East Siberian Sea Shelf Research Articles

Forecast of Natural Variations in Air Temperature and Sea Ice on the East Siberian Sea Shelf for the Coming Centuries

Forecast of Natural Variations in Air Temperature and Sea Ice on the East Siberian Sea Shelf for the Coming Centuries

Newly Acquired Data on the Geologic Structure and Hydrocarbon Potential in the Eastern Part of the East Siberian Sea Shelf

Abstract —We report results of an integrated interpretation of geophysical data (2D CDP seismic acquisition and gravity and magnetic survey) conducted by Gazpromneft Science and Technology Center in the North Wrangel lease block on the East Siberian Sea shelf. The newly obtained geophysical data have allowed us to update the geological model of the sedimentary cover within the North Wrangel lease block, to clarify the limits of the Drem-Khed Trough, to reconstruct the conditions under which sedimentary complexes formed, and to identify possible promising oil and gas accumulation zones. We were able to determine the position of the main stratigraphic unconformities, to propose several approaches to their interpretation, and to identify the clinoform architecture of the Aptian–Albian and Cenozoic complexes within the Drem-Khed Trough. The integrated review of geological and geophysical data has been helpful in refining the framework of the main structures adjoining the trough and predicting the extent of potential reservoirs and oil and gas source rocks throughout the sedimentary cover. The Upper Cretaceous and Cenozoic complexes are regarded as oil and gas exploration targets.

East Siberian Sea: Interannual heterogeneity of the suspended particulate matter and its biogeochemical signature

The East Siberian Sea (ESS) is the largest, shallowest and most icebound Arctic marginal sea. It receives substantial input of terrigenous material and climate-vulnerable old organic carbon from both coastal erosion and rivers draining the extensive permafrost-covered watersheds. This study focuses on the interannual variability and spatial distribution of suspended particulate matter (SPM) in the surface and bottom waters of the ESS during the ice-free period in 2000, 2003, 2004, 2005 and 2008. We report on the composition and variability of particulate organic carbon (POC), total nitrogen (TN), POC/TN ratios, carbon and nitrogen isotopes (δ13C, δ15N) and provide estimates of the contribution of terrestrial organic carbon (terrOC) based on the δ13C isotopic values.The results show that interannual SPM distribution and elemental-isotopic characteristics of POC differ significantly between the western biogeochemical province (WBP; West of 165oE) and the eastern biogeochemical province (EBP; East of 165oE) of the ESS. The SPM mean concentration in the WBP is almost an order of magnitude higher than in the EBP. From west-to-east of the ESS, SPM tends to become more depleted in δ15N, while the δ13C becomes isotopically heavier. This trend can be explained by a shift in organic matter sources from terrigenous origin (erosion of the coastal ice complex and riverine POC) to becoming dominantly from marine plankton.The maximum contribution of terrOC to POC reached 99% in parts of the WBP, but accounts for as low as 1% in parts of the EBP. At the same time, the type of atmospheric circulation and its associated regime of both water circulation and ice transport control a displacement of the semi-stable biogeochemical border between WBP and EBP to the east or to the west if compared to its long-term average position near 165oE. Our multi-year investigation provides a robust observational basis for better understanding of the transport and fate of terrigenous material upon entering the ESS shelf waters. Our results also provide deeper insights into the interaction in the land-shelf sea system of the largest shelf sea system of the World Ocean, the East Siberian Arctic Shelf system.

Fluorescent signatures of autochthonous dissolved organic matter production in Siberian shelf seas

