authigenic Minerals. Authigenic minerals form by in situ
inorganic precipitation on the seafloor and within the sediment
column. Barite is the only mineral so far reported to also form in
the water column (52, 53~. Some authigenic mineral reactions are
bacterially mediated; the bacteria modify the immediate geo-
chemical environment, inducing mineral formation. For Paleo-
environmental studies, the most important marine authigenic
minerals are barite, francolite (a carbonate fluor apatite), evapor-
ites, especially anhydrite/gypsum and halite, and Mn-Fe-
oxyhydroxide that occur as nodules or crusts. The concentrations
and isotopic compositions of oceanic conservative components in
these authigenic minerals that form at the seafloor are extremely
informative for studies of the history of seawater chemistry.
Nonconservative chemical components also reveal information
about distinct water masses, thus about ocean circulation in the
geologic past. In addition to diagenetic opal-CI and quartz that
form from the dissolution of biogenic opal-A, other common
authigenic alumino-silicate minerals are smectites and zeolites.
Dolomite and pyrite are also widespread authigenic minerals.
Proc. Natl. Acad. Sci. USA 96 (1999)
Authigenic alumino-silicates from both close to the sediment-
seawater interface and the sediment column mainly form by
replacement of precursor minerals or mineraloids, but also can
form by direct precipitation from pore fluids. On short time scales
(decadal to millenial) the total amount of oceanic authigenic
silicate remineralization seems to exert an extremely small impact
on the global balance between CO2 degassing rate and atmo-
spheric CO2 removal rate by continental weathering of silicates
(43, 54, 55~. Research on the contribution of silicate remineral-
ization to the global CO2 budget in the modern ocean presently
is being actively pursued (56~.
The important marine clay minerals are smectites. The iron
smectite, nontronite, is widespread in the Pacific ocean floor
where hydrothermal activity is prevalent and sedimentation rates
are slow <1 cm/kyr. It forms at the seafloor from hydrothermal
Fe oxyhydroxides and opal-A (57~. In addition, nontronite and
saponite replace precursor volcanic components, especially vol-
canic glass. Diagenetic illitic clays are celadonite and glauconite.
Celadonite is associated primarily with volcanic matter alteration.
Glauconite forms in subtropical and tropical margins in shallow
to intermediate water depths, near the sediment-water interface
beneath high productivity upwelling regions. These areas have
intermediate to slow sedimentation rates, and both ferric and
ferrous Fe are simultaneously available in pore waters. The
zeolite Phillipsite is common at the seafloor or at shallow burial
(58, 59~. It replaces volcanic material in slow sedimentation
environments such as the South Pacific deep sea. Clinoptilolite is
also common and forms within the sediment column where silica
concentrations are elevated to stabilize it, mostly from opal-A
dissolution (59, 604. Occasionally clinoptilolite forms pseudo-
morphs after radiolaria tests (60~. Analcime is significantly less
abundant and typically forms in volcanic ash-rich sediments (59~.
Authigenic calcite is widespread in the ocean. It forms mostly
from biogenic calcite recyrstalization and is geochemically dis-
tinct in its minor and trace element concentrations from its
precursor. This recrystallization process ultimately transforms
calcareous ooze into chalk and limestone. Dolomite forms where
pore-fluid seawater is modified bacterially or by physical-
chemical processes. In continental margins where pore fluids
become suboxic to anoxic, dolomite forms both as a primary
precipitate and by replacing precursor Ca and Ca-Mg carbonates
(61, 624. It also occurs in carbonate platforms where dolomite
replaces CaCO3 by the thermally driven convective flow of
seawater that is being inorganically and bacterially modified along
the flow path. The chemical and isotopic compositions, primarily
of carbon and oxygen, reveal the origins of the various dolomites.
Rhodochrosite and siderite are uncommon marine minerals.
Manganese and Fe mostly substitute for Mg in the dolomite
structure in suboxic to anoxic pore fluid environments where they
are mobilized. In these environments, only in the presence of
much detrital Mn and Fe, rhodochrosite and/or siderite may
form during late diagenesis where the reactions have driven
Mg/Ca ratios in the pore fluids to levels below dolomite stability.
