Cretaceous Seawater Research Articles

Early Cretaceous seawater Os isotope composition: an indicator of a LIP-OAE link?

Early Cretaceous seawater Os isotope composition: an indicator of a LIP-OAE link?

Scleractinian corals produce calcite, and grow more slowly, in artificial Cretaceous seawater

Scleractinian corals produce calcite, and grow more slowly, in artificial Cretaceous seawater

Scleractinian corals produce calcite, and grow more slowly, in artificial Cretaceous seawater

The mineralogies of most biotic and abiotic carbonates have alternated in synchroneity between the calcite (hexagonal) and aragonite (orthorhombic) polymorphs of CaCO3 throughout the Phanerozoic Eon. These intervals of calcite and aragonite production, or calcite seas and aragonite seas, are thought to be caused primarily by secular variation in the molar magnesium/calcium ratio of seawater (mMg/Ca . 2 5 aragonite 1 high-Mg calcite; mMg/Ca , 2 5 low-Mg calcite), a ratio that has oscillated between 1.0 and 5.2 throughout the Phanerozoic. In laboratory experiments, we show that three species of scleractinian corals, which produce aragonite in modern seawater and which have flourished as important reef builders primarily during aragonite seas of the past, began producing calcite in artificial seawater with an ambient mMg/Ca ratio below that of modern seawater (5.2). The corals produced progressively higher percentages of calcite and calcified at lower rates with further reduction of the ambient mMg/Ca ratio. In artificial seawater of imputed Late Cretaceous composition (mMg/Ca 5 1.0), which favors the precipitation of the calcite polymorph, scleractinian corals produced skeletons containing .30% low-Mg calcite (skeletal mMg/Ca , 0.04). These results indicate that the skeletal mineral used by scleractinian corals is partially determined by seawater chemistry. Furthermore, slow calcification rates, resulting from the production of largely aragonitic skeletons in chemically unfavorable seawater (mMg/Ca , 2), probably contributed to the scleractinians’ diminished reef-building role in the calcite seas of Late Cretaceous and early Cenozoic time.

Modelling the geochemical cycle of boron: Implications for the long-term δ11B evolution of seawater and oceanic crust

The boron geochemical cycle has been simulated using a time-dependent geochemical box model that was coupled to a one-dimension model of seawater–oceanic crust interactions. Boron elemental and isotopic compositions of oceanic rocks as a function of depth were calculated by mass balance, using the temperature and porosity profiles of the crust as well as the available experimental and empirical distribution coefficients and fractionation factors between mineral and water. Ranges of boron elemental and isotopic variations of seawater were calculated for crust–seawater interactions that take place from the ridge-axis to the off-axis closure of the hydrothermal system. The present-day δ 11B of seawater (40‰) could represent a steady-state value. However, depending on crustal permeability, lifetime of water–rock interactions, and expansion rate of the oceanic ridge, the δ 11B of seawater may vary from 30‰ to 50‰ at the 10 million year scale. Some boron isotope compositions of Cretaceous biogenic carbonates and ophiolitic serpentinites from Oman are comparable to modern rock samples, suggesting that the δ 11B of Cretaceous seawater was close to the present-day value. Low δ 11B values of some biogenic carbonates cannot be attributed to low pH values of past seawater, but more probably to δ 11B variations of seawater or diagenetic alteration by crustal aqueous fluids. Boron isotope composition of hydrothermally altered serpentines could be considered as a promising proxy of the seawater composition.

Neodymium isotope evolution of NW Tethyan upper ocean waters throughout the Cretaceous

Neodymium isotope compositions of twenty-four fish teeth, nineteen from the NW Tethys and five from different locations within the Tethys, are interpreted to reflect the evolution of Tethyan upper ocean water composition during the Cretaceous and used to track changes in erosional inputs to the NW Tethys and in oceanic circulation throughout the Cretaceous. The rather high ε Nd (up to − 7.6) of the NW Tethyan upper ocean waters recorded from the Late Berriasian to the Early Aptian and the absence of negative excursions during this interval support the presence of a permanent westward flowing Tethys Circumglobal Current (TCC). This implies that temperature variations during this time period, inferred from the oxygen isotope analysis of fish tooth enamel, were not driven by changes in surface oceanic currents, but rather by global climatic changes. The results presented here represent a significant advance over previously published Cretaceous seawater Nd isotope records. Our newly acquired data now allow the identification of two stages of low ε Nd values in the NW Tethys, during the Early Albian–Middle Albian interval (down to − 10) and the Santonian–Early Campanian (down to − 11.4), which alternate with two stages of higher ε Nd values (up to − 9) during the Late Albian–Turonian interval and the Maastrichtian. Used in conjunction with the oxygen isotope record, the fluctuations of ε Nd values can be related to major climatic, oceanographic, and tectonic events that appeared in the western Tethyan domain.

