Terrestrial Organic Carbon Input Research Articles

Anthropogenic activities control the source dynamics of sediment organic carbon in the lower reach of an inland river

Sediment organic carbon (SeOC) sources with rich information can be used as a "historical archive" reflecting anthropogenic activities in the catchment, which is crucial to carbon management in the watershed. Anthropogenic activities and hydrodynamic conditions significantly influence the river environment and are reflected by the SeOC sources. However, the key drivers of the SeOC source dynamics are ambiguous, which restricts the behavior of regulating the carbon output of the basin. In this study, sediment cores from the lower reach of an inland river were selected to quantify the SeOC sources based on a centennial scale. A partial least squares path model was used to establish the relationship between anthropogenic activities and hydrological conditions with the SeOC sources. Findings showed that the exogenous advantage of SeOC composition was gradually significant (early period: 54.3%; middle period: 81%; later period: 82%) from the bottom layer to the surface layer of the sediments in the lower reach of the Xiangjiang River. Factors related to anthropogenic activities controlled the external input of SeOC (δ13C: r∂ = -0.94, P < 0.001; δ15N: r∂ = -0.66, P < 0.001). Different anthropogenic activities performed different effects. Land use change aggravated soil erosion and brought more terrestrial organic carbon to the downstream. The variation of grassland carbon input was the most obvious (from 33.6% to 18.4%). In contrast, the reservoir construction intercepted upstream sediments, which might have been the main reason for the slow growth of terrestrial organic carbon input in the downstream in the later period. This study provides a specific grafting for the SeOC records - source changes - anthropogenic activities in the lower reach of the river, which provides scientific basis for watershed carbon management.

Changes in Organic Carbon Delivery to the Yangtze River Delta Over the Last 2000 Years

Natural processes and anthropogenic activities are vital in dictating the amount and character of organic carbon (OC) input into large river deltas and adjacent shelves. Previous studies have indicated that sediment from the Huanghe River (HR) has significantly affected the formation of the northern Yangtze River subaqueous delta (YRD) over the past several hundred years. However, whether this process has changed sedimentary OC burial in the YRD remains unclear. A sediment core was collected from the YRD in 2018 CE for optically stimulated luminescence and 210Pb dating as well as grain size, total OC, total nitrogen, and stable-isotope analyses to investigate temporal changes in sedimentary OC over the past 2000 years. The results indicate that changes in terrestrial OC inputs to the YRD have been controlled mainly by the East Asian summer monsoon and anthropogenic influences in the past 2000 years. However, the decreased terrestrial OC inputs after 1385 CE, have been significantly affected by increased contribution of HR sediment to the YRD when the HR lower courses shifted to enter the southern Yellow Sea. This study demonstrates that sediment source changes should not be neglected in analyses of mechanisms and variations in OC burial in estuarine and coastal areas.

Impacts of Climate Change and Human Perturbations on Organic Carbon Burial in the Pearl River Estuary Over the Last Century

Estuaries have experienced significant changes due to global climate change and human perturbations since the last century. However, the climate and anthropogenic influence on the burial of sedimentary organic carbon (OC) in estuaries is still not understood well yet. Here, a 3-meter sediment core was taken from the Pearl River Estuary (PRE) in China. Depth profiles of both bulk OC and lignin biomarker data indicated three stages with different features of buried OC during the 130-year sediment deposition. The 1893-1957 stage showed 20% more burial of marine derived OC, which was mostly adsorbed on finer minerals compared to the years after 1957. The 1957-1980 period witnessed 4.6 times higher burial rate of petrogenic OC, which made the radiocarbon age of total organic carbon 42% older than before due to soil erosion and carbonate rock weathering. The 7-year running average variation of terrestrial OC input based on endmember mixing model was correlated with the Pacific Decadal Oscillation index before 1957, but correlated with the Atlantic Multidecadal Oscillation between 1957 and 1980 in the region. The reduction of land derived OC content after 1980s was mostly affected by human perturbations such as deforestation and dam construction which corresponded to the beginning of Economic Reform and Open Up in China. The overall increase of lignin content from bottom to surface sediment indicated increased vascular plant derived OC due to deforestation activities during the urbanization process. The study suggested different time periods when climate or human disturbance dominantly affected the OC burial in the PRE, which have significant indications for local and global carbon cycling and environmental ecology.

