Shallow-water Carbonate Sediments Research Articles

Carbon and oxygen isotopes in the Frasnian–Famennian section of the Kuznetsk basin ( southern West Siberia)

The first detailed isotope-geochemical study of carbonate deposits has been performed in the Lower Famennian stratotype section of the northwestern Kuznetsk Basin (Kosoy Utyos), which was localized in the middle latitudes of the Northern Hemisphere in the Late Devonian. The δ 13C carb, δ 13C org, and δ 18O variation curves were constructed for the section deposits. Geochemical and petrographic studies of carbonates allowed allocation of samples that underwent postsedimentation alteration and exclude them from further interpretation. Compared with coeval sections in the other world's regions, the Kosoy Utyos section is characterized by higher δ 13C carb values, up to 5.4‰, whereas the maximum value in subequatorial area sections is 4‰. The isotope shift amplitude of the studied section reaches 4.6‰, which is 1.5‰ higher than those in other regions. The δ 18O values are 3‰ lower than the ones of the world's coeval sections. The results obtained show that δ 13C and δ 18O variation trends differ from those of coeval subequatorial sections. The high shift amplitude and maximum δ 13C carb values in the Kosoy Utyos section are due to the shallow-water carbonate sedimentation environments on the Siberian continental shelf and, probably, the lower temperatures of waters in the middle latitudes as compared with the subequatorial areas.

Geophysical investigation of seamounts near the Ogasawara Fracture Zone, western Pacific

Abstract This paper provides an analysis of multi-channel seismic data obtained during 2000–2001 on seamounts near the Ogasawara Fracture Zone (OFZ) northwest of the Marshall Islands in the western Pacific. The OFZ is unique in that it is a wide rift zone that includes many seamounts. Seven units are delineated on the basis of acoustic characteristics and depth: three units (I, II, and III) on the summit of seamounts and four units (IV, V, VI, and VII) in basins. Acoustic characteristics of layers on the summit of guyots and dredged samples indicate that the seamounts had been built above sea level by volcanism. This was followed by reef growth along the summit margin, which enabled deposition of shallow-water carbonates on the summit, and finally by subsidence of the edifices. The subsidence depth of the seamounts, estimated from the lower boundary of unit II, ranges between 1,550 and 2,040 m. The thick unit I of the southern seamounts is correlated with proximity to the equatorial high productivity zone, whereas local currents may have strongly affected the distribution of unit I on northern seamounts. A seismic profile in the basin around the Ita Mai Tai and OSM4 seamounts shows an unconformity between units IV and V, which is widespread from the East Mariana Basin to the Pigafetta Basin.

A sequence stratigraphic approach to a Middle Triassic shelf-slope complex of the Ligurian Alps (Ligurian Briançonnais, Monte Carmo-Rialto unit, Italy)

The Mesozoic sedimentary cover belonging to the Monte Carmo-Rialto unit of the Ligurian Brianconnais domain is composed of Scythian clastics and Anisian to Carnian carbonate rocks over 300 m thick. This paper focuses on the stratigraphy of this carbonate complex, its environmental significance, and its evolution in light of dynamic stratigraphy. Our facies analysis of limestones and dolomites of the Triassic complex allowed us to reconstruct an environmental model. Data support a distally steepened carbonate ramp of Anisian age evolving to a more diversified Ladinian platform with an oolitic sand-bar belt separating the lagoon from the slope. The Monte Carmo-Rialto slope facies are the only witnesses of deep sedimentation in the Triassic terrains of the Ligurian Brianconnais domain, otherwise represented by shallow-water carbonate deposits. On the basis of facies succession, we have identified nine medium-scale cycles (third-order sequences) in the study area, comparable to those evidenced in the Brianconnais s.s. domain by the French authors. Small-scale cycles analysis evidenced mainly shallowing-upward trends in the examined sequences; although a few evidences of transgression-related deposits (deepening upward cycles) have been found at the base three sequences, they have been mostly obliterated by dolomitization and masked by local tectonics. For this reason, we can undoubtedly distinguish only the part of each sequence belonging to HST, while the TST, though present, still remains a partition that cannot be precisely characterized. In the same way, LSTs are not present in the Monte Carmo-Rialto unit, due to the original relative landward position of the examined area. Sequence stratigraphy analysis indicates different long-term dynamics for the two evolutionary stages of the Triassic Ligurian platform: a general landward backstepping to moderate progradation during the Early Anisian and true progradation during the latest Anisian and Ladinian. In addition, a good fit with the sequences proposed by the SEPM chart has been found, indicating a correspondence for the third-order sequences of the Middle Triassic.

