Stephen E. Calvert

Geochemistry of Recent oxic and anoxic marine sediments: Implications for the geological record

Abstract The distributions of certain minor and trace elements in marine sediments should potentially provide forensic tools for determining the redox conditions of the bottom waters at the time of deposition. The ability to identify such conditions in the geological past is important because (1) current models of the conditions of formation of organic-rich rocks require reexamination, (2) a method to determine whether the areal extent of anoxic waters expanded or retracted in response to palaeoceanographic changes is required, and (3) the effects of such environmental changes on the geochemical balance of these elements in the ocean need to be understood. Recent research has suggested that some minor and trace elements are precipitated where free dissolved sulphide is present (Cu, Cd, Ni, Zn) without undergoing a valency change, whereas others undergo a change in valency and are either more efficiently adsorbed onto solid surfaces under oxic (I) or anoxic (V) conditions or are precipitated under anoxic conditions (Cr, Mn, Mo, Re, U, V). Hence, the enrichment of these minor and trace elements relative to their crustal abundances indicates that the host sediments accumulated under anoxic conditions, although not necessarily under anoxic bottom waters. Examination of the chemical composition of the sediments of anoxic basins, continental margin sediments and oxidized deepsea sediments shows that I and Mn enrichments are reliable indicators of bottom water oxygenation, whereas enrichments of the remaining elements reflect either bottom water anoxia or element uptake by subsurface anoxic sediments below a relatively thin surficial oxic veneer. Hence, the absence of metal enrichment in these cases can be taken as firm evidence that the bottom waters of a basin of sedimentation were not anoxic. These behaviours may be used to propose, for example, that the Holocene sapropel in the Black Sea accumulated under oxic bottom waters, whereas the modern facies reflects its formation under the prevailing intensely anoxic conditions, and that the Panama Basin bottom waters were not anoxic during the Last Glacial Maximum when the rate of accumulation of organic carbon increased. Likewise, the enrichment of Mn as a mixed carbonate phase in some ancient black shales strongly suggests that they formed under oxic bottom waters rather than anoxic conditions as is commonly assumed.

Rhenium and molybdenum enrichments in sediments as indicators of oxic, suboxic and sulfidic conditions of deposition

Abstract The trace elements Re and Mo both behave conservatively in seawater yet are strongly enriched in reducing sediments. Their potential for authigenic enrichment above crustal concentrations is greater than for many other elements, due to the high ratio of [metal] sw /[metal] crust . We present sedimentary Re and Mo data from box- and multi-cores spanning a range of redox conditions, from well-oxygenated sites to locations with substantial sulfide concentrations. At the oxic sites, Re and Mo, as expected, accumulate at concentrations close to their crustal abundances. Re shows substantial enrichment in suboxic (absence of O 2 and H 2 S) sediments of the Sea of Japan, in sediments within the oxygen minimum of the Pakistan margin, as well as in sediments underlying the sulfide-bearing waters of the Black Sea and Saanich Inlet. Re enrichment occurs in these cores just below the depths of Fe and U reduction. Only in the sediments underlying the sulfidic waters of the Black Sea and Saanich Inlet is there substantial authigenic Mo accumulation. Absence of Re and Mo enrichments in sediment trap samples from the sulfide-bearing basins suggests that the addition of both metals occurs at or below the sediment-water interface. The Re and Mo concentration profiles are modeled to hindcast the removal depth of Re and Mo in the sediments which, along with the Re/Mo ratio, determine whether oxic, suboxic or sulfidic conditions were present in the water column or in the sediments in the past. Using this approach, historical redox conditions can be inferred even in environments such as continental margins where a substantial lithogenic component can obscure authigenic enrichments of other metals.

Rare earth element geochemistry of oceanic ferromanganese nodules and associated sediments

Abstract Analyses have been made of REE contents of a well-characterized suite of deep-sea (> 4000 m.) principally todorokite-bearing ferromanganese nodules and associated sediments from the Pacific Ocean. REE in nodules and their sediments are closely related: nodules with the largest positive Ce anomalies are found on sediments with the smallest negative Ce anomalies; in contrast, nodules with the highest contents of other rare earths (3 + REE) are found on sediments with the lowest 3 + REE contents and vice versa. 143 Nd 144 Nd ratios in the nodules (∼0.51244) point to an original seawater source but an identical ratio for sediments in combination with the REE patterns suggests that diagenetic reactions may transfer elements into the nodules. Analysis of biogenic phases shows that the direct contribution of plankton and carbonate and siliceous skeletal materials to REE contents of nodules and sediments is negligible. Inter-element relationships and leaching tests suggest that REE contents are controlled by a P-rich phase with a REE pattern similar to that for biogenous apatite and an Fe-rich phase with a pattern the mirror image of that for sea water. It is proposed that 3 + REE concentrations are controlled by the surface chemistry of these phases during diagenetic reactions which vary with sediment accumulation rate. Processes which favour the enrichment of transition metals in equatorial Pacific nodules favour the depletion of 3 + REE in nodules and enrichment of 3 + REE in associated sediments. In contrast, Ce appears to be added both to nodules and sediments directly from seawater and is not involved in diagenetic reactions.

