Seafloor Hydrothermal Activity Research Articles

地球化学的手法による熱水活動変遷の解析

As our understanding of seafloor hydrothermal systems grows, we recognize they are not always stable and sometimes show dramatic changes. In this review, the authors present a compilation of geochemical and geochronological studies that are helpful when investigating the evolving processes of submarine hydrothermal systems. Chapter II describes the systematics and methodology of three dating techniques with discussions on their application to minerals formed by seafloor hydrothermal activities. The K-Ar (Ar-Ar) technique is popular for dating igneous rocks, but it is not appropriate for dating hydrothermal minerals because potassium is a trace component of sulfide/sulfate minerals. Following recent progress, micro-analytical techniques applying laser fusion are applicable for dating fluid inclusions and/or hydrothermal alteration minerals, which could provide important geochronological information. Uranium and thorium series disequilibrium dating have been employed for previous geochronological studies of hydrothermal minerals obtained from submarine ore deposits. To cover a wide time range, it is necessary to use various combinations of parent and daughter nuclides. Applying ESR dating to hydrothermal minerals is a rather new challenge. Although it needs several investigations to establish the methodology, it could be a useful rapid dating technique for a time range of less than one thousand years. Chapter III introduces studies focusing on the evolution of seafloor hydrothermal activities over a short time scale (one week to a few years). Detection of event plumes associated with seafloor lava eruption brought an awareness of episodic hydrothermal activity triggered by magmatic perturbation. Subsequent dive studies revealed evolving geochemical processes, such as major changes of volatiles and elemental species concentrations of venting fluid. With remote real-time monitoring of acoustic T-waves generated by seafloor seismic activities, event detection and response cruises have been conducted successfully to investigate various evolving processes in more detail. Chapter IV introduces studies focusing on the evolution of seafloor hydrothermal activities over a long time scale (tens of thousands of years). Radiometric dating studies of hydrothermal minerals such as sulfide and manganese oxide collected from the TAG mound, which is one of the largest hydrothermal mound structures, reveal an age distribution over at least 15000 years separated by quiescent intervals lasting up to 2000 years. On slow spreading ridges such as the Mid-Atlantic ridge, major fracture systems focus the hydrothermal discharge at one place for more than one thousand years with repeated reactivation. In Chapter V, the authors discuss the direction of future studies. Although hydrothermal systems on mid-oceanic ridges have been well studied, those related to arc-backarc magmatic activities could provide more appropriate fields for studying the evolutionary process of submarine hydrothermal systems. Combining geochronological studies with geochemical and mineralogical studies would be important for reconstructing the evolution process in more detail.

Correction to “Piston core record of Late Paleogene (31 Ma) to recent seafloor hydrothermal activity in the Southwest Pacific Basin”

Correction to “Piston core record of Late Paleogene (31 Ma) to recent seafloor hydrothermal activity in the Southwest Pacific Basin”

Attempt to identify sources of atmospheric methane and carbon dioxide concentrations found in in situ aircraft measurements over Southern Australia

Identifying the sources and sinks of methane and carbon dioxide is important for understanding processes within the Earth's climate system. This paper attempts to use back trajectories to identify sources of atmospheric methane and carbon dioxide as measured by high resolution in situ gas analyzers during aircraft ascents and descents in Southern Australia. Results from the back trajectory analysis were confirmed by also performing a forward trajectory analysis on some of the data. The in situ aircraft measurements were part of a joint Japanese‐Australia field campaign in March and April 2007 near Adelaide, South Australia. The vertical profiles showed considerable variation in methane and carbon dioxide content above the planetary boundary layer. We used back trajectories based on an atmospheric transport model to derive the origin of the air masses which enabled speculation about sources of the gases. We were thus able to identify emission from the volcanoes on Réunion Island in the Indian Ocean and the seafloor hydrothermal activity in the Southeast Indian Ridge, confirming speculations published earlier by other research teams.