Integrating Perspectives on Dissolved Organic Carbon Removal and Whole‐Stream Metabolism

AbstractQuantifying organic carbon (OC) removal in streams is needed to integrate the functional role of inland waters into landscape carbon budgets. To illustrate how in‐stream OC removal measurements can be used to characterize ecosystem and landscape carbon fluxes, we compared two common methods: (a) Bioassays measuring water column dissolved organic carbon (DOC) uptake and (b) daily rates of whole‐stream metabolism and OC spiraling calculated from fluorescent dissolved organic matter, oxygen, and discharge measurements. We then assessed how OC removal rates from these two methods, measured in two low‐productivity heterotrophic streams, affected estimates of terrestrial OC loading and export using a mass balance model. OC mineralization velocities calculated from whole‐stream metabolism (0.06 ± 0.03 m d−1 (mean ± SD)) were greater than water column bioassay DOC uptake velocities (0.01 ± 0.01 m d−1), which resulted in higher in‐stream OC removal estimates (0.5%–15.2% and 0.02%–4.2% removal for whole‐stream metabolism and bioassays, respectively). Furthermore, the terrestrial OC inputs needed to sustain in‐stream OC concentrations differ among methods, with simulated inputs ranging from 79 to 1,300 or 3–350 g OC d−1 for whole‐stream metabolism or bioassays, respectively. We show how in‐stream OC removal can be used to quantify terrestrial‐aquatic linkages by estimating OC inputs needed to fuel whole‐stream metabolism in low‐productivity streams, and offer future directions to better link OC removal with whole‐ecosystem OC budgets. Without appropriate conversions to whole‐stream processes, bioassays systematically underestimate whole‐stream carbon cycling. By integrating whole‐stream metabolism with OC transport, we can better elucidate the role of running waters in landscape carbon budgets and the global carbon cycle.

Modeling Global Riverine DOC Flux Dynamics From 1951 to 2015

AbstractClimate change has a profound impact on the global carbon cycle, including effects on riverine carbon pools, which connect terrestrial, oceanic, and atmospheric carbon pools. Until now, terrestrial ecosystem models have rarely incorporated riverine carbon components into global carbon budgets. Here we developed a new process‐based model, TRIPLEX‐HYDRA (TRIPLEX‐hydrological routing algorithm), that considers the production, consumption, and transport processes of nonanthropogenic dissolved organic carbon (DOC) from soil to river ecosystems. After the parameter calibration, model results explained more than 50% of temporal variations in all but three rivers. Validation results suggested that DOC yield simulated by TRIPLEX‐HYDRA has a good fit (R2 = 0.61, n = 71, p &lt; 0.001) with global river observations. And then, we applied this model for global rivers. We found that mean DOC yield of global river approximately 1.08 g C/m2 year, where most high DOC yield appeared in the rivers from high northern or tropic regions. Furthermore, our results suggested that global riverine DOC flux appeared a significant decrease trend (average rate: 0.38 Pg C/year) from 1951 to 2015, although the variation patterns of DOC fluxes in global rivers are diverse. A decreasing trend in riverine DOC flux appeared in the middle and high northern latitude regions (30–90°N), which could be attributable to an increased flow path and DOC degradation during the transport process. Furthermore, increasing trend of DOC fluxes is found in rivers from tropical regions (30°S–30°N), which might be related to an increase in terrestrial organic carbon input. Many other rivers (e.g., Mississippi, Yangtze, and Lena rivers) experienced no significant changes under a changing environment.

Sources and characteristics of terrestrial carbon in Holocene-scale sediments of the East Siberian Sea

Abstract. Thawing of permafrost carbon (PF-C) due to climate warming can remobilise considerable amounts of terrestrial carbon from its long-term storage to the marine environment. PF-C can be then be buried in sediments or remineralised to CO2 with implications for the carbon–climate feedback. Studying historical sediment records during past natural climate changes can help us to understand the response of permafrost to current climate warming. In this study, two sediment cores collected from the East Siberian Sea were used to study terrestrial organic carbon sources, composition and degradation during the past ∼ 9500 cal yrs BP. CuO-derived lignin and cutin products (i.e., compounds solely biosynthesised in terrestrial plants) combined with δ13C suggest that there was a higher input of terrestrial organic carbon to the East Siberian Sea between ∼ 9500 and 8200 cal yrs BP than in all later periods. This high input was likely caused by marine transgression and permafrost destabilisation in the early Holocene climatic optimum. Based on source apportionment modelling using dual-carbon isotope (Δ14C, δ13C) data, coastal erosion releasing old Pleistocene permafrost carbon was identified as a significant source of organic matter translocated to the East Siberian Sea during the Holocene.