Present-day and fossil rhodolith pavements compared: Their potential for analysing shallow-water carbonate deposits

Coralline red algae represent the main biogenic components in most shallow-water carbonate successions from the Eocene to the Recent. They contribute significantly to sediment production on open platforms. Carbonate sediments formed by unattached coralline algae include rhodolith pavements (RPs) which represent dense accumulations of rhodoliths, as well as maërl which is composed of rhodoliths, coralline algal branches and their detritus. Recent RPs sampled off Sesoko-jima (Okinawa-jima, southern Japan) occur at depths of 50–70 m on a submarine terrace. The taxonomic coralline composition is dominated by melobesioids associated with minor amounts of mastophoroids and sporolithaceans. The rhodoliths are characterised by various nuclei, an encrusting inner arrangement, encrusting to warty outer growth-forms and sub-spheroidal shapes. Bioerosion, encrustation and abrasion are the most prevalent taphonomic features. Possible fossil counterparts were identified in Chattian and Priabonian RPs from middle-ramp depositional systems from the Venetian area, north-east Italy. A direct comparison between Recent and fossil RPs is possible by contrasting the constituent rhodolith characteristics including taxonomic composition, nature of the nucleus, inner arrangement, outer growth-forms, size and shape as well as taphonomic signatures. This allows factors controlling rhodolith formation and growth in RPs to be compared especially with respect to hydrodynamic regimes and substrate type.

Global synchronous changes in the carbon isotopic composition of carbonate sediments unrelated to changes in the global carbon cycle

The carbon isotopic (delta(13)C) composition of bulk carbonate sediments deposited off the margins of four carbonate platforms/ramp systems (Bahamas, Maldives, Queensland Plateau, and Great Australian Bight) show synchronous changes over the past 0 to 10 million years. However, these variations are different from the established global pattern in the delta(13)C measured in the open oceans over the same time period. For example, from 10 Ma to the present, the delta(13)C of open oceanic carbonate has decreased, whereas platform margin sediments analyzed here show an increase. It is suggested that the delta(13)C patterns in the marginal platform deposits are produced through admixing of aragonite-rich sediments, which have relatively positive delta(13)C values, with pelagic materials, which have lower delta(13)C values. As the more isotopically positive shallow-water carbonate sediments are only produced when the platforms are flooded, there is a connection between changes in global sea level and the delta(13)C of sediments in marginal settings. These data indicate that globally synchronous changes in delta(13)C can take place that are completely unrelated to variations in the global carbon cycle. Fluctuations in the delta(13)C of carbonate sediments measured during previous geological periods may also be subject to similar processes, and global synchroniety of delta(13)C can no longer necessarily be considered an indicator that such changes are related to, or caused by, variations in the burial of organic carbon. Inferences regarding the interpretation of changes in the cycling of organic carbon derived from delta(13)C records should be reconsidered in light of the findings presented here.

