Carbon Isotopic Composition Of Seawater Research Articles

A rapid rise of seawater δ13C during the deglaciation of the Marinoan Snowball Earth

The Marinoan Snowball Earth glaciation (~650–635 Ma) was followed by an increase in atmospheric O2 level and the diversification of eukaryotes. These dramatic changes have been assumed to have resulted from the rapid recovery of primary productivity during deglaciation, but this hypothesis has not yet been tested. Carbonate carbon isotope (δ13Ccarb), closely related to organic matter production and burial, has the potential to provide with the most straightforward evidence for the perturbation in marine carbon cycle. However, individual δ13Ccarb record might be less well-suited to test this hypothesis than a large data compilation. Here, we analyze 1590 globally-distributed carbonate carbon isotope data from the Marinoan cap carbonate. The least square sum analysis of the shallow-water cap carbonate δ13Ccarb data indicates that the carbon isotopic composition of seawater dissolved inorganic carbon (DIC, δ13CDIC) was −3.01‰ during cap carbonate precipitation. This result sharply contrasts with the assumed synglacial seawater δ13CDIC value of −5‰, which was verified with a binary mixing model. The 2‰ increase in the seawater δ13CDIC can be attributed to intensive organic matter production and burial during the deglaciation in less than 106 years. Massive organic carbon burial in the wake of the Marinoan Snowball Earth glaciation contributed a postglacial increase of atmospheric O2 level by 3.8% to 13.4% PAL (present atmospheric level) and thus further facilitated diversification of eukaryotes.

Evolution patterns of seawater carbon isotope composition during the Cambrian and their stratigraphic significance

Our current understanding of the Cambrian carbon cycle mostly comes from geologically rapid carbon isotope excursions (CIEs) of marine carbonate. In addition to carbon cycle indicators, stable carbon isotopes of marine carbonates can be used as stratigraphic correlation tools. Previous studies have recognized 10 CIEs in the Cambrian, but only 3 of them appear to have global significance. This limits the application of CIEs in chronostratigraphic correlation of Cambrian successions with the 10 successive stages. Here, we present a continuous carbon isotope record from a stratigraphically complete Cambrian section at Xiaoerbulak, Aksu region in the Tarim Basin, Xinjiang, China. Its comparison with more than 40 sections across the world shows the carbon isotope record of the Cambrian can be divided into 10 units based on the variation patterns. Each pattern has occurrences in at least two Cambrian tectonic plates, suggesting global significance. A generalized curve consisting of the 10 patterns, Carbon Isotope Pattern Curve (CIPC) has been compiled. Based on the fossil records, the 10 patterns are correlated with the 10 stages of the chronostratigraphic system of the Cambrian. The pattern Angularianis correlated with Fortunian Stage and the lower half of Stage 2, Dampingian with the upper part of Stage 2, Boxian with Stage 3, Rabbitian with Stage 4, Chairian with the lower half of Wuliuan, Zigzagian with the upper Wuliuan Stage, Stickian with the Drumian and Guzhangian stages, Towerian with the Paibian Stage and Lower Jiangshan Stage, Shelfian with Upper Jiangshan Stage and the lower half of Stage 10, and Hillian with the upper half of Stage 10.We suggest that the application of CIPC makes the chronostratigraphy of non‐fossiliferous successions realizable.

Authigenic carbonate burial in the Late Devonian–Early Mississippian Bakken Formation (Williston Basin, USA)

AbstractLate Devonian (Famennian) marine successions globally are typified by organic‐rich black shales deposited in anoxic and euxinic waters and the cessation of shelf carbonate sedimentation. This global ‘carbonate crisis’, known as the Hangenberg Event, coincides with a major extinction of reef‐building metazoans and perturbations to the global carbon cycle, evidenced by positive carbon‐isotope excursions of up to 4‰. It has been suggested that authigenic carbonate, formed as cements in sedimentary pore spaces during early burial diagenesis, is a significant mass fraction of the total global carbon burial flux, particularly during periods of low oxygen concentration. Because some authigenic carbonate could have originated from remineralization of organic carbon in sediments, it is possible for this reservoir to be isotopically depleted and thereby drive changes in the carbon isotopic composition of seawater. This study presents bulk isotopic and elemental analyses from fine‐grained siliciclastics of the Late Devonian–Early Mississippian Bakken Formation (Williston Basin, USA) to assess the volume and isotopic composition of carbonates in these sediments. Carbonate in the Bakken black shales occurs primarily as microscopic disseminated dolomite rhombs and calcite cements that, together, comprise a significant mass‐fraction (ca 9%). The elemental composition of the shales is indicative of a dynamic anoxic to sulphidic palaeoenvironment, likely supported by a fluctuating chemocline. Despite forming in an environment favourable to remineralization of organic matter and the precipitation of isotopically depleted authigenic carbonates, the majority of carbon isotope measurements of disseminated carbonate fall between −3‰ and +3‰, with systematically more depleted carbonates in the deeper‐water portions of the basin. Thus, although there is evidence for a significant total mass‐fraction of carbonate with contribution from remineralized organic matter, Bakken authigenic carbonates suggest that Famennian black shales are unlikely to be sufficiently 13C‐depleted relative to water column dissolved inorganic carbon to serve as a major lever on seawater isotopic composition.

