Acid-insoluble Organic Matter Research Articles

Bioturbation supplying young carbon into West Antarctic continental margin sediment

Accelerator Mass Spectrometry (AMS) 14C dating is a widely applied method for establishing high-precision chronologies used to underpin late Quaternary paleoclimatic studies. Feedbacks associated with Antarctic Ice Sheets (AIS) and Southern Ocean variance are thought to have played a critical role in regulating past global climate changes since the Last Glacial Maximum. However, determining the exact role of the AIS and Southern Ocean in these changes has proven difficult because of the sparse occurrence of biogenic carbonate suitable for AMS 14C dating in Antarctica. Consequently, AMS 14C dating is often measured on acid insoluble organic matter (AIOM) from bulk sediment samples, which are often biased towards ages that are older than the depositional age, due to pervasive reworking of old carbon. In this scenario, ages can still potentially provide maximum deposition ages, but such interpretations need to be assessed carefully. Here, we document AIOM and foraminifer AMS 14C dates at the same depth intervals from Antarctic continental margin cores, and we find AIOM AMS 14C dates can range between 2000 and 16,000 years younger than those of foraminifera samples. The younger AIOM 14C dates were found in bioturbated sedimentary facies, which often have extremely low sedimentation rates in Antarctica. In cores with low sedimentation rates, the down sediment contribution of organic carbon from macro-benthos can be quite significant in the sediment column with low organic carbon concentration. After considering possibilities of contamination, foraminifer reworking, and remineralization of secondary carbonate to explain this offset, we conclude that bioturbation plays an important role in the Antarctic carbon cycle as a young carbon supplier to depth into the sediment column. This has clear implications for using condensed sedimentary sections to date post-LGM records in Antarctica. These bioturbation processes also have implications for novel radiocarbon dating methods that have recently been developed to overcome sediment reworking processes and more accurately constrain timing of past AIS retreat, including ramped pyrolysis and compound specific isotopes methods.

Comparison of FT‐IR spectra of bulk and acid insoluble organic matter in chondritic meteorites: An implication for missing carbon during demineralization

AbstractPast studies of the various separable carbonaceous fractions have been unable to account for all of C in primitive chondrites. In particular, up to 20–50% of the C is lost during acid leaching of bulk samples even after the C in carbonates and soluble organic matter is accounted for. To try to better characterize the nature of this “missing C,” we have compared the bulk infrared (IR) absorption spectra of a number of primitive chondrites with those of their previously reported insoluble organic matter (IOM). The aliphatic C–H stretching bands, in particular, allow us to compare the molecular structures of bulk C with that of IOM. The spectral differences between bulk C and IOM reflect “missing C” phases that were lost during acid leaching, although we cannot completely exclude the possibility that the OM was modified after demineralization. Comparing IR spectra of bulk meteorite powder and IOM suggests that the missing C varies in its molecular structure, and that mildly thermally metamorphosed type 3 chondrites tend to be richer in an aliphatic fraction with lower CH2/CH3 ratios, relative to IOM, compared to aqueously altered carbonaceous chondrites (CI/CM/CR). The missing C is most likely released from acid‐labile functional groups, such as esters, acetals, and amides, during demineralization, although it cannot be ruled out that some fraction of the missing C is in small grains that are difficult to recover from suspension, or in water‐soluble compounds trapped in phyllosilicates.

Influence of pH and redox on mobilization of inositol hexakisphosphate from oligotrophic lake sediment

It has been suggested that inositol hexakisphosphate (IP6) contributes to the release of phosphorus (P) from lake sediments, but a mechanistic understanding remains elusive. We investigated the potential mobilization and mineralization of myo- and scyllo-IP6 from the sediment of an oligotrophic Danish lake known to contain high concentrations of inositol phosphates. Solution 31P NMR spectroscopy was used to determine changes in myo- and scyllo-IP6 in laboratory microcosms incubated under either oxic or anoxic conditions. In addition, we incubated sediment slurries adjusted to pH between 4.9 and 6.6, with and without addition of myo-IP6, and induced redox changes by adding starch and sulfate. We observed no significant changes in myo- or scyllo-IP6 after 1 year of incubation under anaerobic conditions. A sequential extraction procedure revealed that one half of the added myo-IP6 was recovered in the humic acid fraction (acid-insoluble organic matter) and the other half in the fulvic acid fraction (acid-soluble organic matter). Reduction in redox potential by starch addition did not mobilize myo-IP6, but myo-IP6 bound to humic acids was released to the pore water when the pH was increased to ≥ 5.8. This pH-induced mobilization of IP6 occurred in parallel with increases in dissolved iron and organic matter, suggesting the release of IP6 bound to humic acids through metal bridges. We conclude that myo-IP6 mobilization from this oligotrophic lake sediment is driven by changes in pH rather than by changes in the redox potential.