Spatial and temporal development of siliceous basin and shallow-water carbonate sedimentation in Oxfordian Northern Calcareous Alps

The Late Jurassic succession of Mount Rettenstein (central Northern Calcareous Alps, Austria) is unique in comparison to all other sections known in the Northern Calcareous Alps because it provides the oldest coexistence of radiolarite basin sedimentation with contemporaneous shallow-water carbonate intercalations. An up to 3.5-m-thick debris flow made up of shallow-water carbonate detritus with a radiolaritic matrix is overlain by thin (calcareous) radiolarite, followed by several hundreds of meters of shallow-water carbonates of the Plassen Formation. Benthic foraminifers (Labyrinthia mirabilis Weynschenk and Alveosepta aff. jaccardi) and the radiolarian associations indicate a depositional age of both the debris flow and the basal Plassen Formation around the boundary of Middle/Late Oxfordian resp. in the Late Oxfordian. This is as yet the first unambiguous evidence of Oxfordian shallow-water sedimentation in the Northern Calcareous Alps. This early neritic stage with the settlement of ooid bars and coral-stromatoporoid-reefs, evidenced by the debris flow resediments in siliceous basin sedimentation, is followed by the definite, rapid progradation of the actual Late Oxfordian/Kimmeridgian–Berriasian Plassen Carbonate Platform with its steep slope configuration. Assumably, this evolution was steered by a mixture of both global environmental and regional tectonic constraints.

Development of carbonate platforms on an extensional (rifted) margin: the Valanginian–Albian record of the Prebetic of Alicante (SE Spain)

Within the upper Valanginian to upper Albian deposits of the easternmost part of the Prebetic Zone of the Betic Cordillera (Iberian Peninsula), seven lithostratigraphic formations made up of shallow-water carbonate and carbonate-siliciclastic sediments and of outer-platform hemipelagic sediments have been recognized. These formations were deposited in the most distal part of a platform that developed on the Southern Iberian Continental Palaeomargin. The geodynamic context was a margin affected by extensional or transtensional faults that produced tilted blocks. The interval studied records three major second-order transgressive-regressive facies cycles: (I) A late Valanginian to earliest Aptian cycle, mostly represented by hemipelagic and condensed sedimentation, with the development of a tectonically controlled high without sedimentation that separated two sectors with different sedimentary evolution and that ended with an episode of shallow-water carbonate platform development; (II) An earliest to latest Aptian cycle, with a transgressive phase represented by a retrogradational shallow-water carbonate platform capped by a drowning event leading to hemipelagic sedimentation, which was affected by an anoxic event (OAE 1a); the regressive phase is represented by progradation and aggradation of shallow-water carbonate deposits. Finally (III) a latest Aptian to early-late Albian cycle that records the expansion of mixed platform deposits in the entire area, ending with a phase of shallow-water carbonate platform development. Extensional tectonics leading to spatial and temporal changes in subsidence patterns is envisaged as the main control on sedimentation at a local scale, resulting in notable lateral changes in thickness as the main signature. Tectonics exerted a strong control on the distribution of sedimentary environments only during Cycle I. At a higher order, sea-level fluctuations are responsible for sequential organization, and environmental factors determined shallow-water carbonate platform development and demise, as well as oceanic anoxic events. The relevant continuity of the stratigraphic record in the distal part of the Prebetic platform has led to the recognition of events related to cycle boundaries, which result mainly from a combination of tectonics and sea-level changes.

New species of Antognathus (Conodonts) from the Famennian deposits of southern Kazakhstan

A new conodont species, Antognathus vjatsheslavi sp. nov., is described from the Famennian shallow-water carbonate deposits of the Bolshoi (Greater) Karatau Range in southern Kazakhstan.