The East Siberian Sea is an area of high biogeochemical activity caused by multiple factors, such as an influence of river runoff, Atlantic, and Pacific waters, formation and melting of sea ice, and internal circulation. Extensive amounts of carbon are accumulated in sub-sea permafrost within the Arctic shelf in East Siberia. Thawing permafrost and resulting microbial decomposition of frozen carbon pool is a potential feedback process affected global climate dynamics. Several studies of dissolved organic carbon (DOC) distribution in the East Siberian Sea have demonstrated untypical for other Arctic shelf seas pronounced non-conservative DOC behavior. Using seawater samples from the 69th cruise of R/V Akademik Mstislav Keldysh conducted in the Laptev and East Siberian seas in September 2017, this study examines the distribution of fluorescent dissolved organic matter (FDOM) along the shelf-crossing transects including the areas affected by the Khatanga, Lena, Indigirka, and Kolyma river runoff. The set of 137 excitation-emission matrices (EEMs), analyzed with Parallel Factor analysis (PARAFAC), was described by a 4-component model demonstrating Tucker’s congruence coefficient above the 0.95 threshold. Spectral characteristics of the resulting components allowed identifying them as well-known humic-like A and C, protein-like B/T, and marine humic-like M fluorophores. Component C1 (A fluorophore) showed a good correlation with salinity for different river plume influenced regions. As well as chromophoric DOM (CDOM) absorption, it can be used as marker of input of terrestrial DOM to the Arctic Ocean. A distinctive feature of the East Siberian Sea shelf waters in comparison with the Laptev Sea is the higher content of the C2 and C4 components, exhibiting protein-like and marine humic-like fluorescence. Component C3 (C fluorophore) which is usually identified as terrestrial-derived material, was found to be produced locally on the East Siberian Sea shelf and in the continental slope region of the Khatanga transect. Destruction of dead algae cells, production of marine biota and dissolved organic matter (DOM) reworking are considered as possible autochthonous FDOM sources in the Siberian shelf seas.

Formation conditions and evolution of the oil and gas strata of the prospective East Siberian oil and gas province

Introduction. East Siberian Sea shelf refers to the Novosibirsk-Chukotka prospective oil and gas province. The definition of the East Siberian shelf as a prospective oil and gas province and its division into areas is based, along with the structural and geological prerequisites, on the identification of numerous bitumen occurrences in the Paleozoic, Triassic and Jurassic sediments of the Novosibirsk Islands.Aim. To construct spatio-temporal digital models of sedimentary basins and hydrocarbon systems for the main horizons of source rocks, as well as to carry out their detailed analysis based on the available information about the oil and gas content, the gas-chemical composition of sediments, the characteristics of the component composition and evolution of source rocks within the studied prospective oil and gas province. The conducted research made it possible to study regional trends in oil and gas content, features of the sedimentary cover formation and the development of hydrocarbon systems in the area under study.Materials and methods. The materials of production reports obtained for individual large objects in the water area were the source of information. A basin analysis was based on a model developed by the Equinor specialists (Somme et al., 2018) [8, 9], which covered the time period from the Triassic to Paleogene inclusive and took into account the plate-tectonic reconstructions performed by Dor’e et al. in 2015. The resulting model included four main sedimentary complexes: pre-Aptian, Apt-Upper Cretaceous, Paleogene, and Neogene-Quaternary.Results. The calculation of numerical models was carried out in two versions with different types of kerogen of oil and gas source strata corresponding to humic and sapropel organic matter. The key factor in controlling the development of hydrocarbon systems was found to be the sinking rate of the basins and the thickness of the formed overburden complexes.Conclusion. The conducted analysis allowed the most promising research objects to be identified. The main foci of hydrocarbon generation in the Aptian-Late Cretaceous and Paleogene complexes were identified, along with the area of their most probable accumulation. Significant hydrocarbon potential is expected in the Paleogene clinoforms of the Eastern Arctic. This complex is currently underestimated, thus requiring further resource assessment study. A detailed mapping of its interior structure should be carried out.

Exploring Five Methods for Estimating Net Community Production on the Siberian Continental Shelf and Slope of the Arctic Ocean