Terrestrial biosphere changes over the last 120 kyr

Abstract. A new global synthesis and biomization of long (> 40 kyr) pollen-data records is presented and used with simulations from the HadCM3 and FAMOUS climate models and the BIOME4 vegetation model to analyse the dynamics of the global terrestrial biosphere and carbon storage over the last glacial–interglacial cycle. Simulated biome distributions using BIOME4 driven by HadCM3 and FAMOUS at the global scale over time generally agree well with those inferred from pollen data. Global average areas of grassland and dry shrubland, desert, and tundra biomes show large-scale increases during the Last Glacial Maximum, between ca. 64 and 74 ka BP and cool substages of Marine Isotope Stage 5, at the expense of the tropical forest, warm-temperate forest, and temperate forest biomes. These changes are reflected in BIOME4 simulations of global net primary productivity, showing good agreement between the two models. Such changes are likely to affect terrestrial carbon storage, which in turn influences the stable carbon isotopic composition of seawater as terrestrial carbon is depleted in 13C.

Mantle plume: The invisible serial killer — Application to the Permian–Triassic boundary mass extinction

The Earth experienced a severe mass extinction at the Permian–Triassic boundary (PTB) about 252 million years ago. This biological catastrophe was accompanied by major changes in geochemical composition of the atmosphere and ocean and the appearance of sedimentary features which had not occurred since the Precambrian time. The eruption of the largest continental flood basalt, the Siberian Traps, overlapped this mass killing. Many hypotheses have been proposed but no definitive conclusion currently exits. Here we present characteristics of three sections from Iran and China and propose that an active mantle plume initiated a series of processes which led to the mass mortality and produced major sedimentological, mineralogical, and geochemical changes observed in the transition from the Paleozoic to the Mesozoic. The injection of mantle plume-related igneous dike swarms into the continental margin facilitated the release of massive amounts of CH 4 primarily from the dissociation of marine gas hydrates and secondarily from the maturation of organic-rich sediments and fracturing of hydrocarbon reservoirs. The bulk of the CH 4 was aerobically oxidized in the water column producing dissolved CO 2 with low δ 13C values. This CO 2-saturated seawater became acidic to the point of dissolution of shelf carbonates promoting precipitation of siliciclastic-rich strata in the transition from the Permian to the Triassic. Methane-derived CO 2 also lowered carbon isotopic composition of seawater leading to the observed decline in δ 13C composition of organic and inorganic marine carbon at the PTB. Gas-charged oceans released large volumes of CO 2 and CH 4 into the atmosphere which created a severe global warming (the end-Permian inferno) causing the release of additional CH 4 from the dissociation of polar gas hydrates. These events lowered δ 13C compositions of terrestrial carbon. Simultaneously, feeder dikes from the mantle plume formed the Siberian Traps flood basalt. Marine mass extinction was the result of a change in seawater composition due to the injection and oxidation of CH 4 in the water column causing low pH, high concentrations of CO 2, Ca 2+ and HCO 3 −, and low CO 3 2− values. Combined with a hot seawater temperature, these changes made calcification of marine organisms difficult and produced major physiological crisis including reduced metabolic rates, high sensitivity to environmental stress, and hampered growth and reproduction. Terrestrial mass extinction can be attributed to severe global warming and soil acidification produced by increased atmospheric CO 2, acid rain that was generated by SO 2 derived from the Siberian trap eruption, and loss of habitat. Cessation of the plume activity during Early Triassic stopped the release of CH 4 into the ocean and terminated continental flood basalt eruption ending the environment of death on land and in sea. The cut off of CO 2 production in the ocean instantly increased carbonate saturation of seawater resulting in extensive seafloor cementation. It also resulted in the deposition of marine carbonates by microbial activities in the hostile post-extinction environment. From the trigger to recovery, the perturbation which included the end-Permian mass mortality could have lasted for at least 2 Myr. Several major mass extinctions of the Phanerozoic are temporally accompanied by flood basalt eruptions. So far, these two events have been interpreted in a cause-and-effect relation: flood basalt eruption causes mass extinction. This study proposes that flood basalts and their time correlative biological crises are themselves the consequence of a complex perturbation caused by mantle plume activities. If so, major disturbances in the near surface of the Earth are ultimately controlled by changes in the mantle. The physical sedimentological observations presented here combined with previous paleontological evidence cast doubt on other interpretations which use geochemical variables, numerical modeling, biomarkers, and Ce anomaly to suggest that Late Permian ocean was anoxic all the way to the photic zone.