Late Pleistocene Variability of the Carbon Isotopic Composition of Organic Matter in the Eastern Mediterranean: Monitor of Changes in Carbon Sources and Atmospheric CO2 Concentrations

The organic carbon isotopic record of the sapropels (S1 and S3–S10) and intercalated marl oozes has been determined in a 12-m piston core from the eastern Mediterranean. The δ13Corganic values are systematically lighter (mean=−21.0±0.82 ‰) in all sapropels and heavier (mean=−18.8±1.07‰) in the marl oozes. These differences are not due to variable marine and terrestrial organic matter mixtures because all values are heavier than modern plankton in the Mediterranean, there is no relationship between the Corganic/N ratios and the isotopic values, and published information on the abundance and distribution of organic biomarkers shows that terrestrial material constitutes a minor fraction of the total organic matter. Temperature effects on isotope fractionation are also discounted because the change in δ13Corganic values between glacial and interglacial horizons is in the opposite sense. Diagenesis, which can produce relatively small changes in the carbon isotopic composition of sedimentary organic matter under certain circumstances, is unlikely to have caused the observed differences because this mechanism would cause an enrichment in 12C, implying that all values were even heavier originally, and there is no secular trend in the δ13Corganic record. The observed differences in δ13Corganic between the two lithologies are probably produced by changes in the isotopic composition and the concentration of dissolved CO2. First, freshwater flooding during the formation of the sapropels caused the isotopic composition of the dissolved inorganic carbon in the surface waters of the Mediterranean to become lighter because of the 13C deficiency in fresh waters. Hence photosynthesis would have produced isotopically lighter organic material. Second, changes in atmospheric pCO2 between glacial and interglacial periods, as shown by the Vostok ice core, caused marked changes in the concentration of free dissolved CO2 in the mixed layer; lower values during glacial maxima caused a smaller fractionation of the carbon isotopes by phytoplankton, whereas levels were less limiting during the interglacials. Concentrations of dissolved CO2 could also have been much higher during the deposition of the sapropels because of the supply of regenerated CO2 to the mixed layer by upwelling, and this could have further lightened the δ13Corganic values in the sapropels themselves. Carbon isotope records may provide an alternative method for estimating atmospheric pCO2 levels over longer time periods than can be obtained from ice cores.

Glacial-Holocene Biogenic Sedimentation Patterns in the South China Sea: Productivity Variations and Surface Water pCO2

A bathymetric transect of cores in the South China Sea extending from 4200-m to less than 1000-m water depth has been examined for glacial-interglacial changes in carbonate and organic carbon sedimentation. Typical “Pacific carbonate cycles” (high carbonate content during glacials and low carbonate content during interglacials) characterize cores from water depths deeper than 3500 m. In contrast, “Atlantic carbonate cycles” (low carbonate during glacials and high carbonate during interglacials) are observed in cores from depths shallower than 3000 m as a result of increased dilution of carbonate by terrigenous material during glacial low stands of sea level. Glacial-interglacial changes in the carbonate chemistry of South China Sea intermediate and deep waters resulted in significant changes in the positions of the carbonate compensation depth (CCD) and the aragonite compensation depth (ACD). During the last glacial the CCD and ACD were at least 400 and 1200 m deeper, respectively, than at present. Organic carbon accumulation rates in the South China Sea were approximately 2 times higher during the last glacial than the Holocene. Carbon isotopic analyses and C/N ratios of the organic matter indicate that only a small fraction of the increase in glacial organic carbon accumulation can be attributed to input of terrestrial carbon. On the basis of this we conclude that surface water productivity in the South China Sea was approximately 2 times higher during the last glacial maximum. This is consistent with previous studies which have demonstrated that glacial productivity was higher in low- to mid-latitude regions of the Atlantic and eastern Pacific. The deglacial decrease in organic carbon accumulation is accompanied by a decrease in δ13Corg. Using the relationship between δ13Corg and [CO2](aq) developed by Popp et al. [1989], we estimate that surface water pCO2 values in the South China Sea during the last 25,000 years were very similar to atmospheric CO2 concentrations.