How important are terrestrial organic carbon inputs for secondary production in freshwater ecosystems?

Abstract Many freshwater systems receive substantial inputs of terrestrial organic matter. Terrestrially derived dissolved organic carbon (t‐DOC) inputs can modify light availability, the spatial distribution of primary production, heat, and oxygen in aquatic systems, as well as inorganic nutrient bioavailability. It is also well‐established that some terrestrial inputs (such as invertebrates and fruits) provide high‐quality food resources for consumers in some systems. In small to moderate‐sized streams, leaf litter inputs average approximately three times greater than the autochthonous production. Conversely, in oligo/mesotrophic lakes algal production is typically five times greater than the available flux of allochthonous basal resources. Terrestrial particulate organic carbon (t‐POC) inputs to lakes and rivers are comprised of 80%–90% biochemically recalcitrant lignocellulose, which is highly resistant to enzymatic breakdown by animal consumers. Further, t‐POC and heterotrophic bacteria lack essential biochemical compounds that are critical for rapid growth and reproduction in aquatic invertebrates and fishes. Several studies have directly shown that these resources have very low food quality for herbivorous zooplankton and benthic invertebrates. Much of the nitrogen assimilated by stream consumers is probably of algal origin, even in systems where there appears to be a significant terrestrial carbon contribution. Amino acid stable isotope analyses for large river food webs indicate that most upper trophic level essential amino acids are derived from algae. Similarly, profiles of essential fatty acids in consumers show a strong dependence on the algal food resources. Primary production to respiration ratios are not a meaningful index to assess consumer allochthony because respiration represents an oxidised carbon flux that cannot be utilised by animal consumers. Rather, the relative importance of allochthonous subsidies for upper trophic level production should be addressed by considering the rates at which terrestrial and autochthonous resources are consumed and the growth efficiency supported by this food. Ultimately, the biochemical composition of a particular basal resource, and not just its quantity or origin, determines how readily this material is incorporated into upper trophic level consumers. Because of its highly favourable biochemical composition and greater availability, we conclude that microalgal production supports most animal production in freshwater ecosystems.

Holocene changes in marine productivity and terrestrial organic carbon inputs into an Icelandic fjord: Application of molecular and bulk organic proxies

This study examines the dynamics of organic carbon contributions from different sources to the sediments of a ~39 m core from Ísafjarðardjúp Fjord, Northwest Iceland, throughout the Holocene. Furthermore, it shows that the variability of terrestrial organic carbon (OCterr) and marine organic carbon (OCmar) is linked to palaeoclimatic change throughout the Holocene. glycerol-dialkyl-glycerol-tetraether (GDGT), alkenone, n-alkane, total OC and total nitrogen analyses were conducted on 326 samples to yield high-resolution branched versus isoprenoid tetraether index (BIT-index), n-alkane/alkenone index and C/N ratio records from ~10,800 to ~300 cal. a BP. These records were used to estimate the OCterr and the OCmar contributions to the sediments. Three different approaches of estimating the OCterr contribution yield different relative amounts, but similar long-term trends. These results indicate that the combination of biomarker records is a good approach to reconstruct OCterr contributions but also highlight the strengths and weaknesses of the individual biomarkers. The OCterr contribution to the total OC inventory continually increases throughout much of the Holocene but does not rise above 30%. It seems to have been driven by changing climate rather than changing sedimentation rates, and during the late Holocene, anthropogenic activity may have been an influence. The reconstructed OCmar contribution to the sediment was used to model changes in palaeoproductivity throughout the Holocene. These changes were likely forced by changes in nutrients supplied both by the catchment area and the Irminger Current.