Carbon isotopic evolution of the terminal Neoproterozoic and early Cambrian: Evidence from the Yangtze Platform, South China

Profound geotectonic, climatic and biological changes occurred during the terminal Neoproterozoic and its transition into the Early Cambrian. These changes are reflected in temporal variations of the carbon isotopic composition of seawater, recorded in a sedimentary succession on China's Yangtze Platform. However, in addition to secular changes in carbon isotopes, the studied succession reflects additional variations in accordance with the deposition in different sedimentary facies, ranging from shallow water platformal to deeper water basinal settings. Likely a consequence of incorporation of variable amounts of 13C depleted bacterial biomass, this regional signal is superimposed on the secular variations of the global carbon cycle.

Trace element abundances in the Rock Canyon Anticline, Pueblo, Colorado, marine sedimentary section and their relationship to Caribbean plateau construction and oxygen anoxic event 2

Rapid eruption of submarine lava flows during formation of the Caribbean plateau correlates closely with ocean anoxic event 2 (OAE2), which bracketed the Cenomanian/Turonian boundary (∼93.5 Ma). These events also correspond with a positive excursion in carbon isotopic composition of seawater. Hydrothermal activity associated with large‐scale submarine volcanism may have been responsible for this abrupt change in ocean chemistry. We determined the distribution of major, minor, and trace element abundances in the Rock Canyon marine sedimentary section (Pueblo, Colorado). After normalizing element concentrations to Zr to remove the variable contribution of terrigenous material to these sediments, we detected an interval of concentrated metal abundance anomalies that coincides with the abrupt beginning of the positive δ13C isotope excursion. The metal abundance anomalies indicate that intermittent hydrothermal activity, in the form of both water/rock exchange and magmatic degassing, introduced large concentrations of trace metals into the Cretaceous ocean at the same time that extinctions of benthic species, turnover in plankton communities, and increases in isotopically light organic carbon burial occurred. The stratigraphic position of this interval of trace metal anomalies matches events associated with OAE2 and indicates that intermittent hydrothermal activity on a massive scale triggered abrupt changes in carbon burial and deep ocean oxygen contents.

Orbitally induced climate and geochemical variability across the Oligocene/Miocene boundary

To assess the influence of orbital‐scale variations on late Oligocene to early Miocene climate and ocean chemistry, high‐resolution (∼5 kyr) benthic foraminiferal carbon and oxygen isotope and percent coarse fraction time series were constructed for Ocean Drilling Program site 929 on Ceara Rise in the western equatorial Atlantic. These time series exhibit pervasive low‐ to high‐frequency variability across a 5‐Myr interval (20.5–25.4 Ma). The records also reveal several large‐scale secular variations including two positive (∼1.6‰) oxygen isotope excursions at 22.95 and 21.1 Ma, suggestive of large but brief glacial maxima (Mi‐1 and Mi‐1a events of Miller et al. [1991]), and a long‐term cyclical increase in the carbon isotopic composition of seawater (shift of ∼1.52‰) that reaches a maximum coincident with peak δ18O values at 22.95 Ma. Lower‐resolution (∼25 kyr) records constructed from benthic and planktonic foraminifera as well as bulk carbonate at a shallower site on Ceara Rise (site 926) for the period 21.7–24.9 Ma covary with site 929 δ18O values reflecting changes in Antarctic ice‐volume. Likewise, covariance among carbon isotopic records of bulk sediment, benthic, and planktonic foraminifera suggest that the low‐frequency cycles (∼400 kyr) and long‐term increase in δ13C values represent changes in the mean carbon composition of seawater ΣCO2. The time series presented here constitute the longest, most continuous, and highest‐resolution records of pre‐Pliocene climate and oceanography to date. The site 929 carbon and oxygen isotope power spectra show significant concentrations of variance at ∼400, 100, and 41 kyr, demonstrating that orbitally induced oscillations have been a normal characteristic of the global climate system since at least the Oligocene, including periods of equable climate and times with no apparent Northern Component Water production.