Major and trace element geochemistry and Os isotopic composition of metalliferous umbers from the Late Cretaceous Japanese accretionary complex

Metalliferous umbers and red shales occur as unique products of the Kula‐Pacific ridge‐forearc collision in the Late Cretaceous Shimanto Supergroup, an accretionary complex in Japan. These umbers are closely associated with greenstones of mid‐ocean ridge basalt (MORB) origin and are regarded as hydrothermal metalliferous precipitates related to MOR‐type volcanism. The umbers and red shales were deposited in the trench area where both terrigenous detritus from land and hydrothermal metalliferous particulates from a MOR were supplied simultaneously. Besides a predominance of Fe and Mn, the umbers exhibit remarkable enrichments in P, V, Co, Ni, Zn, Y, Mo, rare earth elements (REEs), and Os relative to continental crustal abundances. The X/Fe (X = Mn, P, V, Co, Ni, Zn, Y, and REEs) ratios and PAAS‐normalized REE patterns of the umbers are very similar to those of modern hydrothermal plume fallout precipitates deposited on flanks of MOR. This indicates that the umbers preserve primary geochemical signatures of hydrothermal metalliferous sediments that scavenged seawater‐derived elements and thus can be used as a proxy for Late Cretaceous seawater. The marine 187Os/188Os ratios reconstructed from the late Maastrichtian umbers range from 0.42 to 0.56 and are very consistent with recent data obtained from the Pacific and Atlantic pelagic carbonates that record an abrupt decline from 0.55 to 0.4 during the period between 67.0 Ma and 65.7 Ma.

Evolution of the Mg/Ca ratio of Cretaceous seawater: Implications from the composition of biological low-Mg calcite

For the evaluation of variations in the Cretaceous seawater Mg/Ca ratio, 12 shells of rudist bivalves, ranging in age from the Late Barremian (112.2 Ma) to the Early Campanian (81.4 Ma), have been analysed for intra-shell variations in chemical (Mg, Sr, Fe, Mn) and isotopic ( δ 18O, δ 13C) compositions of low-Mg calcite. Preservation of fibrous prismatic ultrastructure, low Fe and Mn concentrations, as well as Sr concentrations within the range of marine biological calcite, indicate only very minor diagenetic alteration. In all studied shells, Mg concentrations and δ 18O-values show a more or less well-defined inverse covariance. This pattern is similar to the conditions observed in modern skeletal calcite, and interpreted to result from a combination of a temperature effect and an unidentified metabolic control on the incorporation of Mg into calcite. Average Mg concentrations in rudist calcite are significantly lower than those of modern bivalves, and increase from the Barremian–Albian to the Campanian, while slopes of δ 18O–Mg regressions are similar to those of modern bivalves. Assuming that the Mg incorporation is linearly dependent on the Mg/Ca ratio of seawater, and that Mg incorporation into calcite of modern bivalve mollusks follows the same metabolic pathway than in their extinct Cretaceous relatives, the Mg/Ca molar ratio of Cretaceous seawater increased from ∼1 in the Late Barremian to ∼3 in the Early Campanian. This trend matches previous model predictions of seawater composition, and experimental results from fluid inclusions in halite. A very low Mg/Ca ratio of seawater is confirmed here to have occurred before the onset of the significant mid-Cretaceous increase of seafloor-production rates in the Late Barremian. Uncertainties in the secular variations of seafloor spreading are discussed, as hydrothermal alteration of young oceanic crust is an important driver of changes in the seawater Mg/Ca ratio.