Assessing C/N and δ13C as indicators of Holocene sea level and freshwater discharge changes in the subaqueous Yangtze delta, China

To examine the applicability of C/N and organic carbon stable isotope (δ13C) in studies of the Holocene sea level and freshwater discharge in the large river mouth of Yangtze, we observed the distribution of carbon, nitrogen and δ13C in a late-Quaternary core (ZK9) collected from the present subaqueous delta. We also collected published data of the two proxies for the suspended particulate matter (SPM) and surficial sediments from the lower Yangtze River to the adjacent East China Sea. The results show that the estuarine front is an important boundary for terrestrial and marine contribution of the organic component in the modern sedimentary environment. In the core ZK9, sediments deposited during c. 13–9 cal. ka BP are characterized by high values of TOC (0.54–1.16%), CaCO3 (0.35% on average), and C/N (&gt;12), which reflect an inner tidal estuarine environment dominated by C3 terrestrial organic carbon input. During c. 9–0.7 cal. ka BP, both TOC content (0.57% on average) and C/N ratio (&lt;10) decrease remarkably while TN increases, indicating a lower estuarine or shallow marine environment. An abrupt sea level rise from c. 9 cal. ka BP resulted in a deeper water environment and reduced terrestrial input at the core location. The low δ13C values (−24.23‰ on average) before c. 6 cal. ka BP reflect a dominantly terrestrial source of organic matter associated with increased freshwater discharge into the estuary during that time. The sediments since c. 6 cal. ka BP are characterized by increasing δ13C up to −24.1 to −23.39‰, reflecting more contribution from marine algae as freshwater discharge fell. We suggest that in the Yangtze River mouth the C/N ratio indicates an abrupt sea level rise at c. 9 cal. ka BP, while δ13C is more useful in reflecting freshwater discharge.

The benthic foraminiferal record of the Medieval Warm Period and the recent warming in the Gullmar Fjord, Swedish west coast

A high-resolution study of benthic foraminiferal assemblages was performed on a ca. eight metre long sediment core from Gullmar Fjord on the west coast of Sweden. The results of 210 Pb- and AMS 14 C-datings show that the record includes the two warmest climatic episodes of the last 1500 years: the Medieval Warm Period (MWP) and the recent warming of the 20th century. Both periods are known to be anomalously warm and associated with positive NAO winter indices. Benthic foraminiferal successions of both periods are compared in order to find faunal similarities and common denominators corresponding to past climate changes. During the MWP, Adercotryma glomerata , Cassidulina laevigata and Nonionella iridea dominated the assemblages. Judging from dominance of species sensitive to hypoxia and the highest faunal diversity for the last ca. 2400 years, the foraminiferal record of the MWP suggests an absence of severe low oxygen events. At the same time, faunas and δ 13 C values both point to high primary productivity and/or increased input of terrestrial organic carbon into the fjord system during the Medieval Warm Period. Comparison of the MWP and recent warming revealed different trends in the faunal record. The thin-shelled foraminifer N. iridea was characteristic of the MWP, but became absent during the second half of the 20th century. The recent Skagerrak–Kattegat fauna was rare or absent during the MWP but established in Gullmar Fjord at the end of the Little Ice Age or in the early 1900s. Also, there are striking differences in the faunal diversity and absolute abundances of foraminifera between both periods. Changes in primary productivity, higher precipitation resulting in intensified land runoff, different oxygen regimes or even changes in the fjord's trophic status are discussed as possible causes of these faunal differences. ► We study an ultra high-resolution marine sediment record covering the last 2500 years. ► We compare foraminifera and δ 13 C of the Medieval Warm Period to the recent warming. ► Both periods display dominance of thin-shelled species but different faunas. ► Gullmar Fjord basin fauna changed to the Skagerrak type during the early 1900s.

Carbon isotopes and lipid biomarker investigation of sources, transport and degradation of terrestrial organic matter in the Buor-Khaya Bay, SE Laptev Sea