Formula omitted] and [formula omitted] ratios in skeletal calcite of Mytilus trossulus: Covariation with metabolic rate, salinity, and carbon isotopic composition of seawater

Minor element and isotopic compositions of marine bivalve calcite are frequently used as proxy records of seawater temperature and salinity. Although molluscan calcite is secreted at or near oxygen isotope equilibrium, the influence of metabolic activity (i.e., vital effects) on skeletal SrCa ratios and δ 13C values is not well known. We present measurements of skeletal chemistry from (a) consecutive samples milled in chronological order from the organism's final year of growth and (b) adjacent samples within three separate growth bands of the marine mussel Mytilus trossulus to investigate chemical disequilibrium effects among different parts of the shell.Seawater temperature and salinity were monitored for one year at Squirrel Cove, British Columbia. At the end of the year, a young, rapidly growing mussel (mussel A) and an old, slowly growing mussel (mussel B) were harvested from this site. Growth bands within the shells were sampled to provide a chronological record of shell carbonate chemistry. Results from consecutive samples show (1) a significantly higher average SrCa ratio in mussel A than that in mussel B and (2) δ 13C values that correlate well with salinity in mussel A but not in mussel B. Results from time-equivalent, adjacent samples show (1) higher SrCa and δ 13C values near the ventral margin than at time-equivalent regions on lateral margins of the shell and (2) little or no variation in δ 18O values.These observations suggest that skeletal chemistry (SrCa and δ 13C) is primarily controlled by rate of mantle metabolic activity and secondarily modified by variation in seawater salinity. Because metabolic activity in the mantle at the site of carbonate precipitation varies with shell curvature, the composition of calcite secreted along lateral margins is influenced to a greater extent by metabolic activity than calcite secreted coevally at the central margin of the shell. Hence, the chemistry of calcite secreted at the ventral margin is precipitated in near-equilibrium with seawater and allows accurate estimation of seawater SrCa ratios and carbon isotopic composition. In contrast, shell precipitation along lateral margins is dominantly controlled by metabolic activity, and calcite chemistry is not in equilibrium with ambient seawater. In this context, we present a biomineralization model where variations in skeletal SrCa and δ 13C values are explained in the context of mantle metabolic activity and seawater salinity.

Do planktic foraminifera accurately record shifts in the carbon isotopic composition of seawater ΣCO 2?

Both the carbon isotopic composition of seawater and physiological “vital effects” such as respiration and symbiont photosynthesis influence the δ 13C value of foraminiferal calcite. The symbiont-bearing planktic foraminifera, Orbulina universa, was grown in a series of laboratory experiments to determine whether vital effects differentially obscure the δ 13C signal such that changes in the δ 13C of seawater ΣCO 2 cannot be quantitatively evaluated. Foraminifera were cultured under three different irradiance levels to elicit a full range of symbiont photosynthetic effects and in seawater with δ 13C values ranging between + 1.77‰ (ambient) and + 5.76‰. Power regression analysis of isotope data from individual shells plotted against culture irradiance yields three curves which are nearly parallel but are offset by the difference between the carbon isotopic composition of the culture waters. When irradiance groups are combined and O. universa δ 13C values are plotted against seawater δ 13C, three parallel linear regressions are obtained with slopes ranging between 0.93 and 0.96. Because the slopes are near unity, it is concluded that the δ 13C of O. universa accurately tracks changes in the carbon isotopic composition of seawater regardless of the magnitude of its physiological vital effect. O. universa oxygen isotope data show that individuals maintained in the low and medium irradiance groups secrete their shells in isotopic equilibrium according to established paleotemperature equations. In contrast, individuals grown under high irradiance levels (maximum symbiont photosynthetic rates) are on average 0.35‰ depleted in 18O relative to equilibrium. Symbiont-enhanced calcification rates in the high light group may be responsible for the observed δ 18O difference.