Seawater chemistry, coccolithophore population growth, and the origin of Cretaceous chalk

The magnesium/calcium ratio (Mg/Ca) and calcium (Ca) con- centration of seawater have oscillated throughout geologic time; our experiments indicate that these variables have strongly influ- enced biomineralization and chalk production by coccolithophores. The high Mg/Ca ratio of modern seawater favors precipitation of high-Mg calcite and/or aragonite. In contrast, the low Mg/Ca ratio of imputed Cretaceous seawater favored precipitation of low-Mg calcite. We have discovered that some coccolithophore species to- day secrete skeletal elements of high-Mg calcite, rather than low- Mg calcite, as conventionally believed. These species incorporated less Mg when the ambient Mg/Ca ratio was lowered, secreting low- Mg calcite in imputed Cretaceous seawater. Calcification stimu- lates coccolithophore population growth by contributing CO2 to photosynthesis. Three extant coccolithophore species multiplied much faster as the composition of ambient seawater was shifted toward that estimated for Cretaceous seas. Two of these species secreted high-Mg calcite in ambient seawater having Mg/Ca . 1, and incorporation of Mg in a calcite crystal inhibits growth. Cal- cification of the third species, which secreted low-Mg calcite at all ambient Mg/Ca ratios, is hindered by the high Mg/Ca ratio and low absolute concentration of Ca of modern seawater. We conclude that the ionic composition of Cretaceous seawater enabled cocco- lithophores to produce massive chalk deposits, and conversely, that the ionic composition of modern seawater inhibits population growth for most extant coccolithophore species, which occupy nu- trient-poor waters and fail to respond to fertilization by nitrate, phosphate, or iron.

A critical assessment of mollusk 18O/ 16O, Mg/Ca, and 44Ca/ 40Ca ratios as proxies for Cretaceous seawater temperature seasonality

Seasonal temperature fluctuations of Tethyan coastal water are recorded in a pristine, ∼80-My-old rudist shell from Turkey. In order to critically assess factors other than seawater temperature that might have influenced the shell geochemical record, three different paleo-temperature proxies ( δ 18O, Mg/Ca and δ 44Ca) were applied. In a qualitative manner, all three proxies reflect the same cyclical trends and thus yield robust evidence for seasonal fluctuations in Late Cretaceous surficial seawater temperature. This suggests the successful application of the new δ 44Ca temperature proxy to fossil mollusk calcite. The direct comparison of the three data sets, however, demonstrates that all proxies are fraught with problems. Similar to other studies, 18O/ 16O ratios point to warmer summer temperature maxima than those in comparable costal settings today. Nevertheless, the 18O/ 16O proxy is subject to environmental factors that might lead to an overestimation of peak temperatures. Moreover, Mg/Ca molar ratios are less sensitive to environmental factors but were strongly affected by the ion regulating capability of the rudist bivalve that responded to the low Mg/Ca ratio of Cretaceous seawater. Similarly, uncertainties of the δ 44Ca composition of Cretaceous seawater and the complex bio-calcification of mollusks presently limit the interpretation of δ 44Ca values in terms of absolute seawater temperature. The multiproxy approach applied here, however, documents that these limitations do not obscure the dominant patterns of seasonal sea-surface temperature variations recorded in the biogeochemical rudist archive.

Extremely high sea-surface temperatures at low latitudes during the middle Cretaceous as revealed by archaeal membrane lipids

The middle Cretaceous (125-88 Ma) greenhouse world was characterized by high atmospheric CO 2 levels, the general absence of polar ice caps, and much higher global temperatures than at present. Both δ 1 8 O-based and model-based temperature reconstructions indicate extremely high sea-surface temperatures (SSTs) at high latitudes. However, there are a number of uncertainties with SST reconstructions based on δ 1 8 O isotope data of foraminifera due to diagenetic overprinting effects and tenuous assumptions with respect to the δ 1 8 O value of Cretaceous seawater, the paleoecology of middle Cretaceous marine organisms and seawater pH. Here we applied a novel SST proxy (i.e., TEXT [tetraether index of 86 carbon atoms], based on the membrane lipids of marine crenarchaeota) derived from middle Cretaceous sedimentary rocks deposited at low latitudes. The TEXT proxy indicates that tropical SSTs in the proto-North Atlantic were at 32-36 °C during the early Albian and late Cenomanian-early Turonian. This finding agrees with SST estimates based on δ 1 8 O paleothermometry of well-preserved foraminifera as well as global circulation model calculations. The TEXT proxy indicates cooler SSTs (27-32 °C) for the equatorial Pacific during the early Aptian, which is in agreement with SST estimates based on δ 1 8 O paleothermometry.