Pyrite forms where sulfate is bacterially reduced. In these
environments Fe+3 is reduced and mobilized, reacting with the
sulfide to form pyrite. Because of the large isotopic fractionation
by the bacterial reduction of sulfate to sulfide (~40%) the amount
of pyrite formation modulates the seawater-sulfate sulfur isotopic
composition and the atmospheric oxygen content, as discussed
below. Greigite, another Fe sulfide, may form kinetically instead
of pyrite, especially in the presence of Fe excess or in S-deficient
anoxic pore waters. The variability in pyrite versus greigite
formation reflects the pH and Fe/S ratio of the pore fluid, which
originally was modified by sulfate-reducing bacteria. Greigite has
distinct magnetic properties.
The most important economic authigenic mineral is francolite,
which forms phosphorite deposits when tectonic-oceanographic
conditions are favorable (63, 64~. Similar to C, about 80% of the
global P burial occurs in continental margins despite the much
greater area of the deep ocean. Understanding the origin of
OCR for page 3383
Colloquium Paper: Kastner
francolite and phosphorites, used principally as fertilizer in de-
tergents and other industrial applications, is essential for effective
exploration for new phosphorite deposits. Oceanic phosphorites
form more than 80% of the world phosphate, whereas guano
deposits are considerably less economically important. Based on
recent research, francolite forms in continental margins at slow
to intermediate sedimentation rates and more efficiently in
suboxic to oxic bottom waters where Fe-redox cycling of P
prevails (64-66~. Its occurrence in this environment is charac-
teristic of high sea-level stands during interglacial periods, where
it is enriched by winnowing and redeposition during glacial
periods (67~. Repeated formation and enrichment of this mineral
is necessary to form economic deposits. Secular variations in the
occurrence of large phosphorite deposits in the geologic record
reflect shifts in global regional patterns of P deposition that are
controlled by tectonics and climate. The chemical and isotopic
signatures in phosphorite francolite that forms at or close to the
sediment-seawater interface are those of the bottom water oce-
anic environment. Because of slow formation and sedimento-
logical reworking and mixing, the chemical paleoceanographic
information stored in phosphorite francolite, although extremely
valuable, is of moderate to low resolution.
Submarine Hydrothermal Sulfide Deposits. Submarine hydro-
thermal massive sulfide deposits forming at divergent plate
boundaries have been discovered only in the past ~20 years.
These deposits primarily consist of the sulfides chalcopyrite,
sphalerite and/or wurtzite, pyrrhotite, pyrite and/or marcasite,
and the associated minerals anhydrite, barite, and opal-A. At the
seafloor in oxidizing low-temperature environments hydrother-
mal sulfides are unstable, they are oxidized to form Fe oxides and
secondary sulfides such as covellite, digenite, and bornite, except
if rapidly covered by new volcanic eruptions (i.e., refs. 68 and 69~.
As yet, observations mainly of surface seafloor sites have been
conducted, and the magnitude and structures of these sulfide
deposits have not yet been thoroughly characterized. The discov-
ery of massive sulfide deposits has had a profound impact on our
understanding of ore-forming processes, but their economic
significance is still uncertain.
At fast- and slow-spreading centers, seawater penetrates sev-
eral km into oceanic basement (maximum depth is above the top
of the magma chamber), where it is heated to 350-400°C and
interacts with oceanic basement and sediments. This fluid as-
cends by buoyancy to form hydrothermal vents and sulfide
deposits. The discharging fluids support rich chemosynthetic,
previously unknown biological communities. The suggestion that
the deep-sea hydrothermal vents environment was possibly the
most conducive environment for the origin of life on Earth is
controversial.