Piston core record of Late Paleogene (31 Ma) to recent seafloor hydrothermal activity in the Southwest Pacific Basin

A large diameter piston core containing 8.35 m of metalliferous sediment has been recovered from a small abyssal valley in the remote Southwest Pacific Basin (31° 42.194′S, 143° 30° 331′W; 5082 m water depth), providing unique insight into hydrothermal activity and eolian sedimentation there since the early Oligocene. A combination of fish‐teeth Sr‐isotope stratigraphy and INAA geochemical data reveals an exponentially decreasing hydrothermal flux 31 Ma to the present. Although hydrothermal sedimentation related to seafloor spreading explains this trend, a complex history of late Eocene/early Oligocene ridge jumps, propagating rifts and plate tectonic reorganization of South Pacific seafloor could have also played a role. A possible hiatus in deposition, as recorded by changes in core composition just below 2 m depth, is beyond the resolution of the fish teeth Sr isotope dating method employed here; however, the timing of this interval may be coincident with extinction of the Pacific‐Farallon Ridge at ∼20 Ma. A low flux eolian component accumulating at this site shows an increase relative to the hydrothermal component above 2 m depth, consistent with dust‐generating continental sources far to the west (Australia/New Zealand). This is the first long‐term paleoceanographic record obtained from within the South Pacific “bare zone” (Rea et al., 2006), an anomalous region where Pacific seafloor has largely escaped sediment accumulation since the Late Cretaceous.

Origin of a native sulfur chimney in the Kueishantao hydrothermal field, offshore northeast Taiwan

Analyses of rare earth and trace element concentrations of native sulfur samples from the Kueishantao hydrothermal field were performed at the Seafloor Hydrothermal Activity Laboratory of the Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences. Using an Elan DRC II ICP-MS, and combining the sulfur isotopic compositions of native sulfur samples, we studied the sources and formation of a native sulfur chimney. The results show, when comparing them with native sulfur from crater lakes and other volcanic areas, that the native sulfur content of this chimney is very high (99.96%), the rare earth element (REE) and trace element constituents of the chimney are very low (Sigma REE < 21x10(-9)), and the chondrite-normalized REE patterns of the native sulfur samples are similar to those of the Kueishantao andesite, implying that the interaction of subseafloor fluid-andesite at the Kueishantao hydrothermal field was of short duration. The sulfur isotopic compositions of the native sulfur samples reveal that the sulfur of the chimney, from H2S and SO2, originated by magmatic degassing and that the REEs and trace elements are mostly from the Kueishantao andesite and partly from seawater. Combining these results with an analysis of the thermodynamics, it is clear that from the relatively low temperature (< 116 degrees C), the oxygenated and acidic environment is favorable for formation of this native sulfur chimney in the Kueishantao hydrothermal field.

Advances in the Investigation Technology of Modern Seafloor Hydrothermal Activities

Seafloor hydrothermal activities are involved with extreme conditions.Therefore traditional deep-sea investigation methods can not be satisfied with the investigation demand.Recent technologies,such as visual-sampling technology,submersible technology,in-situ monitoring technology and sampling with fidelity technology and experimental simulation technology,have been developed and widely used.Based on the advances in the investigation technologies associated with modern seafloor hydrothermal activities,the development of the investigation technologies in this field should be paid much attention to by our country.