Glacial/interglacial variations in production and nitrogen fixation in the Cariaco Basin during the last 580 kyr

The effect of sea level change on nutrient supply to the anoxic Cariaco demonstrates the fundamental importance of nitrogen (N2) fixation and phosphate to oceanic production. As N2 fixation produces biomass of low δ15N and has been reported to be an important component of the nitrogen cycle in the modern Cariaco Basin, we propose that it contributes to the light interglacial δ15N (∼2‰–3‰) values observed in the Ocean Drilling Program (ODP) site 1002 sediment record. During the glacials the sediments are bioturbated (oxic conditions) with low total organic carbon (TOC) contents and sedimentary δ15N values of ∼5‰, suggesting that nitrogen (N2) fixation contributed little to the N nutrition of Cariaco surface waters. The most plausible explanation for the inferred glacial/interglacial changes in N2 fixation in the Cariaco is that they have occurred in response to variations in the N/P ratio of the nutrient supply, driven by changes in denitrification.

Eastern Pacific cooling and Atlantic overturning circulation during the last deglaciation

Surface ocean conditions in the equatorial Pacific Ocean could hold the clue to whether millennial-scale global climate change during glacial times was initiated through tropical ocean–atmosphere feedbacks or by changes in the Atlantic thermohaline circulation. North Atlantic cold periods during Heinrich events and millennial-scale cold events (stadials) have been linked with climatic changes in the tropical Atlantic Ocean and South America, as well as the Indian and East Asian monsoon systems, but not with tropical Pacific sea surface temperatures. Here we present a high-resolution record of sea surface temperatures in the eastern tropical Pacific derived from alkenone unsaturation measurements. Our data show a temperature drop of ∼1 °C, synchronous (within dating uncertainties) with the shutdown of the Atlantic meridional overturning circulation during Heinrich event 1, and a smaller temperature drop of ∼0.5 °C synchronous with the smaller reduction in the overturning circulation during the Younger Dryas event. Both cold events coincide with maxima in surface ocean productivity as inferred from 230Th-normalized carbon burial fluxes, suggesting increased upwelling at the time. From the concurrence of equatorial Pacific cooling with the two North Atlantic cold periods during deglaciation, we conclude that these millennial-scale climate changes were probably driven by a reorganization of the oceans’ thermohaline circulation, although possibly amplified by tropical ocean–atmosphere interaction as suggested before.

SOURCES AND RELATIVE REACTIVITIES OF AMINO-ACIDS, NEUTRAL SUGARS, AND LIGNIN IN AN INTERMITTENTLY ANOXIC MARINE-ENVIRONMENT

A sediment-trap sample, representing an annual average particle flux at 50 m in Saanich Inlet, British Columbia, was analyzed for its elemental, amino acid, neutral sugar, and lignin composition. Amino acid analyses also were performed on underlying sediments which were analyzed previously for organic carbon, nitrogen, neutral sugars, and lignin. The results uniformly indicate primarily marine organic matter sources for all samples, although relatively higher terrigenous contributions are evident in the sediments. The δ13C values of trap materials also point to primarily autochthonous particle fluxes. Comparison of annual average water-column fluxes to sediment accumulation rates indicates under-sampling of sinking particles due to lateral sediment inputs at depth. The anoxic benthic interface appears to be an important site of diagenesis, and selective removal is observed both at compound-class and molecular levels. Cinnamyl and syringyl phenols are selectively removed relative to vanillyl phenols and loss patterns of the monosaccharides, and to a lesser degree the amino acids, strongly indicate preferential preservation of diatom cell-wall materials. Low flux ratios displayed by the nonprotein amino acids are consistent with their diagenetic origin. Preferential loss of marine organic material is indicated by the calculated δ13C value and biochemical composition of the substrate. Concentrations of all measured organic constituents decreased with depth in the uniformly varved 0–14 cm sediment interval, and suggest in situ degradation. Relative reactivities of the biochemical classes indicate a change in diagenetic substrate from that utilized above and at the benthic interface. With the exception of the amino acids, however, diagenesis is generally less selective in the sediments. The amino acid utilization pattern differs from that observed across the benthic interface, and down-core changes in protein and nonprotein amino acid compositions clearly indicate in situ degradation. The sedimentary degraded fraction also appears to be predominantly marine, but lignin yields and sugar compositions suggest a relative increase in the utilization of vascular plant remains. Protein, polysaccharide, and lignin contributions to total organic carbon decrease from 37% in the sediment-trap sample to 22% at the bottom of the 0–14 cm sediment interval. These biochemicals represent over 40 and 50–60% of the degraded carbon and nitrogen, respectively, and thus are important nutrients for the benthic and water-column communities.