About 30% of the oceanic heat budget is removed by hydro-
thermal circulation (70, 71~. Chemical and isotopic exchange
reactions between the heated seawater and oceanic basement
have a profound influence on seawater chemistry (72, 73), and
thus also effect the mineralogy, geochemistry, and physical
properties of the altering oceanic basement (74, 75~. Hot and
acidic, pH 3-4, discharging hydrothermal fluids are strongly
enriched, not only in metal complexes and 3He, but also in Li, K,
Rb, Ca, Ba, and Si, and have no Mg and sulfate (72~. Overall the
fluxes of these elements out and into the midocean ridges are
comparable to those from river fluxes. Precipitation of minerals
occurs when the hot acid, sulfide, and metal-rich fluids mix with
cold oxidizing alkaline seawater. The entire ocean volume circu-
lates through oceanic basement at spreading centers in 5-7
million years. Especially the common newly formed hydrous
silicate minerals, smectite, ordered mixed layer smectite/illite,
epidote, chlorite, serpentine, and amphiboles exert important
controls on the chemical and physical behaviors of the oceanic
plate when it is subsequently subducted. In particular, as a result
of mineral dehydration reactions, these minerals determine the
amount and timing (temperature) of fluids released in the
Proc. Natl. Acad. Sci. USA 96 (1999) 3383
seismogeneic zone in relation to earthquake cycles and arc
volcanism (76-78~.
In 1994, on Ocean Drilling Program Leg 158 drilling into an
actively forming large sulfide mound in the Trans-Atlantic Geo-
traverse (TAG) area of the Mid-Atlantic Ridge, at ~26°N, a
modern analog of fossil ophiolite-hosted deposits such as those
exposed on Cyprus or Oman, revealed that this sulfide deposit has
been forming for at least 20,000 years by successive hydrothermal
episodes, including brecciation and cementation by anhydrite and
quartz of previous precipitates. Brecciation of these deposits is
induced by anhydrite dissolution between mineralization epi-
sodes. The widespread occurrence of anhydrite indicates seawa-
ter entraintment and its heating to >150°C. Upon cessation of the
hydrothermal activity the anhydrite dissolves and does not survive
in the geologic record. Pyrite is abundant in the upper flow region
together with the anhydrite and with quartz. The hottest hydro-
thermal chimneys (black smokers) are composed of mostly
chalcopyrite and anhydrite; the cooler ones (white smokers) are
dominated by sphalerite. Estimates are that 2.7 million tons of
massive sulfides exist above the seafloor and at least 1.2 million
tons beneath it in the fluid upflow zones at TAG (79~. This is
comparable to an average-sized Cyprus-type ophiolite sulfide
deposit (80~. The basalt below is chloritized. In sedimented and
back arc spreading centers, barite is a common cementing mineral
together with anhydrite. Examples are the Guaymas Basin, Juan
de Fuca Ridge, and Mariana Trough. Unlike anhydrite, barite is
more likely to survive burial and resist diagenesis and dissolution.
Examples of Recent Advances in Studies of Oceanic
Minerals and Implications for Chemical Paleoceanography
and Global Change
Changes in ocean chemistry respond to climate-producing vari-
ations in the exogenic cycle and are recorded in oceanic minerals,
but the record is far from simple and bulk chemical analyses are
inadequate for paleoceanographic studies. Having well-dated
materials is essential for placing geochemical records in a firm
chronology. Important advances and greater understandings of
(a) key minor and trace element cycles, (b) isotopic sources and
sinks and fractionation behaviors especially of the light stable
isotopes (with isotope masses <40), (c) the potential diagenetic
problems of mineral diagenesis, and (d) important developments
in high-resolution analytical instrumentation recently have been
achieved. These advances have clearly reaffirmed that neither
gradualism nor uniformitarinism can explain many of the recent
important discoveries in paleoceanography.
Evidence for Rapid Climate Shifts. Recent results obtained
from the oxygen isotopes of foraminifera tests from deep-sea
cores indicate that the evolutions of seawater chemistry, ocean
circulation, biology, and climate are punctuated by periods of
rapid (decadal to millennial) changes. Among the most well-
known and geochemically clearly marked punctuating events are
the rapid cooling and warming events that lasted a few thousand
to a few hundred years during the longer Pleistocene-Holocene
glacial-interglacial cycles, for example, the younger Dryas (i.e.,
refs. 81-84~. Similar results, but at even higher resolution and
frequencies, have been obtained from ice core studies (i.e., refs.
85-87~. A second prime example is the abrupt and major
extinction at the Cretaceous-Tertiary boundary when a bolide
struck Earth (i.e., refs. 18-20~. Oceanic minerals hold the key to
understanding how the records left by such events and the
resulting feedbacks might be used to gain knowledge about how
Earth works. These event studies leave us better prepared to
predict rapid global warming and may allow us to avoid surprises
in climatic response to anthropogenic perturbations.