Abstract. The world's largest continental shelf, the East Siberian Shelf Sea, receives substantial input of terrestrial organic carbon (terr-OC) from both large rivers and erosion of its coastline. Degradation of organic matter from thawing permafrost in the Arctic is likely to increase, potentially creating a positive feedback mechanism to climate warming. This study focuses on the Buor-Khaya Bay (SE Laptev Sea), an area with strong terr-OC input from both coastal erosion and the Lena river. To better understand the fate of this terr-OC, molecular (acyl lipid biomarkers) and isotopic tools (stable carbon and radiocarbon isotopes) have been applied to both particulate organic carbon (POC) in surface water and sedimentary organic carbon (SOC) collected from the underlying surface sediments. Clear gradients in both extent of degradation and differences in source contributions were observed both between surface water POC and surface sediment SOC as well as over the 100 s km investigation scale (about 20 stations). Depleted δ13C-OC and high HMW/LMW n-alkane ratios signaled that terr-OC was dominating over marine/planktonic sources. Despite a shallow water column (10–40 m), the isotopic shift between SOC and POC varied systematically from +2 to +5 per mil for δ13C and from +300 to +450 for Δ14C from the Lena prodelta to the Buor-Khaya Cape. At the same time, the ratio of HMW n-alkanoic acids to HMW n-alkanes as well as HMW n-alkane CPI, both indicative of degradation, were 5–6 times greater in SOC than in POC. This suggests that terr-OC was substantially older yet less degraded in the surface sediment than in the surface waters. This unusual vertical degradation trend was only recently found also for the central East Siberian Sea. Numerical modeling (Monte Carlo simulations) with δ13C and Δ14C in both POC and SOC was applied to deduce the relative contribution of – plankton OC, surface soil layer OC and yedoma/mineral soil OC. This three end-member dual-carbon-isotopic mixing model suggests quite different scenarios for the POC vs SOC. Surface soil is dominating (63 ± 10 %) the suspended organic matter in the surface water of SE Laptev Sea. In contrast, the yedoma/mineral soil OC is accounting for 60 ± 9 % of the SOC. We hypothesize that yedoma-OC, associated with mineral-rich matter from coastal erosion is ballasted and thus quickly settles to the bottom. The mineral association may also explain the greater resistance to degradation of this terr-OC component. In contrast, more amorphous humic-like and low-density terr-OC from surface soil and recent vegetation represents a younger but more bioavailable and thus degraded terr-OC component held buoyant in surface water. Hence, these two terr-OC components may represent different propensities to contribute to a positive feedback to climate warming by converting OC from coastal and inland permafrost into CO2.

Microbial biomass and community composition in boreal lake sediments

We used phospholipid fatty acids (PLFA) to determine microbial biomass and community structure in the sediments of eight boreal lakes with different loadings of allochthonous organic carbon and total phosphorus (TP) in the water during the course of a year. The total concentration of PLFA, an estimate of the microbial biomass, depended more on TP, a proxy for pelagic primary production, but not on dissolved organic carbon, a proxy for terrestrial organic carbon input. The composition of PLFAs varied considerably over time, demonstrating seasonal dynamics in microbial community composition. When PLFA profiles in all lakes and seasons are compared, community composition is more similar within season than within lakes.

High organic carbon deposition in the northern margin of the Aleutian Basin (Bering Sea) before the last deglaciation

High-resolution geochemical, isotope and elemental data from core PC23A in the northern margin of the Aleutian Basin (Bering Sea) were used to reconstruct distinct paleoceanographic features of the last deglaciation (pre-Boreal[PB], Bolling-Allerod[BA], Younger Dryas[YD]). The PB and BA intervals are characterized by increased siliceous (diatom) and calcareous (coccolithophores and foraminifers) productivity represented by high biogenic opal and CaCO3 contents, respectively. The enhanced productivity can plausibly be attributed to an elevated sea-surface nutrient supply from increased melt-water input and enhanced Alaskan Stream injection under warm, restricted sea-ice conditions. High Corg/N ratios and low δ13C values of sediment organic matter during the PB and BA intervals reflect the contribution of terrestrial organic matters. The PB and BA intervals were also identified by laminated sediment layers of core PC23A, characterized by high Mo/Al and Cd/Al ratios, indicating that the bottom water condition remained anoxic. High δ15N values during the same period were attributed mainly to the increased nutrient utilization and subsequent denitrification of seawater nitrate. Part of high δ15N values may also be due to incorporation of inorganic nitrogen in the clay minerals. It is worthy of note that high total organic carbon (TOC) deposition occurred approximately 3,000 years before onset of the last deglaciation. Simultaneous high Corg/N ratios and low δ13C values clearly suggest that the high TOC content should be related to terrestrial organic carbon input. Low δ15N values during the high TOC interval also confirm the contribution of terrigenous organic matter. Although abundant calcareous phytoplankton production under cold, nutrient-poor conditions represented by Baex data was reported for high TOC deposition preceding the last deglaciation in an earlier study of the Okhotsk Sea, the main reason for the enhanced TOC deposition in the Bering Sea is an increased terrigenous input from the submerged continental shelves (Beringia) with a sea-level rise; this is further supported by Al enrichment of bulk sediments during the high TOC deposition.