Influence of water column anoxia and sediment supply on the burial and preservation of organic carbon in marine shales

Abstract Previous work has suggested that the laminated, organic-rich and bioturbated, organic-poor shales of the Camp Run Member of the Late Devonian-Early Mississippian New Albany Shale formed under anoxic and oxygenated bottomwater conditions, respectively, and that the interbedding of the two facies was due to the vertical oscillation of a water-column anoxic/oxic boundary where it impinged on the basin margin. We have extended this analysis by examining the chemical and mineralogical differences between the two shale facies in a single borehole core, by seeking evidence for deposition of the laminated shales under bottom-water oxia or anoxia, and by determining whether the laminated shales formed when the carbon supply to the sea floor was higher. The results of this study show that the laminated and bioturbated shales are mineralogically and chemically distinct; relative to Al, an index of the aluminosilicate content, Si, Ti, Fe, P, Na, Ba, Co, Cr, Cu, Mo, Ni, V, Zn, and Zr are all higher, whereas Mn, Ca, Mg, and Sr are lower in the laminated compared with the bioturbated shales. The differences are due to a higher quartz, feldspar, titanite/ilmenite, and zircon content in the laminated shales, probably indicating a coarser grain-size, and the greater abundance of manganoan calcite in the bioturbated shales. Dissolved oxygen was present in bottom waters during the deposition of some of the laminated shale intervals because of the presence of manganoan calcite, a phase that can only form in sediments with an oxic surface. In addition, the organic matter preserved in the two shale types is isotopically different; δ 13Corganic values are 1.9z.permil; lighter on average in the laminated compared with bioturbated intervals, possibly indicating a larger fraction of terrestrial organic matter in the latter. δ15N values are 1.9z.permil; lighter on average in laminated compared with bioturbated intervals, possibly indicating a larger fraction of terrestrial organic matter in the latter. δ15N values are 1.9z.permil; lighter on average in laminated compared with bioturbated intervals, suggesting that nutrient drawdown was less during the deposition of the organic-rich, laminated shales. The chemical, mineralogical, and isotopic contrasts between the two shale facies of the Camp Run Member indicate that the conditions of sedimentation were different during their deposition. The difference was possibly related to variations in sea level, which would have caused the Camp Run shoreline to move closer to and farther from the core site, causing, in turn, the deposition of coarser and finer grained sediments that contained different mixtures of marine and terrestrial organic matter. Bottomwater conditions were anoxic during deposition of most laminated intervals. Bottom-water anoxia or dysoxia led to decreased burial and preservation of the essential nutrient phosphorus in the laminated, organic-rich shales relative to the rocks deposited beneath better oxygenated bottomwaters. Increased availability of phosphorus in the water column on long timescales leads to increased productivity and a higher settling flux of organic matter, causing bottom-water oxygen levels to fall. This is consistent with the nitrogen isotope evidence suggesting that production was probably higher during the deposition of the organic-rich shales. Thus, production variations coupled with enhanced sedimentary regeneration of phosphorus from sediments related to low oxygen bottom-water concentrations provide a general mechanism for the formation of the alternating facies of this member of the New Albany Shale.

The annual cycle of sedimentation in Saanich inlet, British Columbia: implications for the interpretation of diatom fossil assemblages

Abstract Data derived from monthly sediment traps at three depths near the head and mouth of Saanich Inlet yield a detailed record of the seasonal cycle of production and vertical flux to the sediments. The material is primarily composed of diatom frustules and silt-to-clay-sized lithic fragments, with dinoflagellates and naked algae present in summer samples. Diatoms dominate from April to September while clastics dominate from October to March. Particles occur primarily in loose flocs; pellets are common in early winter and in summer material, and consist of an indiscriminant mixture of the same particles seen in co-occurring flocs. The seasonal succession of taxa is similar at both sites and is transported to the sediment with little modification by dissolution. Carbon flux is a poor indicator or productivity, due to the strong effect of recycling in the surface waters during spring and uncertainty as to the source of the carbon accumulating in the traps. Although diatom flux is an adequate indicator of primary production, transforming each taxon to equivalent cell volume yields a more accurate picture of the seasonal cycle of production. Relative abundance (taxon percentage) data can be as useful as, and in some cases more accurate than, absolute flux (number per unit time), while number-per-gram may be highly misleading as an estimator of production. Lateral advection and benthic resuspension affect monthly data and annual valve-flux data, but appear to have no net effect upon annual percentage data.