Distinguishing between seafloor alteration and fluid flow during subduction using stable isotope geochemistry: examples from Tethyan ophiolites in the Western Alps

Large amounts of fluid, bound up in the hydrated upper layers of the ocean crust, are consumed at convergent margins and released in subduction zones through devolatilization. The liberated fluids may play an integral role in subduction zone processes, including the generation of arc-magmas. However, exhumed subduction zone rocks often record little evidence of large-scale fluid flow, especially at deeper levels within the subduction zone. Basaltic pillows from the high-pressure Corsican and Zermatt-Saas ophiolites show a range of δ¹⁸O values that overall reflect seafloor alteration prior to subduction. However, comparison between the δ¹⁸O values of the cores and rims of the pillows suggests that the δ¹⁸O values of the pillow rims at least have been modified during subduction and high-pressure metamorphism. Pillows that have not undergone high-pressure metamorphism generally have rims with higher δ¹⁸O values than their cores, whereas the converse is the case in pillows that have undergone high-pressure metamorphism. This reversal in the core to rim oxygen isotope relationship between unmetamorphosed and metamorphosed pillows is strong evidence for fluid–rock interaction occurring during subduction and high-pressure metamorphism. However, the preservation of different δ¹⁸O values in the cores and rims of individual pillows and within and between different pillows suggests that fluid flow within the subduction zone was strongly channelled. Resetting of the δ¹⁸O values in the pillow rims was probably due to fluid-hosted diffusion that occurred over relatively short time-scales (<1 Myr).     

Structurally controlled fluid flow associated with breccia vein formation

A breccia vein sampled from a shear zone in greenschist facies metapelites at Mount Isa, Queensland, Australia, shows a systematic variation in vein geometry that is related to the geometry of folding and faulting within the sample. Calcite vein-fill is coarse grained and equigranular, suggesting precipitation in a fluid-filled space. Partially folded veins suggest that veining occurred during folding and faulting. The breccia vein contains a central zone in which dilation has occurred simultaneously in all directions in the plane of section, implying that this was a zone of high fluid pressure and nearly isostatic differential stress during folding and faulting. From these observations, it can be inferred that the breccia vein was a zone of high permeability and a likely fluid channel during deformation. This hypothesis was tested by stable isotope analysis of veins and host rocks. The calcite veins have δ¹³C values of -11.1 ± 0.1% and δ¹⁸O values of 6-10%_o, whereas the host metapelite has δ¹³C values of -10.62 and -10.11% and δ¹⁸O values of 14-15%_o. These values are consistent with an igneous-derived, H₂O-dominated fluid that exchanged little oxygen with the host rocks, but derived much of its carbon from the wall rock. The isotopic disequilibrium between the veins and the wall rock confirms that the fluid was externally derived, and that the breccia vein acted as a channel for large-volume fluid flow within the shear zone.     

Hydrogen, Oxygen and Sulfur Isotope Studies of Seafloor Hydrothermal System at the Desmos Caldera, Manus Back-arc Basin, Papua New Guinea: An Analogue of Terrestrial Acid Hot Crater-lake

Abstract. The Onsen site is an active submarine hydrothermal system hosted by the Desmos caldera in the Eastern Manus Basin, Papua New Guinea. The hydrothermal fluid is very acidic (pH=1.5) and abundant native sulfur is deposited around the vent. The δ³⁴S values of native sulfur range from -6.5 to -9.3 %o. δ³⁴S values of H₂S and SO₄ in the hydrothermal fluid are -4.3 to -9.9 %o and +18.6 to +20.0 %o, respectively. These δ³⁴S values are significantly lower than those of the other hydrothermal systems so far reported. These low δ³⁴S values and the acidic nature of the vent fluids suggest that volcanic SO₂ gas plays an important role on the sulfur isotope systematic of the Onsen hydrothermal system. Relationship among the δ³⁴S values of S-bearing species can be successively explained by the model based on the disproportionation reaction starting from the volcanic SO₂ gas. The predicted δ³⁴S values of SO₂ agree with the measured whole rock δ³⁴S values. δD and δ¹⁸O values of clay minerals separated from the altered rock samples also suggest the contribution of the magmatic fluid to the hydrothermal system. Present stable isotopic study strongly suggests that the Onsen hydrothermal site in the Desmos caldera is a magmatic submarine hydrothermal system.     

