Timothy I. Eglinton

A reassessment of the sources and importance of land-derived organic matter in surface sediments from the Gulf of Mexico

Organic matter in surface sediments from two onshore-offshore transects in the northwestern Gulf of Mexico was characterized by a variety of techniques, including elemental, stable carbon, radiocarbon, and molecular-level analyses. In spite of the importance of the Mississippi River as a sediment source, there is little evidence for a significant terrigenous input based on the low carbon:nitrogen ratios (8 -5) and the enriched d 13 C values of bulk sedimentary organic carbon (219.7‰ to 221.7‰). Radiocarbon analyses, on the other hand, yield depleted D 14 C values (2277‰ to 2572‰) which indicate that a significant fraction of the sedimentary organic carbon (OC) in all these surface sediments must be relatively old and most likely of allochthonous origin. CuO oxidations yield relatively low quantities of lignin products (0.4 -1.4 mg/100 mg OC) along with compounds derived from proteins, polysaccharides, and lipids. Syringyl:vanillyl and cin- namyl:vanillyl ratios (averaging 1.6 and 0.5, respectively) and acid:aldehyde ratios for both vanillyl and syringyl phenols (averaging 0.8 and 1.2, respectively) indicate that the lignin present in sediments originates from nonwoody angiosperm sources and is highly degraded. The d 13 C values of lignin phenols in shelf sediments are relatively depleted in 13 C (averaging 226.3‰) but are increasingly enriched in 13 C at the slope sites (averaging 217.5‰ for the two deepest stations). We interpret these molecular and isotopic compositions to indicate that a significant fraction (

Compound-specific D/H ratios of lipid biomarkers from sediments as a proxy for environmental and climatic conditions

50%) of the lignin and, by inference, the land-derived organic carbon in northwestern Gulf of Mexico sediments ultimately originated from C4 plants. The source of this material is likely to be soil organic matter eroded from the extensive grasslands of the Mississippi River drainage basin. Notably, the mixed C4 and C3 source and the highly degraded state of this material hampers its recognition and quantification in shelf and slope sediments. Our data are consistent with higher than previously estimated inputs of land-derived organic carbon to regions of the ocean, such as the Gulf of Mexico, with significant sources of terrigenous C4-derived organic matter. Copyright

Sources and contribution of terrigenous organic carbon to surface sediments in the Gulf of Mexico

Hydrogen isotope ratios (D/H) of lipid biomarkers extracted from aquatic sediments were measured to determine whether they can be used as a proxy for D/H of environmental water. Values of dD were determined by using a recently developed isotope-ratio-monitoring gas chromatograph-mass spectrom- eter system (irmGCMS) and were confirmed by conventional hydrogen isotopic measurements (i.e., combustion followed by reduction) on individual compounds isolated by preparative capillary gas chromatography. Diverse lipids (alkanes, n-alkanols, sterols, and pentacyclic triterpenols) were analyzed to examine hydrogen- isotopic controls on lipids of varying origin and biosynthetic pathway. For algal sterols (24-methylcholest- 3b-ol, 24-ethylcholest-5,22-dien-3b-ol, and 4,23,24-trimethylcholesterol, or dinosterol), the fractionation between sedimentary lipids and environmental water was 2201 6 10‰ and was similar in both marine and freshwater sites. In a sediment from a small lake in a forested catchment, triterpenols from terrestrial sources were enriched in D by 30‰ relative to algal sterols. Apparent fractionation factors for n-alkyl lipids were smaller than those of triterpenols and were more variable, probably reflecting multiple sources for these compounds. We conclude that hydrogen-isotopic analyses of algal sterols provide a viable means of reconstructing D/H of environmental waters. Results are less ambiguous than reconstructions based on analyses of kerogen or other operationally defined organic matter fractions. Copyright

Distribution and sources of organic biomarkers in arctic sediments from the Mackenzie River and Beaufort Shelf