Discrepant responses of methane emissions to additions with different organic compound classes of rice straw in paddy soil

Crop straw incorporation has become a prevailing agricultural practice that guarantees the food production and security. There is a significant body of work on the effects of straw incorporation on the methane (CH4) emissions in paddy fields. However, it is unclear whether there are diverse links between CH4 emission dynamics and incorporations of different organic compound classes of straw to paddy fields. In this study, soil incubations were conducted to assess the respective effect of incorporations of hydrolysable amino acid (HAA), dilute-acid extractable carbohydrate (DAC), lipid and acid-insoluble organic matter (AIOM) fractions of rice straw on the CH4 emission in paddy soil. It is revealed that incorporations of HAA and DAC fractions exert the greatest intensities to stimulate the CH4 emissions, which mainly takes place in the early period of incubation; on contrary, the incorporation of lipid fraction exerts the lowest intensity and mainly takes place in the late period. The pattern of CH4 emission after incorporation of AIOM fraction occurs peaks both in the early and late periods of incubation. Our findings highlight that the time of occurrence and intensity of effects of rice straw incorporation on CH4 emissions vary significantly depending on the different organic compound classes of rice straw, which may be key to proposing a promising management strategy for mitigating CH4 emissions in paddy fields in the context of straw incorporation.

Circum-Antarctic age modelling of Quaternary marine cores under the Antarctic Circumpolar Current: Ice-core dust–magnetic correlation

Sediments in the belt under the Antarctic Circumpolar Current (ACC) contain high quantities of siliceous microfossils (mainly diatoms) and very little to zero carbonate. This prevents establishment of the age by the usual methods of oxygen isotope stratigraphy at most coring sites. Downcore variation in magnetic susceptibility (MS) from cores from the Scotia Sea closely resembles the temporal pattern of dust concentration and flux in East Antarctic ice cores. The validity of the temporal correlation between marine MS and EPICA Dome C (EDC) dust on the EDC3 time scale is established through 14C dating of the acid insoluble organic matter back to 30 ka, and radiolarian abundance stratigraphy of Cycladophora davisiana back to 180 ka. The correlations are good ( r > 0.80) and give credence to the use of MS-dust as a means of establishing chronostratigraphy in carbonate-free pelagic sediments from the Southern Ocean. This correlation has been established in > 10 cores in the Scotia Sea. The circum-Antarctic validity of the pattern is shown in correlations to MS in sediment cores from the Indian and SW Pacific sectors of the Southern Ocean. During MIS 3 (57–29 ka), when sedimentation rates at correlated core sites exceeded 10 cm ka − 1 , three MS maxima in the marine sediments and dust peaks at EDC, respectively, correspond to the cool periods between Antarctic warm events A1–A4. We have not established the precise cause of the correlation, but its validity around the circum-Antarctic west wind belt suggests that the material was either all transported by wind or was supplemented by wind-driven current transport. The fact that it occurs also in sections with a low terrigenous content suggests a fine grained, high-susceptibility material such as magnetite as the likely source.

High primary productivity and nitrogen cycling after the Paleoproterozoic phosphogenic event in the Aravalli Supergroup, India

The causes of atmospheric oxygenation in the Paleoproterozoic are multifaceted yet poorly understood. An increase in the level of primary productivity however may have been an important factor in producing large amounts of oxygen in the atmosphere. A likely environmental consequence of atmospheric oxygenation is an increased abundance of seawater nitrate, which would have resulted in a fundamental change from the Archean nitrogen cycle. In this study, we investigate these questions using the carbon and nitrogen isotope compositions of carbonaceous shales from the Paleoproterozoic Aravalli Supergroup, India. The shales occur stratigraphically above one of the oldest phosphogenic events that took place around 2.0Ga. Shales in the phosphate domain of the Jhamarkotra Formation from the Lower Aravalli Group have distinct characteristics from coeval shales in the non-phosphate domain. Carbonaceous shales of the phosphate domain contain relatively high V and U/Th, typical of oxygen-deficient environments. These shales also have a high organic content of up to 14wt%, generally homogeneous δ13Corg values around −29‰, C/NWR up to 360 and a large range of δ15N values between −5 and +27‰. Their organic matter has low H/C and O/C and Raman spectra characteristic of lower greenschist to lower amphibolite facies metamorphism, which suggests a moderate alteration of geochemical compositions. Data from these shales also suggest high primary productivity under eutrophic conditions, as well as intense recycling of organic nitrogen through ammonium assimilation and denitrification at the time of sedimentation.In contrast, carbonaceous shales from the non-phosphate domain of the Jhamarkotra Formation contain trace element abundances characteristic of more oxygenated conditions. They contain less than 3wt% organic matter with δ13Corg values often between −13 and −18‰, whole-rock δ15N values between +5 and +12‰ and C/NWR below 56. Acid-insoluble organic matter from these shales has high H/C and O/C, and Raman spectral characteristics typical of lower greenschist facies metamorphism, which suggests only minor alteration of isotope compositions. Data gleaned from carbonaceous shales of the non-phosphate domain are interpreted to indicate high rates of primary productivity as well as the existence of a modern-like nitrogen cycle in this open-marine environment at the mouth of the Udaipur Epicontinental Sea. The interpretation of high primary productivity in carbonaceous shales from both domains of the Jhamarkotra Formation is consistent with stratigraphically underlying 13C-enriched carbonates and with their associated stromatolitic phosphorites. Therefore, increased fluxes of phosphorous in Paleoproterozoic seawater may have played an important role in stimulating increased levels of primary productivity and the rise of atmospheric oxygen.