Tectonic evolution and paleogeography of the Mesozoic Pucará Basin, central Peru

The Pucará Basin of Peru is an elongate trough that subsided landward of a NNW-trending structural high during the Late Triassic–Early Jurassic. It formed as a postrift regional sag as the earlier Triassic fault-controlled Mitu rifts yoked together. The rift and transitional postrift basins were associated with a NW-striking sinistral shear zone that controlled isopachs and facies distributions and resulted in magmatism and mineralization along its trend. A distinct association of later dolomitization and MVT lead–zinc mineralization also occurs with these basin-forming shear zones. Although basaltic and andesitic extrusives are common, there is no evidence that the Pacific margin was a magmatic arc until the upper Pucará, and then only weakly developed in northern Peru. Except in the upper Pucará of northwest Peru, geochemical studies, including whole rock and trace element analyses, indicate that intercalations of volcanic material have intraplate rift affinities. The basin fill has a three-part stratigraphic subdivision, comprising lower and upper carbonate platforms with an intermediate phase of basin overdeepening and sediment starvation that resulted in a regional, organic-rich argillaceous drape. Stratigraphic accumulation was dominated by axial patterns of onlap and progradation, though facies characteristics show it was augmented by periodic flooding of the western basin margin high. Marine invertebrate fossils indicate normal marine salinities. The sedimentological interpretation is based on a SW–NE transect in the southern part of the Pucará Basin. The Chambará (Norian–Rhaetian) and Condorsinga (Toarcian) formations were constructed principally by shallow-water carbonate sedimentation in lagoon-like subtidal, intertidal, and supratidal paleoenvironments. The subtidal carbonate platform is dominated by oolitic grainstones with subordinate bioclastic packstones. Subordinate open-basin facies in the Chambará Formation consist principally of crinoidal packstones and bioclastic wackestones. In the intertidal and supratidal facies, evaporite pseudomorphs are common and generally associated with algal mats and widespread early diagenetic dolomitization. During the Chambará and Condorsinga, subsidence typically was balanced by carbonate production and shallow-water environments prevailed; the basin had the characteristics of an overfilled basin. Conversely, the intermediate late Rhaetian–Sinemurian stage of basin subsidence was marked by underfilled deep water conditions. This widespread transgressive inundation of the Pucará Basin, recorded in the argillaceous Aramachay stratigraphy, correlates with similar events in other Andean basins.

Evolución geológica del sureste mexicano desde el Mesozoico al presente en el contexto regional del Golfo de México

The geologic evolution of southeastern Mexico is analyzed in the regional context of the Gulf of Mexico, which starts it opening with the fragmentation and spreading of Pangea. The sedimentary record in this depression begins with the deposit of continental red beds during the Late Triassic and Early Jurassic, after which, during the Callovian, sea-waters from the Pacifi c invaded an extense area; low circulation and high evaporation of these waters allowed the deposition of large volumes of salt in the central part of the basin. From Late Jurassic to Late Cretaceous, carbonate deposition prevailed, changing to clastic at the beginning of the Paleogene, when the Laramide Orogeny formed the folds and faults of the Sierra Madre Oriental. During the rest of the Paleogene clastic sedimentation was deposited in large depocenters generated in the foreland of the Sierra Madre Oriental, and in the southern and southwestern parts of the Golf of Mexico, where the Chiapas Massif produced large volumes of sediments, whereas in the Yucatan Block the deposition of shallow water carbonates continued. In the Middle Miocene, during the Serravalian, compressive stresses resulting from the lateral movement of the Chortis Block and the subduction of the Cocos Plate, against the southern end of the North American Plate, formed the folds and faults of the Chiapas-Reforma-Akal belt over a decollement at the level of the Callovian salt; later, these structures were

Cyclicity of shallow-water carbonate sediments in different climatic zones

Based on the study of several different-age shallow-water carbonate sediments from different regions, it is established that cyclothems formed in basins of arid and humid climatic zones differ in their composition and structure. It is shown that the influence of climate was indirect. The climate governed salinity (particularly, in extremely shallow-water settings of initial and terminal stages of the formation of cyclothems) and, thus, stimulated changes in the biota. The biota, in turn, controlled the composition of carbonate material and mechanisms of its sedimentation.