The sources and burial processes of organic matter in marine sediments are not well understood, yet they are important if we are to have a better understanding of the global carbon cycle. In particular, the nature and fraction of the terrestrial organic carbon preserved in marine sediments is poorly constrained. Here we use the chemical and stable carbon isotope signatures of oxidation products from a macromolecular component (lignin) of the terrigenous organic matter preserved in offshore surface sediments in the Gulf of Mexico to complement similar data from an existing onshore transect in this region. The complete onshore–offshore data set, along with radiocarbon dates of the bulk organic material at the same sites, allows the differentiation of material originating from plants that photosynthesize using the C4 mechanism from those that undergo C3 photosynthesis. We conclude that the offshore lignins derive from erosion of the extensive grassland (C4) soils of the Mississippi River drainage basin, and that the nearshore lignins originate largely from C3 plant detritus from coastal forests and swamps. This distribution is probably due to the hydrodynamic sorting of the different source materials during their seaward transport. These results suggest that previous studies have significantly underestimated the terrigenous fraction of organic matter in offshore sediments by not recognizing the contribution of C4 vegetation to the carbon-isotope composition. Such an underestimate may force revisions in the assessment of past marine primary productivity and associated organic carbon fluxes, and of organic matter preservation/remineralization and nutrient cycling in marine sediments.

The effect of grain size and surface area on organic matter, lignin and carbohydrate concentration, and molecular compositions in Peru Margin sediments

Abstract Suspended sediments from the Mackenzie River Delta and surface sediments from the Beaufort Shelf were analyzed by alkaline CuO oxidation. In addition, elemental (percentage total organic carbon, inorganic carbon and silica) and stable carbon isotope compositions were determined for all samples. The carbon-normalized yields of over 60 different compounds derived from the oxidative hydrolysis of several biochemicals, including lignin, cutin, proteins, polysaccharides and lipids were quantified and subjected to principal component analyses (PCA). The results of these investigations indicate that most lignin and cutin products originate from non-woody angiosperm vascular vegetation such as that present in the tundra. For example, lignin-derived product compositions are characterized by relatively high syringyl:vanillyl and cinnamyl:vanillyl phenol ratios (exceeding 0.4 and 0.15, respectively). The compositions of these biomarkers, especially the elevated (0.5 to 1.5) acid:aldehyde ratios for vanillyl and syringyl phenols, also suggest that the land-derived organic matter (OM) exported by the Mackenzie River is highly degraded. Non-lignin CuO reaction products derived from proteins, polysaccharides and lipids display distributions that are consistent with a predominant marine (autochthonous) source. The composition of lipid-derived fatty acid products, which is in shelf sediments are dominated by hexanedecenoic acid, suggests a planktonic origin, likely from diatoms. The distribution of these biomarkers across the shelf indicates the presence of relatively fresh algal remains in at least one sample. The relationships between terrigenous biomarker concentrations and bulk 13 C/ 12 C ratios in surface sediments indicate that terrestrial organic carbon dominates in abundance (80% to 50% of total organic carbon) over much of the shelf. Marine/algal-derived carbon represents 20% to 50% of the total carbon in shelf sediments, with the largest fraction being present in the outer mid-shelf. The large variability in the yields of CuO biomarkers from the river suspended sediment samples highlights the heterogeneous nature of the particle load exported by the Mackenzie River. Such variability must be taken into account during the development of quantitative carbon budgets for the Beaufort Shelf.

A field study of the chemical weathering of ancient sedimentary organic matter

A C-rich sediment sample from the Peru Margin was sorted into nine hydrodynamically- determined grain size fractions to explore the effect of grain size distribution and sediment surface area on organic matter content and composition. The neutral monomeric carbohydrate composition, lignin oxidation product yields, total organic carbon, and total nitrogen contents were determined independently for each size fraction, in addition to sediment surface area and abundance of biogenic opal. The percent organic carbon and percent total nitrogen were strongly related to surface area in these sediments. In turn, the distribution of surface area closely followed mass distribution among the textural size classes, suggesting hydrodynamic controls on grain size also control organic carbon content. Never- theless, organic compositional distinctions were observed between textural size classes. Total neutral carbohydrate yields in the Peru Margin sediments were found to closely parallel trends in total organic carbon, increasing in abundance among grain size fractions in proportion to sediment surface area. Coincident with the increases in absolute abundance, rhamnose and mannose increased as a fraction of the total carbohydrate yield in concert with surface area, indicating these monomers were preferentially represented in carbohydrates associated with surfaces. Lignin oxidation product yields varied with surface area when normalized to organic carbon, suggesting that the terrestrially-derived component may be diluted by sorption of marine derived material. Lignin-based parameters suggest a separate source for terrestrially derived material associated with sand-size material as opposed to that associated with silts and clays. Copyright 0 1997 Elsevier Science Ltd