Prebiotic carbon in clays from Orgueil and Ivuna (CI), and Tagish Lake (C2 ungrouped) meteorites

Abstract— Transmission electron microscopic (TEM) and electron energy‐loss spectroscopic (EELS) study of the Ivuna and Orgueil (CI), and Tagish Lake (C2 ungrouped) carbonaceous chondrite meteorites shows two types of C‐clay assemblages. The first is coarser‐grained (to 1 μm) clay flakes that show an intense O K edge from the silicate together with a prominent C K edge, but without discrete C particles. Nitrogen is common in some clay flakes. Individual Orgueil and Tagish Lake meteorite clay flakes contain up to 6 and 8 at% C, respectively. The C K‐edge spectra from the clays show fine structure revealing aromatic, aliphatic, carboxylic, and carbonate C. The EELS data shows that this C is intercalated with the clay flakes. The second C‐clay association occurs as poorly crystalline to amorphous material occurring as nanometer aggregates of C, clay, and Fe‐O‐rich material. Some aggregates are dominated by carbonaceous particles that are structurally and chemically similar to the acid insoluble organic matter. The C K‐edge shape from this C resembles that of amorphous C, but lacking the distinct peaks corresponding to aliphatic, carboxylic, and carbonate C groups. Nanodiamonds are locally abundant in some carbonaceous particles. The abundance of C in the clays suggest that molecular speciation in the carbonaceous chondrites is partly determined by the effects of aqueous processing on the meteorite parent bodies, and that clays played an important role. This intricate C‐clay association lends credence to the proposal that minerals were important in the prebiotic chemical evolution of the early solar system.

Pre-LGM open-water conditions south of the Drygalski Ice Tongue, Ross Sea, Antarctica

AbstractA marine sediment core collected from the Nordenskjold Basin, to the south of the Drygalski Ice Tongue, provides new sedimentological and chronological data for reconstructing the Pleistocene glacial history and palaeoenvironmental evolution of Victoria Land. The core consists of an over consolidated biogenic mud covered with glacial diamicton; Holocene diatomaceous mud lies on top of the sequence. Radiocarbon dates of the acid insoluble organic matter indicate a pre-Last Glacial Maximum age (>24kyr) for the biogenic mud at the base of the sequence. From this we can presume that at least this portion of the western Ross Sea was deglaciated during Marine Isotope Stage 3 and enjoyed open marine conditions. Our results are consistent with recent findings of pre-Holocene raised beaches at Cape Ross and in the Terra Nova Bay area.

Chronology of the Palmer Deep site, Antarctic Peninsula: a Holocene palaeoenvironmental reference for the circum-Antarctic

Palmer Deep sediment cores are used to produce the first high-resolution, continuous late Pleistocene to Holocene time-series from the Antarctic marine system. The sedimentary record is dated using accelerator mass spectrometer radiocarbon methods on acid insoluble organic matter and foraminiferal calcite. Fifty-four radiocarbon analyses are utilized in the dating which provides a calibrated timescale back to 13 ka BP. Reliability of resultant ages on organic matter is assured because duplicates produce a standard deviation from the surface age of less than laboratory error (i.e., ±50 years). In addition, surface organic matter ages at the site are in excellent agreement with living calcite ages at the accepted reservoir age of 1260 years for the Antarctic Peninsula. Spectral analyses of the magnetic susceptibility record against the age model reveal unusually strong periodicity in the 400,–200 and 50-70 year frequency bands, similar to other high-resolution records from the Holocene but, so far, unique for the circum-Antarctic. Here we show that comparison to icecore records of specific climatic events (e.g., the ’Little Ice Age‘, Neoglacial, Hypsithermal, and the Bølling/Allerød to Younger Dryas transition) provides improved focus upon the relative timing of atmosphere/ocean changes between the northern anid southern high latitudes.