The loss of sea ice and changes to vertical stratification in the Arctic Ocean are altering the availability of light and nutrients, with significant consequences for net community production (NCP) and carbon export. However, a general lack of quality data, particular during winter months, inhibits our ability to quantify such change. As a result, two parameters necessary for calculating annual NCP, integration depth (Zint) and pre-bloom nitrate concentration (Npre), are often either assigned or estimated from summer measurements. Vertical profiles of temperature, salinity, nitrate, and dissolved oxygen were collected during three cruises conducted between August and October of 2013, 2015, and 2018 in a data-sparse region of the Arctic Ocean along the Siberian continental slope. Estimates of NCP were calculated from these data using five different methods that either assigned constant values for Zint and/or Npre or estimated these parameters from summer observations. The five methods returned similar mean values of Zint (44–54 m), Npre (5.4–5.7 mmol m–3), and NCP (12–16 g C m–2) across the study region; however, there was considerable variability among stations/profiles. It was determined that the NCP calculations were particularly sensitive to Npre. Despite this sensitivity, mean NCP estimates calculated along four transects re-occupied during the three cruises generally agreed across the five methods with two important exceptions. First, methods with pre-assigned Zint and/or Npre underestimated the NCP when the nitracline shoaled in the Laptev Sea and when high-nutrient shelf waters were advected northward from the East Siberian Sea shelf in 2015. In contrast, the methods that directly estimated both Zint and Npre did not suffer from this bias. These results suggest that assignment of Npre and/or Zint provides reasonable estimates of NCP, particularly averaged over larger spatial scales and/or longer time scales, but these approaches are not suitable for evaluating interannual variability in NCP, particularly in dynamic regions. Combining all methods across the three cruise years indicates NCP in the Laptev Sea and Lomonosov Ridge areas (10–11 g C m–2) was slightly lower than that north of Severnaya Zemlya (13 g C m–2) and in the East Siberian Sea (16 g C m–2).

HBIs and Sterols in Surface Sediments Across the East Siberian Sea: Implications for Palaeo Sea‐Ice Reconstructions

AbstractHighly branched isoprenoids (HBIs) in marine sediments have emerged as promising semi‐quantitative proxies to reconstruct seasonal sea ice in polar oceans. In this work, we examine the distribution of sympagic HBIs (IP25 and HBI‐II), pelagic phytoplankton biomarkers (brassicasterol, dinosterol and HBI‐III) as well as terrestrial sterols (campesterol and β‐sitosterol) in the surface sediments of the East Siberian Sea (ESS) to test their reliability as sea‐ice proxies under continental run‐off influence. Our data suggest that dinosterol performs better than brassicasterol to assess sea ice across the ESS shelf, yet the correlation between PDIP25 and spring sea ice is relatively weak but improves when removing sites with salinity <25. Strongest relationship is found between PIIIIP25 and summer sea ice in regions remote from riverine influence. Overall, our results show that semi‐quantitative estimates of sea ice based on biomarkers can be problematic in Arctic Ocean margins because of biases induced by continental run‐off on biological productivity and sea‐ice production.

Gas-geochemical zoning of seafloor formations of sedimentary basins and geostructures of East Siberian Sea and Arctic Ocean outer shelf

Gas-geochemical zoning of seafloor formations of sedimentary basins and geostructures of East Siberian Sea and Arctic Ocean outer shelf

The effect of estuarine system on the meiofauna and nematodes in the East Siberian Sea

Arctic meiofauna and nematodes were examined at 12 stations in the East Siberian Sea, covering a depth range of 13–59 m and an estuarine-shelf system from the Indigirka and Kolyma rivers to the adjacent shelf. Our data reveal the low diversity of the meiofauna at the East Siberian Sea shelf. The meiobenthos abundance was influenced by river run-off and the sea bottom landscape. The samples comprised a total of 28 families and 72 genera, and the number of genera per station ranged from 15 to 32. The Comesomatidae was the dominant family with genus Sabatieria. Among all factors, depth, water temperature and the total organic carbon appeared to be important variables explaining spatial variations in meiofauna and nematodes abundance. Depth and river run-off were defining in controlling the density of nematodes in the study area.

Peculiarities of Tertiary petroleum systems evolution under prograding shelf environment on the continental margin of the East Siberian Sea