The Mediterranean Salinity Crisis. The notable finding of a
thick evaporative formation of halite and anhydrite/gypsum
underlain and overlain by deep-sea sediments, in the deep
Mediterranean Sea (and also the Red Sea), at >3 km water
depths, indicates that the Mediterranean Sea almost completely
OCR for page 3384
3384 Colloquium Paper: Kastner
dried up in the late Miocene Messinian stage 6-5 Ma when the
Gibraltar isthmus tectonically separated it from the Atlantic
Ocean. When the strait of Gibraltar barrier broke at the end of
the Messinian, the Mediterranean Sea was abruptly filled with
Atlantic Ocean water (within ~100 years) (i.e., ref. 88~.
Minor and Trace Elements in Biogenic Calcite and Aragonite
and Their Significance. To understand the unique geochemical
records stored in oceanic minerals it is necessary to develop an
in-depth understanding of biogeochemical processes in the mod-
ern oceans and establish new and redundant proxies for the most
significant oceanographic processes. To be a reliable monitor, a
mineral must inherit the chemical and isotopic composition of
seawater at the time of its formation, must retain this signal intact,
and remain inert to chemical exchange after burial. Another
prerequisite for an effective paleoceanographic proxy is that the
chemical behavior and oceanic distribution of the elements are
rather well known.
The prime mineral used for paleoceanographic studies is
biogenic calcite. Foraminifera shells in core tops are composed of
low Mg-calcite (<1-4 mol% MgCO3) that contains minor and
trace element concentrations, most importantly, Mg, Sr, Ba, Cd,
(and possibly Na). These elements substitute for Ca. Li, B. REE,
U are important trace elements in biogenic calcite that act as
oceanographic environmental indicators. It has been shown that,
similar to paleothermometry using the oxygen isotope ratios of
these tests (e.g., ref. 23), the concentrations of these tracers in the
foraminifera tests, although not representative of the thermody-
namic distribution coefficients, generally reliably and consistently
reflect the seawater composition and the biogeochemical condi-
tions of formation. The biogeochemical controls on the incorpo-
ration of these minor and trace elements in the biogenic calcite
and aragonite, however, are poorly known. Over the past several
years, through difficult and persistent studies, it has been dem-
onstrated that minor and trace elements in biogenic calcite can be
used as proxies for a variety of important chemical oceanographic
properties. The pioneering work by Boyle (89, 90) on cleaning
procedures for studies of Cd in planktonic and benthonic fora-
minifera as a proxy for seawater phosphate, and for using Cd/Ca
ratios in foraminifera shells for ocean circulation studies opened
the field for new developments of trace element proxies of very
low concentrations in the CaCO3 lattice. Improvements in cul-
turing foraminifera, and especially in analytical techniques for
accurate chemical and isotopic analyses of trace components and
isotope ratios in small purified samples, such as the multicollector
inductively coupled plasma mass spectrometry (MC-ICPMS) or
the ion probe, provide great promise for documenting variations
in seawater chemistry and sea surface temperature (SST) at very
high resolution. Ibe application of such techniques also should
help to distinguish between physical-chemical versus biogeo-
chemical controls on the trace element distributions in the calcites
and aragonites.
Presently, the best developed and most used geochemical
proxies in foraminifera calcite shells are:
Oxygen isotope ratios and Sr/Ca ratios. Both are primarily
important as temperature indicators; the latter also indicates
seawater Sr concentrations. Oxygen isotopes, however, are
influenced by both temperature and ice volume (i.e., refs. 24,
91, and 92~.
Sr isotope compositions hold information about fluctuations in
the inputs of Sr into the ocean, through time, by climate,
tectonics, weathering processes, and hydrothermal activity. It is
very useful for stratigraphic correlation and dating (i.e., refs.
93-95 and references therein).
Cd/Ca and Ba/Ca ratios can be used as indicators of phosphate
and alkalinity concentrations, respectively (i.e., refs. 31, 89, and
90~. The C cycle is directly linked to nutrient concentrations
and productivity, for example to phosphate concentration,
therefore both Cd and Ba are important for paleoceanography
(22~.