The sequestration of terrestrial organic carbon in Arctic Ocean sediments: A comparison of methods and implications for regional carbon budgets

A variety of approaches have previously been developed to estimate the fraction of terrestrial or marine organic carbon present in aquatic sediments. The task of quantifying each component is especially important for the Arctic due to the regions’ sensitivity to global climate change and the potential for enhanced terrestrial organic carbon inputs with continued Arctic warming to alter carbon sequestration. Yet it is unclear how each approach compares in defining organic carbon sources in sediments as well as their impact on regional or pan-Arctic carbon budgets. Here, we investigated multiple methods: (1) two end-member mixing models utilizing bulk stable carbon isotopes; (2) the relationship between long-chain n-alkanes and organic carbon (ALKOC); (3) principal components analysis (PCA) combined with scaling of a large suite of lipid biomarkers; and (4) ratios of branched and isoprenoid glycerol dialkyl glycerol tetraether lipids (the BIT index) to calculate the fraction of terrestrial organic matter components preserved in Arctic marine sediments. Estimated terrestrial organic carbon content among approaches showed considerable variation for identical sediment samples. For a majority of the samples, the BIT index resulted in the lowest estimates for terrestrial organic carbon, corroborating recent suggestions that this proxy may represent a distinct fraction of terrestrial organic matter; i.e., peat or soil organic matter, as opposed to markers such as n-alkanes or long-chain fatty acids which measure higher plant wax inputs. Because of the patchy inputs of n-alkanes to this region from coastal erosion in the western Arctic, the ALKOC approach was not as effective as when applied to river-dominated margins found in the eastern Arctic. The difficulties in constraining a marine δ 13C end-member limit the applicability of stable isotope mixing models in polar regions. Estimates of terrestrial organic carbon using the lipid-based PCA method and the bulk δ 13C mixing model approach varied drastically at each site, suggesting that organic matter fractions such as amino acids or carbohydrates may affect bulk organic matter composition in a manner that is not captured in the lipid-based analysis. Overall, terrestrial organic matter inputs to the Chukchi and western Beaufort Seas using the average of the methods at each site ranged from 11% to 44%, indicating that land-derived organic matter plays a substantial role in carbon dynamics in the western Arctic Ocean.

Holocene organic carbon burial rates in the southeastern Swedish Baltic Sea

To estimate Holocene changes in organic carbon mass accumulation rates (Corg MARs) in the southeastern Swedish Baltic Sea, two sediment cores were studied in combination with biogeochemical modelling. Prior to 11 300 cal. BP, Corg MARs were extremely low, indicating low organic matter production on the catchment of the Baltic Ice Lake. Following a brief regression, the Ancylus Lake stage occurred between 11 100 and 9800 cal. BP. Corg MARs increased substantially during this period because of enhanced washing in of terrestrial organic matter, when boreal forests were initially established. The prominent marine stage, known as the Littorina transgression between 8500 and 3000 cal. BP, is marked by a minor increase in Corg MARs. Our modelling reveals a changing terrestrial organic carbon input between 100 and 1000 g/m2 per yr that accounts for 30—80% of total organic carbon in sediments of the southeastern Swedish Baltic Sea, with maximum values ( c. 20 × 103g/m2 per yr) occurring during the Bronze/Iron Age transition at about 3000 cal. BP. Corg MAR in the entire Baltic basin is estimated at 3.01 × 106 t/yr during the pre-industrial Holocene, comparable with other large inland water bodies. Regardless of the source of carbon, our data indicate that the Baltic basin is an important sedimentary reservoir for organic carbon storage and thus should be included in accounting for global terrestrial carbon cycling during the pre-industrial Holocene.