The upper part of the sedimentary cover within the East Siberian Sea shelf comprises Cenozoic clinoform deposits, which accumulated in passive continental margin settings. In the Eastern Arctic, the productivity of clinoform deposits has been proved on the Alaska North Slope and in the Beaufort–Mackenzie Basin. Considering that Cenozoic clinoform deposits are widely represented in the Russian part of the Eastern Arctic, they undoubtedly attract considerable interest from the standpoint of hydrocarbons prospecting. However, despite increasingly closer attention to this interval of the sedimentary section, it is still poorly understood due to its complicated geology. The lack of drilled wells in the region imposes a considerable limitation on an understanding of sedimentary basins development. In this situation, geophysical data become the primary source of information for building geologic models in the Russian sector of the Eastern Arctic. An assessment of hypothetical Cenozoic petroleum systems of the East Siberian Sea is the main objective of this paper. It is to be said research performed under high uncertainty of input data. The results obtained from basin analysis and numerical modeling indicate the possibility that an active petroleum system may exist in the Cenozoic sedimentary wedge of the East Siberian Sea. The outlook for the clinoform complex largely depends on the source rock maturity, i.e., higher prospects should be expected in areas where the prograding wedge has maximum thickness. Considering all factors (reservoir quality prediction, proximity to a hydrocarbon kitchen, timing), the Eocene–Oligocene part of the sedimentary section appears to offer the greatest promise within the study area. Here, predominantly oil accumulations may be expected at a depth of 2.5–3.5 km below sea bottom.

Distribution and Transport of Water Masses in the East Siberian Sea and Their Impacts on the Arctic Halocline

AbstractWe investigated the transport of Arctic shelf water and its impacts on the downstream hydrographic structures in the Amerasian Basin by combining data collected in the summer of 2016 from two international cruises in the Pacific sector. The East Siberian Sea (ESS), in which cold shelf water with salinity of 31.0–32.8 is produced, was identified to be a major source of the near‐freezing temperature water for the Makarov Basin. This relatively low‐salinity shelf water was transported into the Chukchi Abyssal Plain and occupied the subsurface layer (30–60 m), which led to the formation of a subsurface oceanic front stretching along the Chukchi Plateau and northern side of the Mendeleev Ridge. The offshore transport of waters from the ESS caused an intrusion of the Atlantic Water onto the shelf and promoted the formation of the diapycnally mixed Lower Halocline Water (D‐LHW) at the bottom of the outer shelf. Our findings indicate that the source of the D‐LHW in the Arctic Pacific sector might extend from the Chukchi Plateau to as far as the Makarov Basin. We hypothesize that advection of waters from the ESS into the Makarov Basin promotes development of the subsurface halocline in the Makarov Basin. The volume transport of surface waters (salinity less than 30) on the ESS shelf was estimated to be about 0.6 Sv (106 m3 s−1) eastward to the Canada Basin in 2016, consistent with the cyclonic Arctic circulation regime in 2016.

Taxonomic and functional patterns of macrobenthic communities on a high Arctic shelf: A case study from the East Siberian Sea

The Siberian Arctic is one of the regions, where the ongoing climate change is the most evident. There is limited knowledge available on the distribution of macrobenthic communities in the Siberian Arctic, their stability and functional structure. We used data from two latitudinal transects from the East Siberian Sea shelf to describe the distribution of macrobenthic communities as well as to identify the main environmental drivers responsible for the observed patterns. Almost all the studied area was under the direct influence of river runoff. Macrobenthic communities were dominated by the bivalve Portlandia arctica and were generally characterised by low species and functional diversity. We suggest that the observed pattern is a direct consequence of considerable river runoff combined with low primary productivity. The overall pattern of macrobenthic distribution corresponds well with the data obtained in the previous decades, suggesting long-term stability of these communities.

Community structure and spatial distribution of phytoplankton in relation to hydrography in the Laptev Sea and the East Siberian Sea (autumn 2008)

The Laptev Sea and the East Siberian Sea are remote areas of the Arctic region where detailed data on phytoplankton composition and spatial distribution remain limited. In the context of the ongoing environmental changes (increasing warming and ice melting) and prospective exploration activities (oil and gas production) on the Arctic shelves, understanding of the seasonal and interannual phytoplankton community dynamics is of critical importance. Our study provides new specifying data on species composition of phytoplankton over the vast area of the Laptev Sea shelf and the East Siberian Sea shelf. We found that the outer shelf of the Laptev and East Siberian seas was characterized by typical late spring diatom species (Chaetoceros furcellatus, Chaetoceros diadema, Chaetoceros debilis, Chaetoceros constrictus). On the inner shelf of the Laptev Sea, which is strongly affected by the Lena River water masses, the phytoplankton were characterized by the transition from the summer to an autumn stage of development. Local algal assemblages were composed by mixo- and heterotrophic dinoflagellates (Dinophysis and Protoperidinium genera) together with marine and brackish water-marine diatoms (Thalassiosira hyperborea, Thalassiosira baltica, Thalassiosira gravida, Thalassiosira nordenskioeldii) accompanied by sporadically occurring freshwater riverine planktonic diatom species (Aulacoseira granulata, Aulacoseira italica, Asterionella formosa). These variations in species composition over the Laptev Sea shelf were attributed to differences in the hydrography, marine chemical conditions, and the sea-ice regime.