.
.
.
Proc. Natl. Acad. Scz. USA 96 (1999)
· Carbon stable isotope ratios are used to study ocean pro-
ductivity (i.e., refs. 26 and 444.
· The B isotope ratios of foraminifera (~iB and i°B) reflect
variations in seawater pH, an important parameter influencing
atmospheric CO2 (i.e., refs. 32 and 96), because the boric acid
to borate ratio is strongly pH dependent, and the two dissolved
B species have distinct B isotopic signatures.
U/Th isotopes can be used for dating (i.e., refs. 97 and 98~.
Sr, and possibly Mn, concentrations are excellent indicators of
diagenesis (i.e., refs. 99 and 1004. This is extremely important
for monitoring the long-term integrity of mineral species in
chemical oceanographic studies.
Because Mg readily substitutes for Ca in the calcite structures
and is a major conservative component of seawater, the Mg/Ca
ratio in foraminifera shells may be realized as a valuable
palaeotemperature indicator. This possibility is being pursued
at several laboratories (i.e., ref. 101~.
Strontium, Ba, and Mg can substitute for Ca in aragonite. The
large Sr and Ba are more abundant in aragonite, and the smaller
Mg is more abundant in calcite. As a result, seasonal banded
corals that grow at the sea surface, composed of aragonite, show
great promise for preserving records of ocean chemistry and
temperature over a few centuries with a seasonally to biweekly
resolution. Questions like the "exact" timing of the last glaciation
or the causes and feedbacks of the very rapid climate changes
observed in deep sea and ice cores are important for our
understanding of the dynamics of the large ice sheets, their effect
on ocean circulation, climate, and SST anomalies in the tropics.
The latter is of a key parameter in paleoclimate analysis, thus, for
understanding the causes of past rapid climate fluctuations such
as those responsible for the El Nino Southem Ocean phenome-
non.
For climate studies, U-Th dated bands of coral species that live
in the upper 5-20 m of the ocean are being used. For the SST
record, in addition to oxygen isotope ratios, a measure of both
SST and ice volume, the same coral bands also are analyzed for
Sr/Ca and Mg/Ca ratios. These ratios are thought to provide
more reliable SST data than oxygen isotopes. Corals from the
equatorial Atlantic, Pacific and Indian oceans thus have been
analyzed for these components (i.e., refs. 36, 83, and 102-106~.
The Sr/Ca and Mg/Ca ratios in the aragonite structure are
controlled by the activity of Sr, Mg, and Ca in seawater and the
Sr/Ca and Mg/Ca distribution coefficients between aragonite
and seawater that also depend on temperature. At present it is
assumed that the biological controls on these distribution coef-
ficients are minimal and that the Sr/Ca and Mg/Ca ratios in
seawater are conservative. However, especially for high-
resolution temperature data based on Sr/Ca ratios, the effect of
the precipitation of acantharians SrSO4 shells in the upper water
column on these ratios in corals needs to be better constrained.
The most important result obtained from recent studies of the
3~8O values and Sr/Ca (and Mg/Ca) ratios in tropical corals is
that SST fluctuation between glacial and interglacial periods in
the tropics were considerably larger, by 2-5°C, than previously
recorded by using the oxygen isotopic ratios of foraminifera. The
resulting implications for climate modeling are profound.
Aragonite is unstable at surface seawater and transforms to
calcite. Therefore the above-discussed coral studies are primarily
applicable for Holocene corals. Even in Holocene corals arago-
nite may undergo diagenesis. As yet very little is known about the
behavior of trace elements during diagenesis of coral aragonite.
The absence of calcite is the only reliable test available that
indicates that coral aragonite is pristine; this, however, is inade-
quate. To being able to extend in time the unique and high-
resolution climatic records stored in coral aragonite, several
laboratories have begun to pursue this problem.