Stable Isotope Study of the Langshan Polymetallic Mineral District, Inner Mongolia, China

Abstract: A comprehensive stable isotope investigation was carried out to clarify the geneses of the ore deposits in the Langshan Pb-Zn mineral district. The lead isotope study shows that these deposits were probably formed from 2. 0 to 1. 5 Ga, and were deformed and metamorphosed 1. 45 Ga. Ore lead could be a mixture of mantle lead and crustal lead. The C and S isotope results indicate that these deposits were precipitated in closed or semi-closed rift basins, and the source of sulfur might be Proterozoic ocean sulfate. The H and O isotope results indicate that the δD and δ¹⁸O values of rocks were changed by water-rock interaction during metamorphism and hydrothermal alteration. The scale of δD and δ¹⁸O shift of rocks reflects the grade of metamorphism and alteration as well as the water-rock ratios. However, the water-rock ratios in the metamorphic processes of Langshan mineral district were relatively low, and the source of water during metamorphism is suggested to be ancient meteoric water. Based on isotopic results and the geological background, it is concluded that these deposits may belong to Proterozoic sedimentary exhalative (SEDEX) type.     

Oxygen and hydrogen isotope study of minerals from metapelitic rocks, staurolite to sillimanite zones, Mica Creek, British Columbia

Abstract Oxygen and hydrogen isotope analyses have been made of coexisting quartz, ilmenite, muscovite, and biotite from Late Precambrian metapelitic rocks, staurolite-kyanite to K-feldspar-muscovite-sillimanite zones, from Mica Creek, British Columbia. The δ¹⁸O and †D values of these minerals are generally uniform and do not decrease significantly with increasing metamorphic grade. This implies that there has not been significant infiltration of deep crustal, possibly magmatic, fluids into the metapelites that has been suggested for other high-grade metamorphic terranes. The uniformity of oxygen isotope compositions of the Mica Creek metapelite rocks may reflect isotopic uniformity in the sedimentary protolith rather than widespread exchange with an isotopically homogeneous metamorphic pore fluid.
Temperature estimates based upon ¹⁸O exchange thermometry for samples below the sillimanite zone are in reasonable agreement with the results of garnet-biotite Fe–Mg exchange thermometry. In the higher grade rocks, the oxygen isotope and garnet-biotite thermometry yield results which disagree by about 100°C. The highest temperatures recorded by oxygen isotope thermometry, 595°C, are at least 60°C below the minimum temperatures required by phase equilibria. These discrepancies appear to result from pervasive equilibrium retrograde exchange of oxygen isotopes between coexisting minerals. In addition, there are problems with calibration of garnet-biotite thermometry at higher temperatures. Retrograde oxygen isotope exchange may be a general characteristic of high-grade metamorphic rocks and oxygen isotope thermometry may not usually record peak metamorphic temperatures if they significantly exceed 600°C.     

Stable isotope study of carbonate sediments from the Coorong Area, South Australia

ABSTRACT
The mineralogy and isotope geochemistry of carbonate minerals in the Coorong area are determined by the water chemistry of different depositional environments ranging from seawater to evaporitically modified continental water. The different isotopic compositions of coexisting calcite and dolomite suggest that each of the above two minerals was formed from water of composition and origin unique to that specific mineral. In addition, the dolomite was not formed by simple solid state cation exchange.
The occurrence of two types of dolomite was shown by isotope analysis and SEM observations. The dolomite, which is isotopically light (δ¹³C = -1 to -2% 0 ; δ¹⁸O=+3 to +5%₀) and of fine grain size (˜ 0·5 μm) probably precipitated under the influence of evaporitically modified continental water. Coarser grained dolomite (up to 4 μm) is isotopically heavier (δ¹³C=+3 to +4%₀; δ¹⁸O=+5 to + 6%₀) contains Mg in excess of Ca and was formed in or close to equilibrium with atmospheric CO² probably by the dolomitization of aragonite.     

Oxygen Isotopic Characteristics of Granitic Intrusions Related to Haobugao Zn-Pb-Cu-Sn Skarn Deposit, East Inner Mongolia, China

Abstract. Granitic rocks related to the formation of Haobugao Zn-Pb-Cu-Sn skarn deposit, Inner Mongolia, China, show unusual low whole-rock δ¹⁸O values down to -8.8 % (V-SMOW), whereas separated quartz crystals from those rocks give positive δ¹⁸O values of+4.1 to +9.9 %. Chemical analyses and microscopic observation of those granitic rocks confirm that they suffered hydrothermal alteration. Some skarn specimens and quartz from the Haobugao deposit also show negative δ¹⁸O values. The isotopic evidence indicates that intensive meteoric water circulation occurred at the time of granitic intrusion, and caused the pervasive hydrothermal alteration of granitic rocks and the precipitation of skarn deposit in this area.     