Microbial remains and other carbonaceous forms from the 3.24 Ga Sulphur Springs black smoker deposit, Western Australia

Well-preserved carbonaceous materials including filamentous microbial remains have been isolated from an Archaean sedimentary package overlying the Sulphur Springs volcanic hosted massive sulphide (VHMS) deposit in the Pilbara of Western Australia, one of the oldest, best-preserved VHMS deposits on Earth. The isotopically light carbonaceous material ( δ 13C values of −26.8 to −34.0‰ V-PDB) occurs as finely striated, lenticular to banded organic matter (OM) within fine-grained epiclastic sediments and silicified materials that overlie the deposit. The predominant banded/lenticular carbonaceous material occurs parallel to original sedimentary bedding planes and is distinctly visible in light microscopy. Transmission electron microscopy (TEM) reveals this OM as bundles of filamentous and tubular structure, closely resembling both modern-day and more ancient microbial forms documented from sea floor hydrothermal environments. Total organic carbon (TOC) has a range of <1.0 to 2.3%, while the thermal maturity of the filamentous microbial bundles indicated by reflectance (% R o), points to maximum temperatures since deposition ca. 3.24 Ga of around 90–100 °C, a factor that has enabled the preservation of their morphology. These results, and the intimate association of sulphides with the OM, are suggestive of a well-developed Archaean sediment-hosted microbial community, situated within a basinal environment associated with an active centre of seafloor hydrothermal activity.

Seafloor hydrothermal activity at off-axial seamounts of backarc spreading in southern Mariana Trough

Seafloor hydrothermal activity at off-axial seamounts of backarc spreading in southern Mariana Trough

A natural laboratory to study arsenic geobiocomplexity

Research on seafloor hydrothermal activity has focused primarily on deep‐sea black smoker‐type locations, which are found along volcanically active portions of the mid‐ocean ridges and in deep back‐arc basins. Submarine hydrothermal activity, however, is not confined to deepwater environments. Hydrothermal vents have been documented on the tops of seamounts, on the flanks of volcanic islands, and in other near‐shore environments characterized by high heat flow. Their easy accessibility, relative to deep‐sea hydrothermal systems, makes them excellent natural laboratories to study a wide range of chemical, physical, and biological processes.

Middle Oxfordian–Lower Kimmeridgian chert nodules in the Holy Cross Mountains, south-central Poland

This report presents the results of field and laboratory (micropaleontologic, petrographic, mineralogical, chemical and isotopic) study of Middle Oxfordian–Lower Kimmeridgian chert nodules from the Holy Cross Mountains (south-central Poland). The chert host facies represent low-energy environments characteristic of changing basinal depths with subordinate influxes of coated carbonate grains derived from a prograding platform. The optical microscopic and SEM studies indicate that the chert nodules consist of cryptocrystalline quartz with subordinate microcrystalline quartz fills and LF-chalcedony sponge spicules. No opal-A and opal-CT precursors of the crypto- and microquartz have been found. These cherts formed as a result of an episodic influx of SiO 2-rich fluids and multistage processes of direct precipitation of cryptoquartz, dissolution (etching), reprecipitation (cementation), and recrystallisation of sponge spicules composed of metastable amorphous opal-A. Of the different genetic models of chert formation, the sea-floor hydrothermal activity linked to the Mid-Polish Rift and extentional tectonics in the Carpathian area seems to be a possible hypothesis.

Submarine volcanoes and high-temperature hydrothermal venting on the Tonga arc, southwest Pacific

Submarine hydrothermal vents and associated seafloor mineralization on the Tonga arc have been found for the first time, in the summit calderas of two shallow-water volcanoes, greatly extending the known areas and diversity of seafloor hydrothermal activity in the western Pacific region. The highest temperature vents (245–265 °C) occur at water depths of 385–540 m near the summit of one volcano at 24°S. The vents are spatially related to basaltic dike swarms exposed at a summit cone and in the caldera walls. Clusters of large (to 10 m high) barite, anhydrite, and sulfide chimneys on the summit cone are vigorously discharging clear hydrothermal fluids with temperatures on the seawater boiling curve. There is abundant evidence of phase separation, which can be seen as flame-like jets of steam (H2O vapor) at the chimney orifices. Pyrite, marcasite, sphalerite, and chalcopyrite line the interiors of the highest temperature vents, similar to black smoker chimneys on the mid-ocean ridges.