Content and isotopic composition of organic carbon within a flood layer in the Po River prodelta (Adriatic Sea)

Thirty-three surface sediment samples from cross-shelf transects on the northern Adriatic shelf were collected in December 2000, soon after a 100-yr flood of the Po River, in order to determine the distribution of organic carbon (OC) along the main sediment dispersal system. To evaluate the temporal variability, stations were re-occupied eight times at seasonal intervals until June 2003. Downcore sediment profiles from two sites characterized by high flood deposit thicknesses were also examined to assess the OC variability within the flood layer. In December 2000, the highest contents of OC (up to 1.24 wt%) were measured in front of the main distributary mouths (Pila, Tolle and Gnocca-Goro) where the greatest thicknesses of the flood deposit were recorded. However, the influence of the Po di Gnocca-Goro sediment supply on the OC surface distribution declined after ∼1.5 years from the fall-2000 river flood, probably because these mouths are less active when the water discharge is lower. The δ 13C of organic matter was used to trace the dispersal of fluvial OC on the continental shelf. The δ 13C values ranged from −25.9‰ to −23.1‰. The fraction of fluvially derived organic particles decreased with increasing water depth according to a radial dispersal pattern around the Po River delta. This pattern persisted in all cruises. δ 13C values increased progressively until April 2002, suggesting an increasing marine contribution to the OC content but decreased again following a second minor flood event in November 2002. The molar C/N ratio was on average 10.0±1.6, with slightly lower values in southern and central areas. Assuming contributions from three OC end-members (terrestrial, riverine and marine), a mixing model based on δ 13C and the ratio of N to C (statistically more robust than C/N; Goñi, M.A., Teixeir, M.J., Perkley, D.W., 2003. Sources and distribution of organic matter in a river-dominated estuary (Winyah Bay, SC, USA). Estuarine, Coastal and Shelf Science 57, 1023–1048) was applied in order to quantitatively assess the OC sources for Po shelf sediments. δ 13C is significantly and positively correlated with the marine OC fraction. The terrestrial fraction is inversely correlated with N/C, while the riverine fraction is positively correlated with N/C. The terrestrial OC source was the most abundant end-member (>70%) showing only little temporal variability regardless of the Po River water discharge. Temporal and spatial changes in OC composition suggest that: (a) the Po River prodelta is always dominated from terrestrial OC input; (b) the Po della Pila supplies most terrestrial OC, whilst other tributaries (e.g., Po di Gnocca-Goro) are secondary sources. However, these mouths are as important as the Po della Pila in affecting the riverine OC signature; (c) offshore, biological primary production raises the marine OC contribution. At two sites on the Po River prodelta, the 2000-flood deposit shows slight but consistent compositional changes of organic matter (N/C and δ 13C) which can help to recognize other flood events in the sedimentary record. The OC budget for the 2000-flood deposit accounts for a terrestrial+riverine OC supply of 68–162 Gg (10 9 g) against an OC deposition of 106–162 Gg (excluding the marine fraction), which implies a rapid and efficient sedimentation of the flood material, and scarce or negligible export out the study area. Flood events may thus enhance terrestrial carbon burial, whereas marine carbon arrives more slowly and may be largely mineralized at the sediment–water interface.

Air‐sea exchange of carbon dioxide in ocean margins: A province‐based synthesis

In determining global sea‐to‐air CO2 flux from measurements or models, the ocean margin has not been resolved from the land or the open ocean. Recent studies have indicated that shelves can be either a large sink or a source for atmospheric CO2. This CO2 sink/source term may substantially alter our current view of the global carbon budget for land and oceans. However, past fieldwork and synthesis have focused on a few shelves in the northern temperate zone while the vast majority of other shelves are ignored. By dividing the highly heterogeneous shelves into seven provinces, we suggest that the continental shelves are a sink for atmospheric CO2 at mid‐high latitudes (−0.33 Pg C a−1) and a source of CO2 at low latitudes (0.11 Pg C a−1). Warm temperature and high terrestrial organic carbon input are most likely responsible for the CO2 release in low latitude shelves.