The isotopic composition of oxygen and carbon in the subfossil mollusc shells of the Baltic Sea as an indicator of palaeosalinity

The carbon (δ¹³ C) and oxygen (δ¹⁸O) isotopic composistion in mollusc shells in mainly determined by the isotopic composition of water and dissolved bicarbonate. The δ¹⁸O values of water show a good correlation with the salinity of the Baltic. This correlation served as a basis for reconstructing palaeosalinity and for stratifying the marine sediments according to the δ¹⁸O values of the carbonate skeletons of subfossil shells. The δ¹³C values in shells are mainly determined by the isotopic composition of land-originating bicarbonate, especially in the carbonate skeleton of Lymnaea balthica , which inhabits the immediate coastal zone. According to the δ¹⁸O data, salinity in the investigated area (the coastal area of W and NW Estonia) was highest (about 9–11%) during the Littorina stage. The Limnae a stage had, in general, a salinity similar to the contemporary one, but during some phases possibly exceeding it by 2–3%.     

Foraminiferal changes and geochemical profiles across the Cenomanian/Turonian boundary in central and south-east Poland

Two sections of the Upper Cenomanian and Lower Turonian in central and south-east Poland were investigated for foraminifers, CaCO₃content, carbon content insoluble in HCl (C_org) and in the carbonates (C_carb), carbon and oxygen isotopic composition of bulk-rock carbonates and elemental abundances. The Cenomanian/Turonian boundary interval is characterized by the appearance of more marly facies, a δ¹³C and δ¹⁸O stable isotope anomaly, a considerable increase in C_org content and decrease in C_carb content and substantial changes in the foraminiferal assemblages. A major carbon stable isotope excursion with a shift of +2 (PDB) occurs in the lowermost Whiteinella archaeocretacea Zone. The late Cenomanian δ¹³C anomaly is associated with heavy δ¹⁸O values. The peak value of δ¹³C corresponds to the minima in P/B ratio and in diversity of foraminiferal assemblages. A late Cenomanian anoxic event is thought to be responsible for changes in foraminiferal assemblages. However, elemental abundance analyses do not show changes in the concentrations of trace elements. This may be explained by the long distance between studied area and a source of enrichment which was probably located in the western hemisphere.     

Stable isotopic signature of a palaeoaquifer, Mississippian Alamogordo Member limestones, Sacramento Mountains, New Mexico, USA

Nine stratigraphic sections, each ≈5 m thick, were sampled from the Alamogordo Member limestones of the Lake Valley Formation, Sacramento Mountains, New Mexico, USA. Four stratigraphic sections consist entirely of lime mudstone and wackestone, whereas the other five sections have a prominent layer of crinoidal packstone about 1 m thick at their base. Stable isotopic analyses reveal that the lime muds in the sections with basal packstone layers show a downward decrease in δ¹⁸O and constant δ¹³C values, whereas those in the sections solely composed of lime mudstone and wackestone have, in general, relatively uniform δ¹⁸O and δ¹³C values. The diagenesis of the Alamogordo Member limestones was previously believed to have been governed by the downward percolation of meteoric water from a regional pre-Pennsylvanian exposure surface ≈100 m above this unit. However, the uniform δ¹³C and downward decrease in δ¹⁸O values in the lime muds in the sections with basal packstones indicate that the meteoric water ascended within the Alamogordo Member, rather than descended from the overlying exposure surface. This indicates that the basal packstones were probably a conduit for meteoric water. This is further supported indirectly by the relatively uniform δ¹⁸O and δ¹³C values of the lime mud in the sections without basal packstones. The implications are that the oxygen isotopic gradients may be used to identify palaeoaquifers, flow directions within these aquifers and that meteoric diagenesis below an exposure surface could be governed by flow through a palaeoaquifer.     