Stable isotopic compositions of methane and carbon monoxide in the Suiyo hydrothermal plume, Izu–Bonin arc: Tracers for microbial consumption/production

Effluent hydrothermal water samples in and around the water column of the Suiyo seamount caldera (ca. 2 km diameter), Izu–Bonin arc, were taken to determine concentrations and stable carbon isotopic compositions ( δ 13C) of methane (CH 4) and carbon monoxide (CO) in the plume. Venting fluids on the caldera floor (ca. 1370 m depth) were also examined, using the manned submersible Shinkai 2000, to compare the chemical composition and isotopic composition of CH 4 and CO with those in the hydrothermal plume. Strong CH 4 enrichment was detected not only in the water column within the caldera, but also on those outside the caldera at the depth of ca. 1100 m, the sill depth of the caldera wall. Within the plume, we also detected significant CO enrichment. The δ 13C of CO in the plume, however, exhibited highly 13C-depleted values (− 110‰ VPDB to − 60‰ VPDB) compared with those in seafloor venting hydrothermal fluids (around − 31‰ VPDB). Besides, the plume samples exhibited higher CO / CH 4 ratios depending on the distance from the seafloor venting site. We conclude that in situ microbial activity in the plume causes the CO enrichment in the hydrothermal plume. The present results suggest that CO enrichment in deep sea water could act as a new tracer for unidentified seafloor hydrothermal activities, especially for detecting plumes distant from venting sites, as well as for quantifying in situ microbial activity related to CO production in hydrothermal plumes.

Carbonatization of oceanic crust by the seafloor hydrothermal activity and its significance as a CO 2 sink in the Early Archean

Early Archean (3.46 Ga) hydrothermally altered basaltic rocks exposed near Marble Bar, eastern Pilbara Craton, have been studied in order to reveal geological and geochemical natures of seafloor hydrothermal carbonatization and to estimate the CO 2 flux sunk into the altered oceanic crust by the carbonatization. The basaltic rocks are divided into basalt and dolerite, and the basalt is further subdivided into type I, having original igneous rock textures, and type II, lacking these textures due to strong hydrothermal alteration. Primary clinopyroxene phenocrysts are preserved in some part of the dolerite samples, and the alteration mineral assemblage of dolerite (chlorite + epidote + albite + quartz ± actinolite) indicates that the alteration condition was typical greenschist facies. In other samples, all primary minerals were completely replaced by secondary minerals, and the alteration mineral assemblage of the type I and type II basalts (chlorite + K-mica + quartz + carbonate minerals ± albite) is characterized by the presence of K-mica and carbonate minerals and the absence of Ca-Al silicate minerals such as epidote and actinolite, suggesting the alteration condition of high CO 2 fugacity. The difference of the alteration mineral assemblages between basalt and dolerite is probably attributed to the difference of water/rock ratio that, in turn, depends on their porosity. Carbonate minerals in the carbonatized basalt include calcite, ankerite, and siderite, but calcite is quite dominant. The δ 13C values of the carbonate minerals are −0.3 ± 1.2‰ and mostly within the range of marine carbonate, indicating that the carbonate minerals were formed by seafloor hydrothermal alteration and that carbonate carbon in the altered basalt was derived from seawater. Whole-rock chemical composition of the basaltic rocks is essentially similar to that of modern mid-ocean ridge basalt (MORB) except for highly mobile elements such as K 2O, Rb, Sr, and Ba. Compared to the least altered dolerite, all altered basalt samples are enriched in K 2O, Rb, and Ba, and are depleted in Na 2O, reflecting the presence of K-mica replacing primary plagioclase. In addition, noticeable CO 2 enrichment is recognized in the basalt due to the ubiquitous presence of carbonate minerals, but there was essentially neither gain nor loss of CaO. This suggests that the CO 2 in the hydrothermal fluid (seawater) was trapped by using Ca originally contained in the basalt. The CaO/CO 2 ratios of the basalt are generally the same as that of pure calcite, indicating that Ca in the basalt was almost completely converted to calcite during the carbonatization, although Mg and Fe were mainly redistributed into noncarbonate minerals such as chlorite. The carbon flux into the Early Archean oceanic crust by the seafloor hydrothermal carbonatization is estimated to be 3.8 × 10 13 mol/yr, based on the average carbon content of altered oceanic crust of 1.4 × 10 -3 mol/g, the alteration depth of 500 m, and the spreading rate of 1.8 × 10 11 cm 2/yr. This flux is equivalent to or greater than the present-day total carbon flux. It is most likely that the seafloor hydrothermal carbonatization played an important role as a sink of atmospheric and oceanic CO 2 in the Early Archean.