Marine Barite as a Monitor of Seawater Sr and S Isotope
Compositions and Ocean Productivity. Marine barite is an ubiq-
uitous minor phase in Pelagic sediments (52, 53), particularly
OCR for page 3385
Colloquium Paper: Kastner
underlying regions of high biological productivity where it reaches
concentrations in the sediment of over 2 weight percent (carbon-
ate free). The exact mechanism of barite formation is not known,
although there are indications that it precipitates inorganically
directly from seawater, in micro-environments containing decay-
ing organic matter, acantharian shells, and siliceous frustules and
tests (i.e., ref. 53~. This authigenic barite forms micro-crystals or
aggregates ranging from 0.5 to 5 mm that is very different in habit
from hydrothermal barite (Fig. 14. The water column is slightly
undersaturated with respect to barite (107), therefore, some
barite must dissolve at greater depths in the water column. This
is observed in dissolved Ba concentration-depth profiles in the
ocean. In most sediments, especially those beneath high produc-
tivity zones, pore-waters rapidly become saturated with respect to
barite; hence, most of this barite is preserved in oxic marine
sediments (108, 109~. Sr isotope analyses of marine barite and its
crystal habit suggest no significant diagenetic alteration of this
mineral occurs after burial in oxic pelagic sediments (109~. The
presence of morphologically and geochemically unaltered marine
barite in calcareous sediments with clear indications of carbonate
diagenesis (for example, Deep Sea Drilling Project Leg 85 Site
574, at >250 m burial depth), further supports this suggestion.
Moreover, the 226Ra activities of marine barite indicate that the
mineral behaves as a closed system in oxic environments (110~.
Thus, marine barite is a highly suitable mineral for a range of
chemical paleoceanographic studies, for example, for seawater Sr
and S isotopic compositions and paleo-productivity.
Minor amounts of particulate barite also form in submarine
hydrothermal systems (1114. Barite dissolves in sulfate-reducing
sediments and reprecipitates at the oxic-anoxic boundaries of
such sediments. These hydrothermal and diagenetic barites may
have Sr isotope ratios that are different from contemporaneous
sea water; they therefore are unsuitable for chemical paleoceano-
graphic studies, but they are relatively easily recognized (Fig. 1~.
As discussed earlier, the strontium isotope ratios in sea water
are influenced by climate, tectonics, weathering, and hydrother-
mal activity at ocean ridges. The evolution of this ratio through
time, determined primarily by measuring the strontium isotope
composition of marine carbonates, holds information about
variations in these processes. It is also useful for stratigraphic
correlation and age dating. Carbonates, however, are absent from
some marine sediments, such as siliceous oozes and red clays, and
can be significantly diagenetically altered in others, especially in
Eocene and older sediments. Like calcite marine barite is an
effective monitor of seawater 87Sr/86Sr. The microcrystals of
marine barite separated from Holocene Pacific, Atlantic, and
Indian ocean sediments all record the modern seawater 87Sr/86Sr
value, and the 87Sr/S6Sr values of barite from sediment samples
spanning the past 35 million years all fall within the range of
published data for carbonates over this time period (109~.
Global changes in climate and atmospheric chemistry also are
intimately related to the sedimentary sulfur cycle, and seawater
sulfate is one of the main reservoirs of the sulfur exogenic cycle.
Proc. Natl. Acad. Sci. USA 96 (1999J 3385
Hence, knowledge of the chemical and isotopic composition of
sulfate is of great importance for understanding coupled geo-
chemical cycles on Earth, especially of C, S. O. and P. Evidence
for large-scale transfers of S between different sedimentary
reservoirs is provided by the evaporite-based isotope record for
oceanic sulfate (112, 113~. Because the geological record of
marine evaporites is episodic, with gaps of tens of million years
and evaporites are susceptible to diagenesis, a continuous sea-
water sulfate sulfur isotope curve for the Cenozoic recently has
been generated at ~1 million year resolution (114~. The new
curve fills significant gaps in the evaporite-based data set. A
comparison between the new sulfate sulfur isotope data and the
carbon isotope record of marine carbonates does not reveal the
clear systematic coupling between the S and C cycles assumed in
the existing major global geochemical cycles models. The new
data indicate that changes in pyrite sulfur and organic carbons
burial rates did not balance each other over a few to tens of
millions of years. This has important implications for modeling of
the atmospheric oxygen concentration over Phanerozoic time.