Origin of dolomites in the Cretaceous Maha Sarakham evaporites of the Khorat Plateau, northeast Thailand

The Khorat Plateau of northeast Thailand and Laos was an area of widespread deposition of evaporites and siliciclastics (Maha Sarakham Formation) during the Cretaceous and three types of dolomites are associated with this formation: (1) limpid dolomite, (2) coarse, subhedral and euhedral dolomite, and (3) saddle dolomite. Limpid dolomite is present as isolated crystals in recrystallized, clear halite beds while the other two types are only present as authigenic clusters and individual large crystals within the widespread Basal Anhydrite Unit (BAU) (1.1 m), which defines the lowest member of the formation. The δ 18O PDB isotopic values and the 87Sr/ 86Sr ratios of the limpid dolomite are within the range of Cretaceous seawater. Petrography and carbon and oxygen isotopes suggest that limpid dolomite was formed under shallow burial during times of freshening in a mostly marine-fed aggrading halite salina. Dolomite formation was possibly influenced by bacterial metabolism. The coarse crystalline and saddle dolomite types float within a matrix of coarse, recrystallized anhydrite in the Basal Anhydrite. The δ 18O isotopic values of the coarse, subhedral and euhedral dolomite types are more negative than the limpid dolomite, and the 87Sr/ 86Sr ratios are more radiogenic than typical marine dolomite of the Cretaceous. Saddle dolomite has even more negative δ 18O isotopic values than the coarse euhedral dolomite. The 87Sr/ 86Sr ratios of the saddle dolomite are also radiogenic. The isotopic and petrographic data indicate that the dolomites in the Basal Anhydrite Unit are late and authigenic but do not appear to replace a precursor carbonate. Rather, they appear to have precipitated within the deep burial environment along a 1–2-m-thick subsurface mixing interface between warm upwelling basinal brines and dense highly saline brines created by dissolving the underside of the overlying impervious halite bed. The upwelling basinal waters presumably rose through the underlying siliciclastics of the Khorat Group and then moved laterally along the base of the Maha Sarakham salt. The result of this inferred hydrologic process apparently is an accumulation of burial dolomite and recrystallized residual anhydrite in a petrographically complex unit that ultimately appear to be a solution residue, the Basal Anhydrite Unit.

Origin of quartz geodes from Laño and Tubilla del Agua sections (middle–upper Campanian, Basque‐Cantabrian Basin, northern Spain): isotopic differences during diagenetic processes

AbstractQuartz geodes and nodular chert have been found within middle–upper Campanian carbonate sediments from the Laño and Tubilla del Agua sections of the Basque‐Cantabrian Basin, northern Spain. The morphology of geodes together with the presence of anhydrite laths included in megaquartz crystals and spherulitic fibrous quartz (quartzine‐lutecite), suggest an origin from previous anhydrite nodules. The anhydrite nodules at Laño were produced by the percolation of marine brines, during a period corresponding to a sedimentary gap, with δ34S and δ18O mean values of 18.8‰ and 13.6‰ respectively, consistent with Upper Cretaceous seawater sulphate values. Higher δ34S and δ18O mean values of 21.2‰ and 21.8‰ recorded in the Tubilla del Agua section are interpreted as being due to a partial bacterial sulphate reduction process in a more restricted marine environment. The idea that sulphates may have originated from the leaching of previously deposited Keuper sulphate evaporites with subsequent precipitation as anhydrite, is rejected because the δ34S, δ18O and 87Sr/86Sr values of anhydrite laths observed at both the Tubilla del Agua and Laño sections suggest an origin from younger marine brines. Later calcite replacement and precipitation of geode‐filling calcite is recorded in both sections, with δ13C and δ18O values indicating the participation of meteoric waters. Synsedimentary activity of the Peñacerrada diapir, which lies close to the Laño section, played a significant role in the local shallowing of the basin and the formation of quartz geodes. In contrast, eustatic shallowing of the inner marine series of the Tubilla del Agua section led to the generation of morphologically similar quartz geodes. Copyright © 2002 John Wiley & Sons, Ltd.