Geochemical Process of Gas Hydrate Formation in the Nankai Trough Based on Chloride and Isotopic Anomalies in Interstitial Water

Abstract: Interstitial water expelled from gas hydrate-bearing and -free sediments in the Nankai Trough are analyzed in terms of Cl-, SO₄²-, δ¹⁸O and δD. The baselines for the Cl- concentration and δ¹⁸O value are close to seawater values (530 mM and 0%), indicating that the interstitial water is of seawater origin. The δD values decrease with depth, implying isotopic exchange of hydrogen between upwelling biogenic methane depleted in D and interstitial water. The Cl- concentrations in gas hydrate-bearing sediments are anomalously low, while the δ¹⁸O and δD values are both high, suggesting that the water forming these gas hydrates was poor in Cl- and enriched in ¹⁸O and D during gas hydrate formation. Calculation of the gas hydrate saturations using Cl "and δ¹⁸O anomalies gives results of up to 80 % in sand, and shows that the δ¹⁸O baseline is not consistent with the Cl" baseline. The δ¹⁸O baseline increases by +1% in gas hydrate-free clay and silt. This is considered to be caused by clustering of water molecules after gas hydrate dissociation in response to the upward migration of the base of gas hydrate stability, as indicated by the presence of a double bottom-simulating reflector at this site. The water clusters enriched in ¹⁸O are responsible for the increase in the δ¹⁸O baseline with normal Cl". The abrupt shallowing of the base of gas hydrate stability may induce the dissociation of gas hydrates and the accumulation of gases in the new stability zone, representing a geological process that increases gas hydrate saturation.     

Very high 18o enrichment in Archean cherts from South India: implications for Archean ocean temperature

Stable carbon and oxygen isotopic compositions of essentially unmetamorphosed Archean (> 2.6 Gyr old) cherts and carbonates of the Dharwar Sequence of southern India, from the northernmost part of the Dharwar-Shimoga supracrustal belt (Kalche and Nagargali), have been determined. The cherts from the Nagargali area, which preserve oolitic texture and cryptocrystalline silica, show highly enriched δ¹⁸O values ranging from 28 to 31.4%o relative to SMOW. Such values are the highest yet reported from Archean nondetrital sediments, but are similar to those of modern marine cherts. On the assumption of a seawater δ¹⁸O of 0%₀, calculation of temperature based on the maximum δ¹⁸O value of 31.4%₀ yields a value of 40°C. This is significantly less than 70–80°C reported for the Archean oceans based on cherts and chert-phosphate pairs. Diagenetically recrystallized microcrystalline chert-dolomite pairs of Kalche area exhibit a range of oxygen isotopic ratios similar to those reported for Archean cherts and carbonates from other parts of the world. The temperature of diagenesis is estimated to be about 68°C.     

Geochemical Characteristics of the Akiyoshi Limestones, Japan and Their Bearing on Exploration for Blind Skarn Deposits

Abstract. Carboniferous-Permian limestones of the Akiyoshi Plateau, in the Inner Zone of southwestern Japan, are composed of essentially pure calcium carbonate containing only small amounts of other elements, and they are accompanied by marble and copper skarn deposits near the contact with late Cretaceous granitoids. The δ¹⁸O values of the Akiyoshi limestones range widely from 7.6 to 28.3% and are mostly lower than those of other areas of the same age (23–29%), whereas the differences among the δ¹³C values are small. The δ¹⁸O values are negatively correlated with Mn and Fe contents. Samples with high δ¹⁸O (>25%) and δ¹³C (>2%) values do not contain Fe, Zn, or Pb, but those with low δ¹⁸O values tend to be rich in these elements, indicating that these elements were introduced by interaction with H₂O dominant fluids, possibly of magmatic origin. Potential scores for evaluating the degree of interaction with hydro thermal fluids were calculated for δ¹⁸O, δ¹³C, Fe, Mn, Zn, Pb, and Sr. Higher scores implying much hydrothermal interaction were evident in the Mt. Hananoyama area, where there are many skarn deposits, and along faults oriented mainly NNW-SSE. Therefore, these are promising areas for exploring for blind deposits. It is likely that the hydrothermal fluid traveled through the limestones along fractures at the time of the granitic intrusions. However, the potential scores here are much smaller than those in the Pb-Zn mineralized area of the Kamioka mine, so more detailed petrological and mineralogical investigations are necessary.     

Seasonal and interannual variability of Siberian river discharge in the Laptev Sea inferred from stable isotopes in modern bivalves

Stable oxygen and carbon isotope profiles from modern bivalve shells were investigated in order to reconstruct short-term hydrographical changes in the river-shelf system of the Laptev Sea. Oxygen isotopic profiles obtained from the aragonitic species Astarte borealis exhibit amplitude cycles interpreted as annual hydrographical cycles. These records reflect the strong contrast between summer and winter bottom water conditions in the Laptev Sea. The seasonal variations in δ¹⁸O are mainly controlled by the riverine freshwater discharge during summer with 0.5‰ per salinity unit. Corrected for a defined species-dependent fractionation offset of -0.37‰, time-dependent salinity records were reconstructed from these δ¹⁸O profiles. They indicate a good correspondence to seasonal hydrographic changes and synoptical data. Persistent trends with shell growth towards more negative δ¹³C values are observed in all specimens and appear to be related to metabolic changes of the bivalves during ontogeny. In contrast, short-term fluctuations are likely linked to seasonal variabilities of the river water outflow patterns and enhanced phytoplankton productivity during summer. This is corroborated by a clear watermass-related distinction of the various δ¹³C records made on the basis of water depth and distance from the riverine source.     