Ocean productivity influences organic carbon supply to the
sediment and its burial efficiency, thus affecting the partitioning
of CO2 between the ocean and atmosphere and influencing
climate. Large glacial to interglacial fluctuations in atmospheric
CO2 concentrations have been observed in ice cores and related
to variations in ocean productivity (85, 87~. To discern the
coupling between ocean circulation, productivity, and climate, it
is important to be able to estimate past ocean productivity and to
reconstruct its history from the record of marine sediments.
Upwelling of CO2-rich waters in the equatorial Pacific provides
the largest natural source of CO2 to the atmosphere.
As indicated above, because of its relative stability in the oxic
pelagic environment marine barite accumulation rate in sedi-
ments is suitable for reconstructing changes in ocean biological
productivity. Fluctuations in barite accumulation rates down-core
(shown in Fig. 2) indicate that during glacial periods of the past
45 x 104 years, the productivity in the central and eastern
equatorial Pacific was about two times that during intervening
interglacial periods (115~. This result is consistent with other
evidence that productivity was high in the eastern and central
equatorial Pacific during the last glacial.
Ocean Dynamics and Climate Records in Hydrogenous Fer-
romanganese Crusts and Nodules. In the modern ocean, the
thermohaline "conveyor" system circulation (in ~1,500 years)
controls the transfer of heat and solutes between the ocean basins
and thus modulates climate. Dense cold surface seawater, formed
in the North Atlantic, is carried throughout the ocean basins at
depth and is replaced by northward transport of warmer water.
Because circulation pattems most likely have been different in the
past, the resulting global environmental, thus geochemical
changes, are stored in biogenic and authigenic marine minerals.
Hydrogenous Mn-Fe oxyhydroxide nodules and crusts effec-
tively scavenge and incorporate geochemically important trace
elements from seawater, including Be and Th-U, making them
B
FIG. 1. Scanning electron micros-
copy micrographs of (A ) marine barite
separated from late Miocene (9.6 Ma)
equatorial Pacific calcareous sedi-
ment, Deep Sea Drilling Project, Site
575. (B) Hydrothermal barite from a
submarine sulfide chimney at Juan de
Fuca Ridge.
OCR for page 3386
3386 Colloquium Paper: Kastner
6
rat
c'
-
o
to
m
1
o
-- - TT01 3-PC72-1 40°W
VNTR01-8PC-110°W
1,
Hi,
1 .
4
1:
_ \/ ~ ~
--I [~$-l i ~6 ~i~ ~-~ --: ~i- i --'40 - ]
1 i 1 1 1 1 1
ft. ~ Air
~.
/1
~1
~ /tl t
-
i t
item
0 50 100 150 200 250 300 350 400 450
Age (kyr)
suitable for age dating. Recent high-resolution studies especially
of Sr, Nd, and Pb isotopes (i.e., refs. 116-119), of well-dated
samples by ~0Be/9Be isotope ratios (i.e., refs. 120-122) using a
high-precision laser ablation, multicollector inductively coupled
plasma mass spectrometry have successfully recorded important
oceanic circulation changes. For example, the closure of the
Central American isthmus, which changed an E-W ocean circu-
lation to a N-S pattern, had clear geochemical consequences with
evidence stored in Mn-crusts and nodules (123~. Because of the
slow growth of these nodules (1 to <10 mm/million year)
high-precision sampling of them permits much higher resolution
studies of paleoclimatic fluctuations than previously possible.
Especially because of the very short residence time of Pb, the
significant input variations in Pb isotopes in these minerals (i.e.,
refs. 117 and 122), on time scales similar to oceanic oxygen
isotopes in sediments, provide important insights on the history
of coupling between climate and input of Pb into the ocean, thus
on continental erosion, atmospheric CO2, and climate-driven
changes in ocean circulation through time.
The Role of Biology in Oceanic Authigenic Minerals Forma-
tion. Questions about the role of biology in the formation of
oceanic authigenic minerals have been raised. As yet, there is no
evidence for a direct biological involvement in the formation of
any of the known oceanic authigenic minerals. Bacterial processes
induce the formation of the oceanic authigenic minerals franco-
lite, pyrite, and/or greigite, and of the carbonates dolomite,
siderite, and rhodochrosite, by modifying their immediate geo-
chemical environment. Biology also controls the preservation of
bacterially produced magnetite. For example, in organic-rich
sediments with bacteria! reduction of Fe3+ to Fez+, single domain
magnetite crystals are unstable and the remanent magnetization
of these sediments is not preserved.