Significance of Halimeda bioherms to the global carbonate budget based on a geological sediment budget for the Northern Great Barrier Reef, Australia

Since the correlation between carbon dioxide (CO2) levels and global temperatures was established in the ice core records, quantifying the components of the global carbon cycle has become a priority with a view to constraining models of the climate system. The marine carbonate budget is still not adequately constrained and the quantitative significance of the calcareous green alga Halimeda still remains particularly poorly understood. Previously, it has been suggested that Halimeda bioherms on the shelf of the Great Barrier Reef may contain a volume of carbonate equal to or greater than that contained within the shelf edge coral reefs. This study uses published datasets to test this hypothesis in the Northern Great Barrier Reef (NGBR) province. It is estimated that Halimeda bioherms on the outer shelf of the NGBR contain at least as much (and up to four times more) CaCO3 sediment as the adjacent ribbon reef facies. Globally, if these findings are even only partially applicable, the contribution of shallow water carbonate sediments to the global carbon budget based on coral reefs alone is currently substantially underestimated.

Enriched stable carbon isotopes in the pore waters of carbonate sediments dominated by seagrasses: Evidence for coupled carbonate dissolution and reprecipitation

Studies of the δ 13C of pore water dissolved inorganic carbon (δ 13C-DIC) were carried out in shallow water carbonate sediments of the Great Bahamas Bank (GBB) to further examine sediment–seagrass relationships and to more quantitatively describe the couplings between organic matter remineralization and sediment carbonate diagenesis. At all sites studied δ 13C-DIC provided evidence for the dissolution of sediment carbonate mediated by metabolic CO 2 (i.e., CO 2 produced during sediment organic matter remineralization); these observations are also consistent with pore water profiles of alkalinity, total DIC and Ca 2+ at these sites. In bare oolitic sands, isotope mass balance further indicates that the sediment organic matter undergoing remineralization is a mixture of water column detritus and seagrass material; in sediments with intermediate seagrass densities, seagrass derived material appears to be the predominant source of organic matter undergoing remineralization. However, in sediments with high seagrass densities, the pore water δ 13C-DIC data cannot be simply explained by dissolution of sediment carbonate mediated by metabolic CO 2, regardless of the organic matter type. Rather, these results suggest that dissolution of metastable carbonate phases occurs in conjunction with reprecipitation of more stable carbonate phases. Simple closed system calculations support this suggestion, and are broadly consistent with results from more eutrophic Florida Bay sediments, where evidence of this type of carbonate dissolution/reprecipitation has also been observed. In conjunction with our previous work in the Bahamas, these observations provide further evidence for the important role that seagrasses play in mediating early diagenetic processes in tropical shallow water carbonate sediments. At the same time, when these results are compared with results from other terrigenous coastal sediments, as well as supralysoclinal carbonate-rich deep-sea sediments, they suggest that carbonate dissolution/reprecipitation may be more important than previously thought, in general, in the early diagenesis of marine sediments.

Inhibition of dissolution within shallow water carbonate sediments: impacts of terrigenous sediment input on syn‐depositional carbonate diagenesis

AbstractThe early diagenetic chemical dissolution of skeletal carbonates has previously been documented as taking place within bioturbated, shallow water, tropical carbonate sediments. The diagenetic reactions operating within carbonate sediments that fall under the influence of iron‐rich (terrigenous) sediment input are less clearly understood. Such inputs should modify carbonate diagenetic reactions both by minimizing bacterial sulphate reduction in favour of bacterial iron reduction, and by the reaction of any pore‐water sulphide with iron oxides, thereby minimizing sulphide oxidation and associated acidity. To test this hypothesis sediment cores were taken from sites within Discovery Bay (north Jamaica), which exhibit varying levels of Fe‐rich bauxite sediment contamination. At non‐impacted sites sediments are dominated by CaCO3 (up to 99% by weight). Pore waters from the upper few centimetres of cores show evidence for active sulphate reduction (reduced SO4/Cl− ratios) and minor CaCO3 dissolution (increased Ca2+/Cl− ratios). Petrographic observations of carbonate grains (specifically Halimeda and Amphiroa) show clear morphological evidence for dissolution throughout the sediment column. In contrast, at bauxite‐impacted sites, the sediment is composed of up to 15% non‐carbonate and contains up to 6000 μg g−1 Fe. Pore waters show no evidence for sulphate reduction, but marked levels of Fe(II), suggesting that bacterial Fe(III) reduction is active. Carbonate grains show little evidence for dissolution, often exhibiting pristine surface morphologies. Samples from the deeper sections of these cores, which pre‐date bauxite influence, commonly exhibit morphological evidence for dissolution implying that this was a significant process prior to bauxite input. Previous studies have suggested that dissolution, driven by sulphate reduction and sulphide oxidation, can account for the loss of as much as 50% of primary carbonate production in localized platform environments. The finding that chemical dissolution is minor in a terrigenous‐impacted carbonate environment, therefore, has significant implications for carbonate budgets and cycling, and the preservation of carbonate grains in such sediment systems.