Deep-sea paleotemperature record of extreme warmth during the Cretaceous

Research Article| February 01, 2002 Deep-sea paleotemperature record of extreme warmth during the Cretaceous Brian T. Huber; Brian T. Huber 1Department of Paleobiology, NHB-121, Smithsonian Institution, Washington, D.C. 20560, USA Search for other works by this author on: GSW Google Scholar Richard D. Norris; Richard D. Norris 2Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA Search for other works by this author on: GSW Google Scholar Kenneth G. MacLeod Kenneth G. MacLeod 3Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA Search for other works by this author on: GSW Google Scholar Geology (2002) 30 (2): 123–126. https://doi.org/10.1130/0091-7613(2002)030<0123:DSPROE>2.0.CO;2 Article history received: 19 Jun 2001 rev-recd: 01 Oct 2001 accepted: 17 Oct 2001 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Brian T. Huber, Richard D. Norris, Kenneth G. MacLeod; Deep-sea paleotemperature record of extreme warmth during the Cretaceous. Geology 2002;; 30 (2): 123–126. doi: https://doi.org/10.1130/0091-7613(2002)030<0123:DSPROE>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Oxygen isotope analyses of well-preserved foraminifera from Blake Nose (30°N paleolatitude, North Atlantic) and globally distributed deep-sea sites provide a long-term paleotemperature record for the late Albian–Maastrichtian interval that is difficult to reconcile with the existence of significant Cretaceous ice sheets. Given reasonable assumptions about the isotopic composition of Cretaceous seawater, our results suggest that middle bathyal water temperatures at Blake Nose increased from ∼12 °C in the late Albian through middle Cenomanian to a maximum of 20 °C during the latest Cenomanian and earliest Turonian. Bottom waters were again ∼12 °C during the middle Campanian and cooled to a minimum of 9 °C during the Maastrichtian. Correlative middle bathyal foraminifera from other ocean basins yield paleotemperature estimates that are very similar to those from Blake Nose. Comparison of global bottom-water temperatures and latitudinal thermal gradients suggests that global climate changed from a warm greenhouse state during the late Albian through late Cenomanian to a hot greenhouse phase during the latest Cenomanian through early Campanian, then to cool greenhouse conditions during the mid-Campanian through Maastrichtian. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Strontium isotope stratigraphy of Turonian–Campanian Gosau-type rudist formations in the Northern Calcareous and Central Alps (Austria and Germany)

Shells of hippuritid rudist bivalves from 16 localities of Upper Cretaceous rudist formations in Gosau-type sedimentary basins of the Eastern Alps (Northern Calcareous Alps and Austroalpine Units) have been analysed for87Sr/86Sr values. The extent of diagenetic alteration of original isotopic compositions was assessed by analyses of concentrations of Sr, Mg, Fe and Mn, and proved to be minor in the samples investigated. A comparison of87Sr/86Sr values of rudist shells with previously published variations of87Sr/86Sr in Late Cretaceous sea water allows correlation with biostratigraphical standard zonations. Considerable stratigraphic revisions are the result for localities where previous biostratigraphical evidence was scarce or imprecise, while existing stratigraphical concepts are confirmed for many other localities where such evidence exists. A Late Turonian to earliest Coniacian age of several shells of Vaccinites inaequicostatus at localities in the environs of Lake St. Wolfgang is confirmed by similar isotopic ratios of shells of the same species in Greece and northern Italy. Previous identifications of hippuritids at the localities studied are discussed, and four characteristic associations are identified.