Recent sediment remolding on a deep shelf, Ross Sea: implications for radiocarbon dating of Antarctic marine sediments

Coarse, bioclastic rich sands have been widely reported from the banks of the Antarctic continental shelf but their origin is still poorly known. We report on a suite of coarse sediments recovered from the top of the Mawson Bank in the northwestern Ross Sea. Radiocarbon ages of biogenic calcite, for modern and apparently late Pleistocene deposits, range from 1085±45 to 20,895±250 yr B.P.. Discovery of soft tissue (Ascidian) preserved as an incrustation on a pebble at 2 m depth indicates aggregation of the sediment within several months or a year of core recovery. Radiocarbon ages of acid insoluble organic matter (aiom) are less than those of the foraminifera calcite. The aiom ages are also reversed in sequence, indicating reworking of the sediment during deposition. These observations and a review of recently published literature suggest that much of the bank top sediment in Antarctica is presently undergoing remobilization, under the influence of strong currents and/or icebergs even under interglacical (high-stand) sea levels. These observations point out the need for careful, integrated studies on high latitude marine sediment cores before resultant “ages” alone are used as the foundation for paleoglacial reconstructions.

Geomagnetic secular variations (inclination) of high latitude fiord cores: eastern Canadian Arctic

Paleomagnetic measurements are reported from 11 piston cores, from the fiords and shelf of eastern Baffin Island, N.W.T., between latitudes 66 and 72 degrees north. The majority of the measurements arc from bioturbated, massive, or laminated mud, with some drop-stones and graded sand beds. Corrected radiocarbon dates on the acid-insoluble organic matter fraction, supplemented by AMS dates on in situbivalves, indicate that all cores extend into the early Holocene, and three extend into the latest Pleistocene Sedimentation rates averaged between 0.2m/ka and 1.4m/ka. Because of varying sedimentation rates, the depth scales are converted to 100 or 200 yr/sample time series. The results indicate a scries of geomagnetic secular oscillations with amplitudes in inclination of c. 10 degrees. A stacked record from four piston cores suggests seven major oscillations in inclination. Times when inclinations consistently exceeded 80° occurred c. 1,400, 4,500, and 8,000 B.P. The most characteristic oscillation occurred c.1,400 ± B.P., when inclinations were nearly vertical. Inclination errors are associated with gravity flows and/or with an increase in sand content, or changes in physical properties. In one core an interval of reverse polarity is attributed to a slump.

Geomagnetic secular variations (inclination) of high latitude fiord cores: eastern Canadian Arctic

Paleomagnetic measurements are reported from 11 piston cores, from the fiords and shelf of eastern Baffin Island, N.W.T., between latitudes 66 and 72 degrees north. The majority of the measurements arc from bioturbated, massive, or laminated mud, with some drop-stones and graded sand beds. Corrected radiocarbon dates on the acid-insoluble organic matter fraction, supplemented by AMS dates on in situbivalves, indicate that all cores extend into the early Holocene, and three extend into the latest Pleistocene Sedimentation rates averaged between 0.2m/ka and 1.4m/ka. Because of varying sedimentation rates, the depth scales are converted to 100 or 200 yr/sample time series. The results indicate a scries of geomagnetic secular oscillations with amplitudes in inclination of c. 10 degrees. A stacked record from four piston cores suggests seven major oscillations in inclination. Times when inclinations consistently exceeded 80° occurred c. 1,400, 4,500, and 8,000 B.P. The most characteristic oscillation occurred c.1,400 ± B.P., when inclinations were nearly vertical. Inclination errors are associated with gravity flows and/or with an increase in sand content, or changes in physical properties. In one core an interval of reverse polarity is attributed to a slump.

Chemical properties of soil and water-stable aggregates after sixty-seven years of cropping to spring wheat

Three non-replicated, unfertilized, dryland grain rotations—continuous wheat, wheat-fallow, and wheat-wheat-fallow—were established in 1912 on a Dark Brown Chernozemic (Typic Haploboroll) soil. The effect of long-term cropping on the chemical constituents of total water-stable aggregates was assessed. There was a loss in percentage of total water-stable aggregates and a shift in aggregate size distribution with time. Together with an increase in the 100 μm diameter fraction, there was an increase in the sand component of this fraction. These sand particles are probably held together by alkaline-soluble, acid-insoluble organic matter. Organic carbon, polysaccharides, polyuronides, phenols, and chloroform/methanol-extractable organic matter were all associated with the >250 μm diameter fractions. Although the aggregates had generally the same suite of aliphatic carboxylic acids, the relative proportions changed with cultivation.