Paleocene coralgal reefs of the western Pyrenean basin, northern Spain: New evidence supporting an earliest Paleogene recovery of reefal ecosystems

The rate of recovery of photic reefal ecosystems after the Cretaceous–Tertiary crisis is controversial. Most reports from earlier authors concluded that Paleogene reef systems did not completely recover from that crisis at least until Oligocene–Miocene times. Several other authors, however, have pointed out that such conclusion was biased by poor preservation and/or inaccessibility of many early Paleogene successions and that the recovery of reefs might have been a faster process. The latter possibility is here strongly supported with data from the Pyrenean basin, where Paleocene shallow-water carbonate deposits are thickly developed and outcrop widely. Five growth phases of reef development have been recognized and age dated with calcareous nannofossils, which together define a complete sequence of expansion-reduction of reef ecosystems during the Paleocene epoch. Reef growth phase 1 (early Danian, lower–middle NP3 calcareous nannofossil Zone), the initial step of that sequence, demonstrates that coral-dominated reef systems came back into existence less than 2 Ma after the K/T boundary, a comparatively short time considering the magnitude of the end-Cretaceous biological crisis. The climax of the sequence is recorded by growth phase 3 (late Danian, NP4 Zone), when a thick barrier-reef complex was developed, which was at least 200 km in length (but probably much larger) and made up by a well diversified coralgal assemblage. Reef growth phases 4 and 5 (early and middle Thanetian, NP6 to NP8 Zones) were characterized by low-relief reef banks dominated by encrusting calcareous algae and had less well-diversified coral assemblages, features that point to a deterioration of environmental conditions, tentatively interpreted in terms of a climatic cooling. Reef bioconstruction almost disappeared in the Pyrenean basin during late Thanetian times, although in all likelihood this is a regional effect of no evolutionary relevance. The new data suggest that the Cenozoic recovery of reefs was rapid but punctuated, with a phase of important reef expansion already in the late Danian.

Stratigraphic evolution of Oligocene–Miocene carbonates and siliciclastics, East Java basin, Indonesia

The Oligocene–Miocene of East Java is characterized by multiple stages of isolated carbonate mound growth surrounded by deeper marine off-mound sediments or by shallow-marine siliciclastics. Three stratigraphic intervals are recognized: Kujung (carbonate mound and off-mound), Tuban (mixed carbonate-siliciclastic), and Ngrayong (siliciclastic). Exposures of the Kujung unit (28–22 Ma) are limited to a few isolated outcrops. At the base, the Kujung is represented by a high-energy, extensive, shallow-marine carbonate facies that grades laterally into deep-marine off-mound sediments of calcareous mudstone and chalk (lower Kujung). In other locations, shallow-water carbonate deposition was restricted to faulted topographic highs in the middle–upper Kujung. The shallow-marine sediments of the lower Kujung were covered by thick chalk and marl sediments of the middle–upper Kujung. The Tuban unit (22–15 Ma) consists of widely exposed shallow-marine mixed carbonate and siliciclastic and poorly exposed open-marine shale and chalk facies. The Tuban consists of at least six stacked cycles that reflect deltaic deposition with episodes of shallow-marine carbonate mound growth. The Ngrayong unit (15–12 Ma) represents a period of regional siliciclastic influx and progradation of tidally influenced deltas and grades into turbidites, basinal shale, mudstone, and chalk. Ngrayong beds are truncated by Bulu carbonates (Serravallian–Tortonian). This is consistent with the tectonic evolution of the region.