Geochemistry of the Margi umbers, Cyprus, and the Os isotope composition of Cretaceous seawater

Research Article| November 01, 1999 Geochemistry of the Margi umbers, Cyprus, and the Os isotope composition of Cretaceous seawater G. Ravizza; G. Ravizza 1Woods Hole Oceanographic Institution, MS 22, Woods Hole, Massachusetts 02543-1541, USA Search for other works by this author on: GSW Google Scholar R. M. Sherrell; R. M. Sherrell 2Institute of Marine and Coastal Sciences, P.O. Box 321, Rutgers University, New Brunswick, New Jersey 08903, USA Search for other works by this author on: GSW Google Scholar M. P. Field; M. P. Field 2Institute of Marine and Coastal Sciences, P.O. Box 321, Rutgers University, New Brunswick, New Jersey 08903, USA Search for other works by this author on: GSW Google Scholar E. A. Pickett E. A. Pickett 3Department of Geology and Geophysics, University of Edinburgh, Grant Institute, West Mains Road, Edinburgh EH9 3JW, UK Search for other works by this author on: GSW Google Scholar Author and Article Information G. Ravizza 1Woods Hole Oceanographic Institution, MS 22, Woods Hole, Massachusetts 02543-1541, USA R. M. Sherrell 2Institute of Marine and Coastal Sciences, P.O. Box 321, Rutgers University, New Brunswick, New Jersey 08903, USA M. P. Field 2Institute of Marine and Coastal Sciences, P.O. Box 321, Rutgers University, New Brunswick, New Jersey 08903, USA E. A. Pickett 3Department of Geology and Geophysics, University of Edinburgh, Grant Institute, West Mains Road, Edinburgh EH9 3JW, UK Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1999) 27 (11): 971–974. https://doi.org/10.1130/0091-7613(1999)027<0971:GOTMUC>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation G. Ravizza, R. M. Sherrell, M. P. Field, E. A. Pickett; Geochemistry of the Margi umbers, Cyprus, and the Os isotope composition of Cretaceous seawater. Geology 1999;; 27 (11): 971–974. doi: https://doi.org/10.1130/0091-7613(1999)027<0971:GOTMUC>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The major element, rare earth element, and platinum-group element chemistry of the Margi umbers have been investigated in order to evaluate these sediments as recorders of past variations in the Os isotopic composition of seawater. Because of the compositional similarity of the Margi umbers to metalliferous sediments and hydrothermal-plume particles from the modern ocean, we conclude that these supraophiolite sediments record the Os isotope composition of Cretaceous seawater. This conclusion requires that the Margi umbers have in large part retained their compositional integrity throughout burial diagenesis and subsequent uplift. That the 187Os/188Os ratio (0.52–0.55) of ca. 90 Ma seawater is similar to, or more radiogenic than, seawater throughout the Paleogene provides evidence of decoupling of the marine Sr and Os isotope records. Beyond constraining the 187Os/188Os of Cretaceous seawater, these results are significant in that they provide a set of chemical criteria that can be used to evaluate the potential of other metalliferous sediment sequences as recorders of the 187Os/188Os ratio of ancient seawater. Establishing these criteria represents an important step toward extending the marine Os isotope record into the Mesozoic and Paleozoic. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Porewater sulphur geochemistry and fossil preservation during phosphate diagenesis in a Lower Cretaceous shelf mudstone

Lower Cretaceous mudstones exposed at Speeton in North Yorkshire, UK, contain lobsters and burrows preserved in diagenetic phosphate concretions. Isotopic compositions of sulphur in both diagenetic sulphide and structural sulphate in diagenetic phosphate have been measured in an attempt to constrain diagenetic porewater chemistry. The occurrence of phosphatized and pyritized lobsters and similarly preserved burrows, allows a detailed comparison of these microenvironments with the host sediments. Host sediments are extensively bioturbated and characterized by very light sulphide isotopic compositions (mean sulphide δ34S = –48·3 ± 3‰ (1σ, n = 19)) and sulphate isotopic compositions that are lighter than Lower Cretaceous seawater sulphate (mean sulphate δ34S = +8·7 ± 3·2‰ (1σ, n = 19)). These isotopic values can be explained by the action of bioturbating macrofauna; the introduction of oxygen in ventilating seawater results in the oxidation of early formed isotopically light pyrite, resulting in porewater sulphate that is enriched in 32S. Subsequent pyrite formation via bacteriogenic reduction of isotopically light porewater sulphate leads to the formation of isotopically light pyrite, whilst residual porewater sulphate apparently remains relatively enriched in the 32S isotope. Sulphur isotopic values for the burrows are very different; sulphide isotopic compositions average –34·4 ± 0·4‰ (1σ, n = 3) and sulphate isotopic compositions average –14·4 ± 6·6‰ (1σ, n = 4). These isotopic compositions are the result of rapid development of closed system conditions in burrows, resulting in the build‐up of acidity necessary for phosphate precipitation and coprecipitation of isotopically light sulphate (formed by oxidation of isotopically light sulphide surrounding the burrows). Lobster shell isotopic compositions fall between these two groups. On the basis of their isotopic compositions, some lobsters appear to have died close to the sediment–water interface, whilst others appear to have been buried (in burrows) prior to death.