In continental margin sediments beneath high productivity
regions with moderate sedimentation rates and burial of reactive
organic matter, bacterial nitrate, Fe3+, and sulfate reduction
occur, and reactive Fe3+ is bacterially reduced to Fez+, setting in
motion the Fe-redox cycling of P. This increases the availability
of P for francolite and eventually promotes phosphorite forma-
tion, especially where bottom waters are oxic (64-66~. Fe-redox
cycling occurs in the uppermost sediment column close to the
sediment-seawater interface. Below this zone, sulfide, produced
by bacterial sulfate reduction, combines with the bacterially
mobilized ferrous ion, which precipitates mostly as pyrite, but
occasionally first as greigite, because of kinetic constraints. The
first appearance of greigite occurs especially in areas of excess Fe
and/or deficient S in sediments. Pore water pH, modulated by
sulfate-reducing bacteria, also controls the formation of greigite
versus pyrite. The sulfur isotope ratios of these sulfides indicate
the important role of bacterial mediation in their formation.
Proc. Natl. Acad. Sci. USA 96 (1999
1 145 "c'
o
1 000 ~
855 3
710 ~
Cry
565
420
-
FIG. 2. (A) Accumulation rate of barite
(AR BaSO4) and productivity in the equatorial
Pacific in the past 45 x 104 years; data from
two piston cores VNTRO1-08PC at 110°W
(o) and TTO13-72PC at 140°W (a). Modified
from figure 3 in ref. 115.
In addition to P- and S-bearing minerals, biology also affects
the formation of dolomite. Dolomite, although the thermody-
namically stable carbonate mineral in seawater, does not form in
the pelagic environment because the activation energy required
for the formation of this ordered mineral at the prevailing low
temperatures in the ocean is very high. It does, however, form in
modified seawater with higher than normal alkalinity and/or
Mg/Ca ratios that help to overcome the activation energy barrier.
Dissolved sulfate kinetically inhibits the precipitation of carbon-
ate minerals, especially of dolomite. Both its removal by bacterial
sulfate reduction and the simultaneous alkalinity production
promote dolomite formation (61, 62~. As a result, dolomite forms
inorganically in the marine environment in evaporative sub-
basins where its formation is kinetically enhanced by high Mg/Ca
ratios and elevated temperatures. It also forms by bacterial
mediation in continental margins where most of the buried
organic matter in the ocean occurs and pore fluid alkalinity is
high. Consequently, on many margins authigenic calcite forms
and pore fluid Mg/Ca ratios increase to between 6 to >20,
compared with the seawater ratio of 5.4. Subsequently, at even
higher alkalinity and lower sulfate concentrations dolomite forms
instead of calcite. Dolomite also forms from precursor carbonates
in carbonate platforms where thermally driven convective fluid
flow drives reactions between modified pore fluids by the above
mentioned bacterially mediated reactions. Bacterial sulfate re-
duction and alkalinity production is also widespread in evapora-
tive sub-basins and is an important mechanism for dolomite
formation in these environments.
An important consequence of the vertical sequence of bacterial
redox reactions on other authigenic carbonate mineral formation
is the continuous mobilization of available reactive Mn2+ and
Fez+ by iron-reducing bacteria. This can cause the formation of
rhodochrosite, ankerite, and/or siderite, respectively, instead of
dolomite, in pore fluids with lowered Mg/Ca (~1-2) from earlier
carbonate and silicate diagenetic reactions. Because of the high
Mg content in seawater siderite is less common as an authigenic
oceanic mineral than in continental environments. In addition to
carbonates, biological involvement also has been invoked in
marine barite formation (i.e., refs. 52 and 53), to explain its
formation in the ocean water column, which is undersaturated
with respect to barite solubility (107~; oxidation of organic S to
sulfate in organic-rich micro environments was proposed (i.e.,
refs. 52, 53, and 108), but the biological involvement in marine
barite formation is yet to be demonstrated unequivocally.
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Representative terms from entire chapter:
oceanic minerals