Terrigenous organic matter in Holocene sediments from the central Baltic Sea, NW Europe

The Baltic Sea is an epicontinental sea surrounded by a catchment in boreal and sub-boreal vegetation zones. The isotopic composition (δ 13C) of organic carbon was in the range of −30‰ to −20‰, but, contrary to our expectations, had no statistically significant relation to lignin oxidation product yields (0.4 to 10 mg g −1 TOC) in a large set of Baltic Sea surface sediments. The isotopic ratio thus may be inappropriate as an indicator for marine versus terrestrial organic carbon input into the Baltic Sea. As in surface sediments, we find no correspondence between lignin biomarker abundance (0.3 to 6 mg g −1 TOC) and δ 13C (−30‰ to −24.5‰) of organic carbon in sediments from a core which brackets the late glacial and the Holocene. The relative contribution of terrestrial carbon, as estimated from lignin oxidation product analyses, increased steadily as soils and forests developed. Transgressive events of lake and marine stages are marked by enhanced input of terrestrial organic carbon. There was no evidence that the terrestrial organic matter was altered significantly after sedimentation because, although (Ad/Al) V and (Ad/Al) S varied between 0.3 and 0.7, there was no trend of increasing values downcore. The amount and the composition of the sedimentary lignin as indicated by S/V (0.3 to 0.9) and C/V (0.2 to 5.6) changed according to the development of the vegetation after deglaciation and the effects of human activity. In the Late Holocene, signals in lignin input and composition thus correlate with the development of the vegetation, with hydrographic changes and with human activities, particularly with deforestation events.

Late Pleistocene-Holocene Paleoclimatic History of the Southern Kerala Basin, Southwest India

The South Kerala Sedimentary Basin (SKSB) constitutes one of the most significant landward extensions of the southwest offshore sedimentary basins of India, and is situated between 8 o45 ' and 10 o15 ' N latitudes. With a maximum width of about 20 km and incorporating a 700 m thick sedimentary succession ranging in age from Early Miocene to Holocene, this belt lies almost entirely under water or alluvium-covered coastal plains. In this study, we use two continuously cored bore holes at Eruva (7.25m deep) and Muthukulam (3m deep) separated by a distance of about 7km to investigate the depositional environment as well as paleo ecology of the depocentre and climatic aspects during the Late Pleistocene and Holocene. Results from C 14 dating of shell fragments from Eruva (depth zone: 2.10–6.64m) yielded ages in the range of 36.2 to 42.5 kyBP corresponding to the late Pleistocene. Wood fragments in the Muthukulam core sample (depth zone: 1.27–3.00m) gave C 14 ages in the range of 3.7 to 7.2 kyBP indicating a Holocene history. The lower half of the Eruva bore hole indicates a marginal marine environment with an abundant supply of terrestrial carbonaceous debris probably corresponding to a period of abnormally high rainfall recorded in many parts of the globe covered by the Asian summer monsoon. The sediments in the upper part of this bore hole indicate a continuation of this environment but with much less input of terrestrial organic carbon. The lower part of the bore hole from this locality, corresponding to the Holocene transgression, is similar to the lower part of Eruva bore hole in the case of TOC. Deposition took place in water bodies with considerable marine influence but receiving high amounts of terrestrial plant debris-mostly in the form of finely divided particles mixed with mud. This transgressive sequence was also deposited during a time when the Asian summer monsoon was abnormally high in intensity as indicated by many examples in India, Africa, Madagascar and elsewhere. It is significant that during the deposition of this part, the sea level was probably the same as present or higher suggesting possible lowering of the stream velocities and resultant deposition of only muddy sediments laden with terrestrial organic material along the coast. The upper part of the section shows a progressively reduced rainfall pattern culminating in a period of very low precipitation with the development of a paleosol, which is traceable all over the SKSB where Late Holocene sediments are available. This period also witnessed aeolian activity modifying the sand ridges in the ridge-runnel systems formed by the Holocene regression.

The importance of terrestrial organic carbon inputs on Kara Sea shelves as revealed by n-alkanes, OC and δ 13C values

Organic carbon (OC) and n-alkanes were investigated in the suspended matter (SM) and surficial sediments of the Arctic Ob and Yenisei Rivers and the adjacent Kara Sea. Organic nitrogen and δ 13C values were determined as additional parameters in the sediments. OC and n-alkane concentrations were highly variable in space. This variability was interpreted as the result of a complex sedimentation pattern involving the discharges of these two rivers and the seasonal occurrence of a shore-ice front along the coast. Using the constant ratio of high molecular weight odd n-alkanes to OC in both rivers, we estimated that more than 70% of the OC preserved in shelf sediments is terrestrially-derived. This result was substantiated by the overall dominance of land-derived n-alkanes. A second approach using δ 13C values and assuming binary dilution of riverine and marine OC led to comparable estimates.