Composition of the early Oligocene ocean from coral stable isotope and elemental chemistry

A sectioned and polished specimen of the coral Archohelia vicksburgensis from the early Oligocene Byram Formation ( ∼ 30 Ma) near Vicksburg, Mississippi, reveals 12 prominent annual growth bands. Stable oxygen isotopic compositions of 77 growth-band-parallel microsamples of original aragonite exhibit well-constrained fluctuations that range between − 2.0 and − 4.8‰. Variation in δ 18 O of coral carbonate reflects seasonal variation in temperature ranging from 12 to 24 ° C about a mean of 18 ° C. These values are consistent with those derived from a bivalve and a fish otolith from the same unit, each using independently derived palaeotemperature equations. Mg/Ca and Sr/Ca ratios were determined for 40 additional samples spanning five of the 12 annual bands. Palaeotemperatures calculated using elemental-ratio thermometers calibrated on modern corals are consistently lower; mean temperature from Mg/Ca ratios are 12.5 ± 1 ° C while those from Sr/Ca are 5.8 ± 2.2 ° C. Assuming that δ 18 O-derived temperatures are correct, relationships between temperature and elemental ratio for corals growing in today’s ocean can be used to estimate Oligocene palaeoseawater Mg/Ca and Sr/Ca ratios. Calculations indicate that early Oligocene seawater Mg/Ca was ∼ 81% (4.2 mol mol − 1 ) and Sr/Ca ∼ 109% (9.9 mmol mol − 1 ) of modern values. Oligocene seawater with this degree of Mg depletion and Sr enrichment is in good agreement with that expected during the Palaeogene transition from ‘calcite’ to ‘aragonite’ seas. Lower Oligocene Mg/Ca probably reflects a decrease toward the present day in sea-floor hydrothermal activity and concomitant decrease in scavenging of magnesium from seawater. Elevated Sr/Ca ratio may record lesser amounts of Oligocene aragonite precipitation and a correspondingly lower flux of strontium into the sedimentary carbonate reservoir than today.

Generation of petroleum-like hydrocarbons involving with seafloor hydrothermal activity and origin of the Green Tuff petroleum

Generation of petroleum-like hydrocarbons involving with seafloor hydrothermal activity and origin of the Green Tuff petroleum

Redox speciation of chromium in the oceanic water column of the Lesser Antilles and offshore Otago Peninsula, New Zealand

Chromium redox speciation analysis was carried out onboard ship in several water column profiles off the islands of Dominica, St Lucia and Grenada (Lesser Antilles), and at two stations offshore from the Otago Peninsula (New Zealand). The catalytic adsorptive stripping voltammetry with diethylenetriaminepentaacetic acid as complexing reagent and a hanging mercury drop as a working electrode were used. In the samples from the Lesser Antilles, Cr(VI) concentrations ranged around a relatively constant background of 1.5–2.5 nM, except for one higher value of 4.3 nM, whereas Cr(III) was highly variable and reached maximum concentrations of 12 nM. Variable environmental influences, such as hydrothermal activity and atmospheric input, are probable sources for reduced chromium species. In contrast, the Cr(III) contribution was less than 50% of total Cr in subantarctic non-hydrothermal water offshore from the Otago Peninsula and the presence of Cr(III) was restricted to the upper 100 m of the water column. In these depth profiles, Cr (VI) (up to 3.7 nM) was clearly the dominating species.Onboard ship redox speciation was shown to be an effective means to avoid storing artefacts in species determination. We conclude that seafloor hydrothermal activity contributes significantly to the reduced Cr species chemistry of the surrounding seawater.