Distribution and Relationship between Heterotrophic Organisms and Viruses on the East Siberian Sea Shelf

The distribution of heterotrophic bacteria, viruses, and heterotrophic nanoflagellates was studied in the shelf waters of the East Siberian Sea along the meridional transect from the Kolyma River delta in September. The production of heterotrophic bacteria and their mortality as a result of protozoan grazing and lysis of viruses were also determined. The patterns of the spatial distribution of microorganisms and viruses varied: the number of bacterioplankton decreased with increasing distance from the Kolyma delta, while, on the contrary, its size and biomass increased to the shelf areas with negative water temperatures. The highest abundance and biomass of virioplankton were recorded at the extreme southern and northern stations of the transect and the abundance and biomass of heterotrophic nanoflagellates increased from the south to the north. The studied shelf waters were characterized by a high concentration of fine-textured detrital particles, and, as a consequence, a relatively large number of viruses attached to detritus. The mortality of heterotrophic bacterioplankton accounted for most of its daily production. Protozoan grazing was the main cause of bacterial mortality that significantly exceeded their mortality due to viral lysis.

Subsea permafrost as a potential major source of dissolved organic matter to the East Siberian Arctic Shelf.

Arctic subsea permafrost contains more organic carbon than the terrestrial counterpart (~1400 Pg C vs. ~1000 Pg C) and is undergoing fast degradation (at rates of ~10 to 30cmyr-1 over the past 3 decades) in response to climate warming. Yet the flux of organic carbon sequestered in the sediments of subsea permafrost to overlying water column, which can trigger enormous positive carbon-climate feedbacks, remain unclear. In this study, we examined the dissolved organic matter (DOM) diffusion to bottom seawaters from East Siberian Sea (ESS) sediments, which was estimated at about 943-2240g C m-2yr-1 and 10-55g C m-2yr-1 at the continuous-discontinuous transition zone of subsea permafrost and the remainder shelf and slope sites, respectively. The released DOM is characterized by prevailing dominance (≥ 98%) of low molecular weight (Mn<350Da) fractions. A red-shifted (emission wavelength >500nm) fluorescence fingerprint, a typical feature of sediment/soil DOM, accounts for 4-6% and 7-8% in the fluorescence distributions of seawaters and pore waters, respectively, on ESS shelf. Statistical analysis revealed that seawaters and pore waters possessed similar DOM composition. The estimated total benthic efflux of dissolved organic carbon (DOC) was ~0.7-1.0 Pg C yr-1 when the estimate was scaled up to the entire Arctic shelf underlain with subsea permafrost assuming the width of continuous-discontinuous transition zone is 1 to 10m. This estimation is consistent with the established ~10-30cmyr-1 degradation rates of subsea permafrost by estimating its thaw-out time. Compiled observation data suggested that subsea permafrost might be a major DOM source to the Arctic Ocean, which could release tremendous carbon upon remineralization via its degradation to CO2 and CH4 in the water column.

Early diagenesis and accumulation of redox-sensitive elements in East Siberian Arctic Shelves