Kaolinite and dickite formation during shale diagenesis: isotopic data

X-ray diffraction, scanning electron microscopy and O-isotope geochemistry have been used to investigate the origin and possible controls on polymorphic transformation of kaolin minerals filling veins in Cretaceous shales from the Gibraltar Strait area (southern Spain). The mineralogy of the enclosing shales indicates that kaolin minerals formed from smectite dissolution, a process that silmultaneously originated I/S mixed-layers and quartz. Kaolinite and dickite δ 18O values suggest that an increase in the water isotopic composition, from Cretaceous sea water values (−1%) to values of about 3%, occurred parallel to smectite dissolution, the intensity of this process being the main factor controlling the isotopic composition of kaolin minerals. The minimum formation temperature ranges from 62°C for kaolinite to 86–96°C for dickite, indicating that the depth of burial was the main control on polymorph formation. This temperature range agrees with that deduced for illite/smectite ordering. The passage from kaolinite- to dickite-rich veins was accompanied, as deduced from SEM examination, by a morphologic evolution characterized by the division of large vermiculae, dominant in kaolinite samples, and the formation of short stacks and platy crystals, which are predominant in dickite. The mechanism of dickite formation, however, remains uncertain.

High-resolution dating of deep-sea clays using Sr isotopes in fossil fish teeth

Strontium isotopic compostitions of ichthyoliths (microscopic fish remains) in deep-sea clays recovered from the North Pacific Ocean (ODP holes 885A, 886B, and 886C) are used to provide stratigraphic age control within these otherwise undatable sediments. Age control within the deep-sea clays is crucial for determining changes in sedimentation rates, and for calculating fluxes of chemical and mineral components to the sediments. The Sr isotopic ages are in excellent agreement with independent age datums from above (diatom ooze), below (basalt basement) and within (Cretaceous-Tertiary boundary) the clay deposit. The 87Sr/ 86Sr ratios of fish teeth from the top of the pelagic clay unit (0.708989), indicate an Late Miocene age (5.8 Ma), as do radiolarian and diatom biostratigraphic ages in the overlying diatom ooze. The 87Sr/ 86Sr ratio (0.707887) is consistent with a Cretaceous-Tertiary boundary age, as identified by anomalously high iridium, shocked quartz, and sperules in Hole 886C. The 87Sr/ 86Sr ratios of pretreated fish teeth from the base of the clay unit are similar to Late Cretaceous seawater (0.707779-0.707519), consistent with radiometric ages from the underlying basalt of 81 Ma. Calculation of sedimentation rates based on Sr isotopic ages from Hole 886C indicate an average sedimentation rate of 17.7 m/Myr in Unit II (diatom ooze), 0.55 m/Myr in Unit IIIa (pelagic clay), and 0.68 m/Myr in Unit IIIb (distal hydrothermal precipitates). The Sr isotopic ages indicate a period of greatly reduced sedimentation (or possible hiatus) between about 35 and 65 Ma (Eocene-Paleocene), with a linear sedimentation rate of only 0.04 m/Myr The calculated sedimentation rates are generally inversely proportional to cobalt accumulation rates and ichthyolith abundances. However, discrepancies between Sr isotope ages and cobalt accumulation ages of 10–15 Myr are evident, particularly in the middle of the clay unit IIIa (Oligocene-Paleocene).