Eastern Pacific cooling and Atlantic overturning circulation during the last deglaciation

Weathering profiles developed on organic carbon-rich black shales were studied to examine the loss and degradation of organic matter (OM) during weathering and its role in the geochemical carbon cycle. Analysis of weathered shales reveals between 60 and nearly 100% total organic carbon (TOC) loss in highly weathered samples relative to initial, unweathered TOC content. Pyrite loss coincides with or precedes organic carbon loss. Elemental analysis and flash pyrolysis–gas chromatography (Py–GC) of kerogen concentrates indicate that there is little or no selective enrichment or depletion of Norg-containing, Sorg-containing, alkylaromatic, branched alkyl or long-chain n-alkyl moieties in most profiles during weathering. Kerogen O/C ratios consistently increase with TOC and pyrite loss. Infrared spectroscopy (IR) reveals an increase in the relative abundance of CC and CO bonds relative to alkyl C–H bonds in progressively weathered samples. These results suggest a two component model for kerogen weathering: largely non-selective oxidation and hydration, followed by cleavage/dissolution of oxidized kerogen fragments. The extent of weathering in a given outcrop is likely limited by a combination of the rate of physical erosion and exposure of the rock to oxidizing surface waters, with OM type/composition playing a lesser role.

Asynchronous alkenone and foraminifera records from the Benguela Upwelling System

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.

The origin of n-alkanes in Santa Monica Basin surface sediment: a model based on compound-specific Δ14C and δ13C data

Abstract Radiocarbon stratigraphy is an essential tool for high resolution paleoceanographic studies. Age models based on radiocarbon ages of foraminifera are commonly applied to a wide range of geochemical studies, including the investigation of temporal leads and lags. The critical assumption is that temporal coupling between foraminifera and other sediment constituents, including specific molecular organic compounds (biomarkers) of marine phytoplankton, e.g. alkenones, is maintained in the sediments. To test this critical assumption in the Benguela upwelling area, we have determined radiocarbon ages of total C37-C39 alkenones in 20 samples from two gravity cores and three multicorer cores. The cores were retrieved from the continental shelf and slope off Namibia, and samples were taken from Holocene, deglacial and Last Glacial Maximum core sections. The alkenone radiocarbon ages were compared to those of planktic foraminifera, total organic carbon, fatty acids and fine grained carbonates from the same samples. Interestingly, the ages of alkenones were 1000 to 4500 yr older than those of foraminifera in all samples. Such age differences may be the result of different processes: Bioturbation associated with grain size effects, lateral advection of (recycled) material and redeposition of sediment on upper continental slopes due to currents or tidal movement are examples for such processes. Based on the results of this study, the age offsets between foraminifera and alkenones in sediments from the upper continental slope off Namibia most probably do not result from particle-selective bioturbation processes. Resuspension of organic particles in response to tidal movement of bottom waters with velocities up to 25 cm/s recorded near the core sites is the more likely explanation. Our results imply that age control established using radiocarbon measurements of foraminifera may be inadequate for the interpretation of alkenone-based proxy data. Observed temporal leads and lags between foraminifera based data and data derived from alkenone measurements may therefore be secondary signals, i.e. the result of processes associated with particle settling and biological activity.

Biomarker records of late Neogene changes in northeast African vegetation

Δ14C and δ13C values were measured for individual long-chain n-alkanes (C24–33) from Santa Monica Basin sediments. The data were then simulated using a three-component mixing model designed to represent the contributions of different sources. The three selected end members were petroleum, modern plant wax, and shale-derived alkanes. The model was optimized to fit the data and to determine the fractional contribution of each component. The results indicated that petroleum accounted for 12% of the alkanes in 0–2.5 cm sediment and 5% in 2.5–7.5 cm sediment. Modern plant waxes contributed 80% (0–2.5 cm) and 87% (2.5–7.5 cm), and the remaining 8% of each sample was attributed to the shale source. The 14C concentration of the modern terrestrial end member was also determined from the model. Δ14C values of ∼+235‰ for the 0–2.5 cm (post-bomb) horizon and ∼ 0‰ for the 2.5–7.5 cm (pre-bomb) horizon indicate that plant leaf waxes have a continental residence time of decades in the southern California region.