A Mid Mesozoic Revolution in the regulation of ocean chemistry

The Phanerozoic has seen fundamental changes in the global biogeochemical cycling of calcium carbonate (CaCO 3), particularly the advent of biomineralization during the early Cambrian when the products of weathering could first be removed through metabolic expenditure, and the subsequent ecological success of planktic calcifiers during the Mesozoic which shifted the locus of deposition from the continental shelves to the deep open ocean. These biologically-driven CaCO 3 depositional ‘mode’ changes along with geochemical and tectonic variations in boundary conditions such as sea-level and calcium ion concentrations all affected the carbonate chemistry of the ocean. I employ a model of atmosphere–ocean–sediment carbon cycling to explore the impact of these factors on the saturation state and carbonate chemistry of the global ocean during the Phanerozoic. The model results highlight that overall; the time evolution and regulation of Phanerozoic ocean chemistry are dominated by a Mid Mesozoic Revolution in the marine carbonate cycle. Prior to this transition, it was possible for the ocean to attain states of extreme saturation during the Permian and Triassic as well as during the late Precambrian. This is primarily a consequence of low sea-level in restricting the potential area for the deposition of shallow water carbonates, thus requiring a more saturated ocean and higher rate of precipitation per unit area in order to balance weathering input. This is consistent with the occurrence of mineralogically ‘anomalous’ carbonates during these periods but not commonly at other times. That the modern carbon cycle does not respond to similar tectonic forcings is due to the ecological success of calcifying planktic taxa during the Mesozoic, which in facilitating the creation of a responsive deep-sea carbonate sink enabled a much greater degree of regulation of saturation state than was previously possible. The model results also highlight the primary role of changes in the concentration of CO 2 in the atmosphere and of Ca 2+ in the ocean in determining surface pH. The uncertainty inherent in paleo CO 2 estimates then translates into sufficient uncertainty in reconstructions of Phanerozoic temperature variability that one can only deduce from the carbonate δ 18O record that the Cretaceous was generally warmer and the Carboniferous colder than average. The substantially enhanced oceanic carbon inventory predicted for the Paleozoic suggests that previous calculations of methane hydrate release may have substantially underestimated the quantity of clathrate carbon required to explain observed carbon isotopic excursions. In both cases the importance of quantifying Phanerozoic marine chemistry is clear.

Sea level, sediments and the composition of seawater

The composition of seawater changed significantly during the course of the Phanerozoic in concert with long-term changes in sea level, which were largely the result of changes in the volume of mid-ocean ridges. These reflect changes in the rate of seafloor spreading and determine the rate of seawater cycling through mid-ocean ridges. Spencer and Hardie (1990) and Hardie (1996) proposed that the changes in the cycling rate of seawater through mid-ocean ridges have been responsible for the changes in the composition of seawater during the Phanerozoic. This explanation cannot account for the major changes in the composition of seawater during the Cenozoic, which was a time of nearly constant seafloor spreading rate. However, changes in the rate of deposition of shallow water carbonates and their penecontemporaneous dolomitization can explain the Cenozoic changes in seawater composition. This indicates that sedimentologic parameters have played a major role in the chemical evolution of seawater. The more rapid cycling of seawater through MORs during the Mesozoic can explain semiquantitatively the fairly minor differences between the composition of seawater in the Eocene and in the Jurassic. Changes in the composition of seawater during the Paleozoic were probably determined both by changes in the rate of dolomite and gypsum/anhydrite deposition and by changes in the rate of seawater cycling through mid-ocean ridges. If the causes of these compositional changes are the same as those during the Cenozoic and Mesozoic, then sedimentologic factors have been significantly more important than the rate of cycling of seawater through MORs in shaping the chemical evolution of seawater during the Phanerozoic.