Strontium isotopic variations in Jurassic and Cretaceous seawater

A high-resolution seawater strontium isotope curve has been generated through the analysis of well-dated and well-preserved belemnites and oysters from the Middle and Upper Jurassic and the Lower Cretaceous of Great Britain. Analysis of Fe and Mn concentrations in these fossils has yielded criteria for eliminating samples that are diagenetically altered. The strontium isotope curve remains relatively flat through the Aalenian and early Bajocian, rapidly descends through the late Bajocian and Bathonian, and reaches a minimum in the Callovian and Oxfordian. It then begins a rapid increase in the Kimmeridgian and Portlandian that continues through much of the early Cretaceous. The curve levels off in the Barremian, suddenly dips downwards in the Aptian, and recovers gradually through the Albian. The strontium isotopic variations are sufficiently large and the data are presented with sufficient stratigraphic detail to allow precise correlation to the classic ammonite zones and lithologic sections of Great Britain using the techniques of strontium isotope stratigraphy. Model results indicate that much of the variation in seawater 87Sr 86Sr between 120 and 40 Ma can be explained by changing the intensity of mid-ocean ridge hydrothermal fluxes proportionally to estimated mid-ocean ridge crustal generation rates. It is also possible that the variations during the rest of the Mesozoic and the Permian are primarily reflections of changing hydrothermal inputs. The model results have several important implications. First, they provide an example in which the variations in the strontium isotope curve are not necessarily driven by changes in fluvial inputs. Second, they suggest that from at least the Aptian through the Eocene variations in continental weathering were minimal. This heightens the importance of the rapid rise in seawater 87Sr 86Sr beginning ~ 40 Ma as a significant transition to an extended period of increasing fluvial 87Sr fluxes continuing to the present. Finally, the results suggest that several documented short-term excursions towards lower 87Sr 86Sr in the latest Triassic, Pliensbachian-Toarcian, Callovian-Oxfordian, Aptian-Albian, and Cenomanian-Turonian are interpretable as pulses of seafloor hydrothermal activity. If so, the strontium isotope record offers a means of constraining the timing, duration, and magnitude of known or proposed hydrothermal events in the geological record.

A sulfur isotopic profile through the troodos ophiolite, Cyprus: Primary composition and the effects of seawater hydrothermal alteration

The sulfide mineralogy, sulfur contents, and sulfur isotopic compositions were determined for a section through the Troodos ophiolite, as represented by drillcore from Holes CY1A, CY2A, and CY4, plus selected outcrops, with the goals of understanding the geochemistry of S during hydrothermal alteration of the crust and obtaining a mass balance for sulfur in altered ophiolitic crust. Primary δ34S values of the ophiolite are 0–1‰, consistent with a MORB- or BAB-like sulfur source. The volcanic rocks have low sulfur contents (mean = 40 ppm) and generally negative δ34S values (to −26.1‰). These are the results of sulfur loss through oxidation by seawater at low temperatures (<100°C) and isotopic fractionation during partial oxidation of igneous Sulfides. The sheeted dike complex is uniformly enriched in 34S (δ34S = 5.4‰) as the result of mixing of reduced Cretaceous seawater sulfate (17‰) with primary crustal sulfide. Sulfate was reduced through oxidation of ferrous iron in the dikes and conversion of igneous pyrrhotite to secondary pyrite. Decreasing water/ rock ratios with depth in the plutonic section led to a general decrease in δ34S values downward to igneous values in the lower gabbros. Seawater effects are present locally to the base of the section, however. Cooling of diffuse upwelling hydrothermal fluids (T > 250°C) produced S and metal enrichments in the upper dikes (1.26 wt% S), whereas sulfur was lost from the lower dikes and upper plutonic rocks (which contain 200 ppm and 10–1640 ppm S, respectively) through higher temperature (>350°C) alteration reactions. Epidosites at the dikes-gabbro transition contain 30 ppm S with δ34S = 5.2–6.5‰. These rocks lost sulfur and metals during reaction with 34S-enriched deep hydrothermal fluids at high temperatures (350–400°C) and high water/rock ratios in basal hydrothermal upflow zones. The ultimate effect of alteration of ophiolitic crust is redistribution of igneous sulfur within the crust and exchange of crustal sulfur for seawater sulfur, with little net change in the sulfur content of the crust overall. The bulk altered Troodos crust is enriched in 34S, having δ34S = 3.6‰. Fluxes of sulfur between seawater and ophiolitic crust are an order of magnitude smaller than modern riverine input to and sedimentary sulfide output from the oceans. The amount of seawater sulfur that reacted with Troodos ophiolite crust is approximately twice that for a composite section of oceanic crust, consistent with the generally greater intensity of alteration of ophiolitic crust compared to modern oceanic crust.