Helium and carbon gas geochemistry of pore fluids from the sediment-rich hydrothermal system in Escanaba Trough

Ocean Drilling Program (ODP) Leg 169, which was conducted in 1996 provided an opportunity to study the gas geochemistry in the deeper part of the sediment-rich hydrothermal system in Escanaba Trough. Gas void samples obtained from the core liner were analyzed and their results were compared with analytical data of vent fluid samples collected by a submersible dive program in 1988. The gas geochemistry of the pore fluids consisted mostly of a hydrothermal component and was basically the same as that of the vent fluids. The He isotope ratios (R/R A=5.6–6.6) indicated a significant mantle He contribution and the C isotopic compositions of the hydrocarbons [δ 13C(CH 4)=−43‰, δ 13C(C 2H 6)=−20‰] were characterized as a thermogenic origin caused by hydrothermal activity. On the other hand, the pore fluids in sedimentary layers away from the hydrothermal fields showed profiles which reflected lateral migration of the hydrothermal hydrocarbons and abundant biogenic CH 4. Helium and C isotope systematics were shown to represent a hydrothermal component and useful as indicators for their distribution beneath the seafloor. Similarities in He and hydrocarbon signatures to that of the Escanaba Trough hydrothermal system were found in some terrestrial natural gases, which suggested that seafloor hydrothermal activity in sediment-rich environments would be one of the possible petroleum hydrocarbon generation scenarios in unconventional geological settings.

Lithology, geochemistry, and structure of Moke Creek sulphide deposit host rocks, Otago Schist, New Zealand

Three types of greyschist (fissile, dark grey, and light grey thickly segregated types) and subordinate greenschist make up most of the Otago Schist hosting the Moke Creek sulphide deposit. Greyschist in the study area is mostly of Torlesse Terrane affinity, with the exception of a sliver of Caples Terrane affinity schist preserved in the footwall of the Moonlight Fault. The three greyschist types are geochemically indistinguishable; however, the former two types contain higher contents of graphite, which may explain their darker field appearance. The greenschists are shown to represent metamorphosed MORB and within‐plate basalts. Schist immediately hosting the Moke Creek sulphide deposit consists of both muscovite‐rich and subordinate epidote‐rich varieties. The former variety contains up to 6.4 wt% K2O, whereas the epidote‐rich variety is depleted in K2O but contains >5 wt% CaO. It is suggested that sericitisation during hydrothermal alteration associated with sulphide deposition produced the mica‐rich schist. The Moke Creek sulphide deposit shares many similarities with Besshi‐type sulphide deposits formed by seafloor hydrothermal activity. The Moke Creek sulphide deposit may have been formed as vein‐dominated roots to exhalative sulphide deposits. The sulphide layers and associated hydrothermal alteration zones were subsequently deformed into parallelism with lithological layering in the Otago Schist.

Carbonate Minerals in the Warrawoona Group, Pilbara Craton: Implications for Continental Crust, Life, and Global Carbon Cycle in the Early Archean