Unravelling the microfacies signatures of parasequences using computer-optimized similarity matrices

The microfacies of a Lower Cretaceous carbonate drillcore from Oman are characterized using optimizing matrices of Jaccard's similarity coefficients of community. Other than systems tract boundaries, there is no obvious evidence of individual parasequences in the core. However, diagnostic patterns in microfossil distribution identify environmental gradients recording changes in water depth. These gradients are used to define individual parasequences, parasequence sets, stacking patterns and key surfaces. The patterns suggest that deposition was controlled by successive fourth- to fifth-order (high-frequency) relative sea-level cycles superimposed on an underlying third-order relative sea-level rise. Although the correlation of these depositional subunits to systematic changes in water depth and the rate of carbonate accumulation alone is not incontrovertible proof of such a sea-level control, concurrent multiorder relative sea-level cyclicity provides by far the most likely explanation. A microfacies deposited when the water depth was shallowing is characterized by a relay of microfossils with affinities that shallow upwards. Conversely, a microfacies that records a gradual increase in water depth has a relay of microfossils with affinities that deepen upwards. Microfacies characterized by an assemblage of microfossils with similar affinities record deposition when the benthic environmental conditions remained stable, either because of an equilibrium between shallow water carbonate deposition and rising sea level, or in deeper water where sediment composition was relatively insensitive to changes of water depth. Microfacies characterized by mixed affinity assemblages record syndepositional reworking. During periods of embedded multiorder sea-level changes, individual parasequences within systems tracts are shown to record more complex environmental gradients than simply the repetition of successive shallowing-up units as traditionally represented in carbonate sequence stratigraphic models. The microfacies of an individual parasequence may shallow up, or may record both deepening-up and shallowing-up depositional phases, as well as periods of sedimentation when benthic environments remained stable. Individual parasequence boundaries may be submarine or subaerial unconformities, or be conformable, as part of a predictable stratigraphic pattern related to the temporal position of an individual parasequence within the underlying third-order cycle of relative sea-level change. The hitherto ubiquitous use of assemblages to describe carbonate microfacies, coupled with the widespread use of the metre-scale shallowing-up template to identify parasequences, may have led to such complexities previously being overlooked.

Organic matter diagenesis in shallow water carbonate sediments

Muddy carbonate deposits near the Dry Tortugas, Florida, are characterized by high organic carbon remineralization rates. However, approximately half of the total sedimentary organic matter potentially supporting remineralization is occluded in CaCO 3 minerals (intracrystalline). While a portion of nonintracrystalline organic matter appears to cycle rapidly, intracrystalline organic matter has an approximately constant concentration with depth, suggesting that as long as its protective mineral matrix is intact, it is not readily remineralized. Organic matter in excess of intracrystalline organic matter that is preserved may have a variety of mineral associations (e.g., intercrystalline, adsorbed or detrital). In surface sediment, aspartic acid contributed ∼22 mole % and ∼50 mole % to nonintracrystalline and intracrystalline pools, respectively. In deeper sediment (1.6–1.7m), the composition of hydrolyzable amino acids in both pools was similar (aspartic acid ∼40 mole %). Like amino acids, intracrystalline and nonintracrystalline fatty acids have different compositions in surface sediments, but are indistinguishable at depth. These data suggest that preserved organic matter in the nonintracrystalline pool is stabilized by its interactions with CaCO 3. Neutral lipids are present in very low abundances in the intracrystalline pool and are extensively degraded in both the intracrystalline and nonintracrystalline pools, suggesting that mineral interactions do not protect these compounds from degradation. The presence of chlorophyll- a, but absence of phytol, in the intracrystalline lipid pool demonstrates that chloropigments are present only in the nonintracrystalline pool. Sedimentary chloropigments decrease with depth at similar rates in Dry Tortugas sediments as found in alumino-silicate sediments from the Long Island Sound, suggesting that chloropigment degradation is largely unaffected by mineral interactions. Overall, however, inclusion and protection of organic matter by biominerals is a major pathway for organic matter preservation in this low-organic carbon, biomineral-rich regime.