Dolomites in the Tazekka Pb–Zn district, eastern Morocco: polyphase origin from hydrothermal fluids

Many different types of water and processes have been proposed for the formation of dolomites. The three phases of hydrothermal dolomites in the Middle Atlas Causse were investigated to elucidate their formation processes. The first two of these are associated with sphalerite and galena in stratiform and open space-filling deposits. These formed early in the history of the deposition of the Pb–Zn mineralization and commonly reveal a paragenetic overlap. A later phase, post-dating Pb–Zn mineralization, is reflected in saddle dolomite.
All three phases show a decrease in δ¹⁸O and δ¹³C values passing from sterile (unmineralized) to mineralized rocks, and isotopic signatures are independent of the carrier facies. However, early-formed dolomites can be separated into two distinct groups on the basis of δ¹⁸O values. Type 1 dolomites host stratiform ore deposits, whereas type 2 dolomites host an open space-filling ore-body. Later saddle dolomites are more depleted in ¹⁸O than either of these.
The early hydrothermal and saddle dolomites precipitated from similar fluids during three distinct events, but formed by two mechanisms: replacement (hydrothermal dolomite) and cement precipitation (saddle dolomite). They show different isotopic signatures and apparently formed at different temperatures. Field data, petrographic and stable isotope results suggest a continuum of replacement, during the Carixian for the early hydrothermal dolomite 1, and during the Toarcian for early hydrothermal dolomite 2, followed by a cement precipitation phase for saddle dolomite.     

Genesis of Kanggur Gold Deposit in Eastern Tianshan Orogenic Belt, NW China: Fluid Inclusion and Oxygen and Hydrogen Isotope Constraints

Abstract. Primary fluid inclusions in quartz and carbonates from the Kanggur gold deposit are dominated by aqueous inclusions, with subsidiary CO₂-H₂O inclusions that have a constant range in CO₂ content (10–20 vol %). Microthermometric results indicate that total homogenization temperatures have a wide but similar range for both aqueous inclusions (120 to 310C) and CO₂-H₂O inclusions (140 to 340C). Estimates of fluid salinity for CO₂-H₂O inclusions are quite restricted (5.9∼10.3 equiv. wt% NaCl), whereas aqueous inclusions show much wider salinity ranging from 2.2 to 15.6 equivalent wt %NaCl.
The 6D values of fluid inclusions in carbonates vary from -45 to -61 %, in well accord with the published δD values of fluid inclusions in quartz (-46 to -66 %). Most of the δ¹⁸O and δD values of the ore-forming fluids can be achieved by exchanged meteoric water after isotopic equilibration with wall rock by fluid/rock interaction at a low water/rock ratio. However, the exchanged meteoric water alone cannot explain the full range of δ¹⁸O and δD values, magmatic and/or meta-morphic water should also be involved. The wide salinity in aqueous inclusions may also result from mixing of meteoric water and magmatic and/or metamorphic water.     

Two-dimensional patterns of metamorphic fluid flow and isotopic resetting in layered and fractured rocks'pa

One-dimensional advection-dispersion models predict that characteristic δ¹⁸O vs. distance and δ¹⁸O vs. δ¹³C profiles should be produced during isothermal metamorphic fluid flow under equilibrium conditions. However, the patterns of isotopic resetting in rocks that have experienced fluid flow are often different from the predictions. Two-dimensional advection-dispersion simulations in systems with simple geometries suggest that such differences may be as a result of fluid channelling and need not indicate disequilibrium, high dispersivities, or polythermal flow. The patterns of isotopic resetting are a function of: (1) the permeability contrast between more permeable layers ('channels') and less permeable layers ('matrix'); (2) the width and spacing of the channels; (3) the width and spacing of discrete fractures; and (4) the orientation of the pressure gradient with respect to layering. In fractured systems, the efficiency of isotopic transport depends on the fracture aperture and the permeability of the surrounding rock. Resetting initially occurs along and immediately adjacent to the fractures, but with time isotopic resetting because of flow through the rock as a whole increases in importance. Application of the one-dimensional advection-dispersion equations to metamorphic fluid flow systems may yield incorrect estimates of fluid fluxes, intrinsic permeabilities, dispersivities, and permeability contrasts unless fluid flow occurred through zones of high permeability that were separated by relatively impermeable layers.     