Abstract: The occurrences of the Early Archean carbonate minerals are compiled and their precipitation processes are investigated for the Warrawoona Group, Pilbara Craton. Sedimentary carbonate rocks such as limestone and dolostone are very rare, and only a small amount of sedimentary carbonate minerals are sometimes contained in the hydrothermal bedded chert, implying that a sink of CO2 was minor in the Early Archean sediments. Moreover, it is very likely that the activity of cyanobacteria forming stromatolites was considerably low in the Early Archean. Microfossils and carbonaceous matter in the hydrothermal cherts are probably derived from a non‐photosynthetic microorganisms related to the seafloor hydrothermal activity. Their preservation in sediments may play a very minor role in carbon sink of the Earth's surface.On the other hand, carbonatized volcanic rocks subjected to seafloor hydrothermal alteration occur ubiquitously in the Early Archean greenstone belts such as the Warrawoona Group, suggesting that the hydrothermally altered oceanic crust had large amounts of CO2 as carbonate minerals. Global carbon cycle in the Early Archean is considered to have been controlled by the intense seafloor hydrothermal alteration. Large amounts of CO2 were sunk into the oceanic crust by the alteration. The carbonatized oceanic crust was partly accreted to the continents and/or island–arcs, and partly subducted into the mantle without decomposition. Significant amounts of carbonate minerals in the carbonatized oceanic crust were very likely to store in the accretionary prisms and mantle, consequently giving rise to a decrease of atmospheric and oceanic CO2.

Seawater strontium isotopes, oceanic anoxic events, and seafloor hydrothermal activity in the Jurassic and Cretaceous

There were three negative seawater strontium-isotope excursions (shifts to lower 87 Sr/ 86 Sr values) during the Jurassic and Cretaceous that were of relatively short duration (5-13 my) and showed a relatively quick recovery to pre-excursion 87 Sr/ 86 Sr ratios. These excursions occurred in the Pliensbachian-Toarcian (Early Jurassic), Aptian-Albian, and Cenomanian-Santonian (Early and Late Cretaceous respec- tively). Each excursion coincided closely in time with an Oceanic Anoxic Event (OAE) marked by sediments unusually rich in organic carbon. The Jurassic OAE occurred at the end of the strontium-isotope excursion, whereas the two Cretaceous OAEs oc- curred at the onset of the accompanying strontium-isotope excursions. The possible causes of these excursions were evaluated by successively examining the changes in the riverine strontium fluxes, riverine 87 Sr/ 86 Sr ratios, or hydrothermal strontium fluxes required to produce each excursion. A range of seawater strontium budgets was used to encompass the uncertainties in modern and ancient cycles. To produce the excursions, we calculate that the riverine strontium fluxes would have had to decrease by 6 to 15 percent or the fluvial 87 Sr/ 86 Sr ratios by 0.00019 to 0.00046. The uncertainties largely stem from the assumed magnitude of the hydrothermal strontium flux at the onset of each excursion. Alternatively, increases in sea-floor hydrothermal activity of 7 to 104 percent could also have produced the strontium-isotope excursions. This large range is due mostly to uncertainties in the relative flux of strontium from axial high-temperature hydrothermal systems and low-temperature off-axis systems. Only a small portion of this range stems from uncertainties in the riverine strontium terms. The possible causes of the excursions were further evaluated by examining several geologic factors that could have affected riverine strontium, including climate change, sealevel, and the eruption of flood basalts. We conclude that neither variations in riverine strontium fluxes nor in 87 Sr/ 86 Sr ratios is the likely cause of the strontium- isotope excursions. The most probable explanation is increased rates of hydrothermal activity related to increased ocean-crust production at the mid-ocean ridges. The close correlation in time between the strontium-isotope excursions and the major Oceanic Anoxic Events (OAEs) is compatible with a causal linkage. We propose that increased ocean-crust production led to enhanced CO2 outgassing and global warming, which in turn led to several processes that acted to make surface ocean waters more productive. However, because OAEs did not occur throughout the proposed periods of enhanced hydrothermal activity, it appears that these processes only preconditioned the oceans for the OAEs: sealevel rise may have been the final trigger. This model explains why all three OAEs did not occur at the same time relative to the onset of excess hydrothermal activity and why OAEs are not associated with every sealevel rise documented in the stratigraphic record.