The evolution of fluorine-rich felsic magmas: source dichotomy, magmatic convergence and the origins of topaz granite

Topaz granite is alkali-feldspar granite that contains essential albite, quartz, K-feldspar, lithium-mica, and topaz. As a group topaz granites are characterized by their extreme enrichment in F (up to 3 wt%) and a wide variety of lithophile elements. They can be subdivided into a 'low-P₂O₅ subtype' (P₂O₅ < 0.1 wt%, Al₂O₃ < 14.5 wt%, SiO₂ > 73 wt%) and a 'high-P₂O₅ subtype' (P₂O₅ > 0.4 wt%, Al₂O₃ > 14.5 wt%, SiO₂ < 73 wt%), the δ¹⁸O values of which indicate a dichotomy of source rock: the low-P₂O₅ subtype (δ¹⁸O < 10‰) having a meta-igneous protolith and the high-P₂O₅ subtype (δ¹⁸O > 10 ‰) a source with a significant component of pelitic material. The unusually high F contents enhance the efficacy of melt segregation and crystal-melt fractionation and so facilitate extreme differentiation in topaz granite magmas. Very low melt volumes restrict the bulk composition of the partial melts regardless of the nature of the source; and extreme fractionation forces them along a path of magmatic convergence, to produce a group of granitic rocks with near-minimum compositions so enriched in a variety of lithophile elements (Li, Nb, Ta, Sn) that economic mineralization often results.     

Oxygen isotope geothermometers for metamorphic rocks

The Chicago mineral-carbonate oxygen isotope fractionation curves have been combined with mineral-water fractionation data for jadeite, zoisite and rutile and new data for grossular-water to provide a set of self-consistent mineral-pair calibrations. The A coefficients in the equation 1000 In α= A × 10⁶T^-2 of the new mineral-pair fractionations are
Jadeite Zoisite Grossular Rutile
Quartz 1.69 2.00 3.03 5.02
Jadeite 0.31 1.34 3.33
Zoisite 1.03 3.02
Grossular 1.99
The isotopic fractionation properties of natural pyralspite garnet [(Ca, Fe, Mg, Mn)₃Al₂Si₃O₁₂] can be approximated by those of the grossular end-member. Appropriate substitutions also yield coefficients for the solid-solution minerals: sodic pyroxene and epidote, e.g.
A _{quartz-sodic pyroxene}= 2.75 - 1.06X_jd,
A _{quartz-epidote}= 2.00 + 0.75X_ps
where X _Jd and X _Ps are the mole fractions of the jadeite and pistacite components, respectively.
The new data set is particularly suitable for the geothermometry of metamorphic rocks. δ¹⁸O data from minerals of the high-pressure metamorphic rocks of the Sesia Zone of Italy and Cyclades Complex of Greece yield well-constrained mean temperatures of 572 and 478 C, respectively. Type III blueschist metabasalts of the Franciscan Formation of California give mean quartz-garnet temperatures of 354 C.     

Oceanic crust production and climate during the last 100 Myr

In order to evaluate the possible influence of oceanic crust production on climatic changes during the past 100Myr variations in total oceanic crust for this period including production at mid-ocean ridges, oceanic plateaus, and back-arc basins were calculated using the most recent and accurate time-scales. The rates presented here differ from those of Larson (1991a, b) on Cenozoic fluctuations and show that (1) maximum production values occurred during the Cenomanian, Palaeocene, and late Oligocene-early Miocene and (2) minimum values occurred in Campanian-Maastrichtian, late Eocene, and middle Miocene. Significantly, variations of oceanic crust production correspond with variations in the δ¹⁸O of deep-water benthic foraminifera: maximum values of oceanic crust production correspond with minimum values of δ¹⁸O, and minimum production values with maximum values of δ¹⁸O. This latter synchronism suggests that changes in land-sea relationships and atmospheric CO₂ related to major fluctuations in oceanic crust production were the main cause of mid-Cretaceous warming and Late Cretaceous cooling, and of climatic quasi-cycles having a periodicity of 33–38 million years over the last 100 Myr. This is the first report showing variations of ocean crust production synchronized with the Cenozoic climate changes.