The Arctic shelf makes up half of the area within the Arctic Ocean. It plays an important role in the global carbon cycle, whilst its part in the recycling process of redox-sensitive elements (RSEs) is still not clear. This study has selected nine sediment cores, 30 to 40 cm in length, covering a wide area over the East Siberian Arctic Shelves (ESAS) and analyzed a suite of RSEs (S, Mn, Mo, U, V, Cd) in the sediments, along with Al, Fe, total organic carbon (TOC), grain size, and sedimentation rate. The redox environment in surface sediments was clearly divided based on the S and Mn contents of the cores, ranging from being more oxidized on the western East Siberian Sea shelf, to more reduced on the Chukchi Sea shelf. TOC also showed an increasing trend eastward in the study region. Statistically significant correlations were found between Cd and S, V and Fe, Mo and Mn, and U and TOC, implying their close relationships during early diagenetic processes at the sediment-water interface. Across the study area, authigenic accumulation occurred for all RSEs, except Mn. The estimated authigenic accumulation fluxes for Mo, U, V, and Cd over the entire ESAS are 0.21 ± 0.09, 0.086 ± 0.043, 11.0 ± 9.2 and 0.019 ± 0.014 × 108 mol y−1, respectively, which are comparable to values reported in suboxic marginal sediments for Mo, U, and Cd, but exceptionally high for V. Although the sampling resolution was coarse and there are uncertainties in the sedimentation rates and detrital background values of V, we propose the authigenic accumulation flux for V on the broad and shallow ESAS may be a significant but missing sink term for the global V cycle.

Heterotrophic Bacterioplankton of the Laptev and East Siberian Sea Shelf Affected by Freshwater Inflow Areas

The present study examined the hydrochemical and biological parameters influencing the spatial distribution of bacterioplankton in the Laptev Sea and the East Siberian Sea shelf regions affected by the Khatanga, Indigirka and Kolyma rivers runoff during September 2017. The highest bacterial abundance and the amount of actively respiring bacteria were determined in rivers delta and in surface water along the edge of continental shelf. Despite the expectations based on previous studies salinity was not the determining factor affecting spatial distribution and activity of bacterioplankton. Bacterial abundance and biomass were significantly and positively correlated with chlorophyll “a” and pheophytin content.

Role of plankton in the vertical flux in the East Siberian sea shelf

Role of plankton in the vertical flux in the East Siberian Sea was studied in the 69 cruise of the RV Akademik Mstislav Keldysh in September 2017. Vertical fluxes were measured in sediment traps samples collected in the area of Indigirka river plume and in the marine shelf area. Mass vertical flux and particulate organic carbon flux varied from 80 to 530 mg/м2/d and from 16 to 49 mgС/м2/d, accordingly. Phytoplankton in sediment traps was dominated by cysts and spores of diatoms and dinoflagellates. Phytoplankton flux increased with depths from 0.220.33 to 1.21.3 мgС/м2/d. Fecal pellet fluxes (712 mgС/м2/d) was almost similar at two studied stations and did not change with depth. Zooplankton in the traps was dominated by houses of larvacean and carcasses of copepods Jashnovia tolli and Calanus glacialis Flux of zooplankton varied from 3 to 17 mgС/m2/d. The influence of the continental runoff reflected in a decrease of the proportion of planktonogenic components in the vertical flux of organic carbon. In the river plume area their total contribution to organic carbon flux did not exceed 30%; on the marine shelf it reached 80%.

The Role of Plankton in the Vertical Flux in the East Siberian Sea Shelf

The Role of Plankton in the Vertical Flux in the East Siberian Sea Shelf

Carbon mineralization in Laptev and East Siberian sea shelf and slope sediment

Abstract. The Siberian Arctic Sea shelf and slope is a key region for the degradation of terrestrial organic material transported from the organic-carbon-rich permafrost regions of Siberia. We report on sediment carbon mineralization rates based on O2 microelectrode profiling; intact sediment core incubations; 35S-sulfate tracer experiments; pore-water dissolved inorganic carbon (DIC); δ13CDIC; and iron, manganese, and ammonium concentrations from 20 shelf and slope stations. This data set provides a spatial overview of sediment carbon mineralization rates and pathways over large parts of the outer Laptev and East Siberian Arctic shelf and slope and allows us to assess degradation rates and efficiency of carbon burial in these sediments. Rates of oxygen uptake and iron and manganese reduction were comparable to temperate shelf and slope environments, but bacterial sulfate reduction rates were comparatively low. In the topmost 50 cm of sediment, aerobic carbon mineralization dominated degradation and comprised on average 84 % of the depth-integrated carbon mineralization. Oxygen uptake rates and anaerobic carbon mineralization rates were higher in the eastern East Siberian Sea shelf compared to the Laptev Sea shelf. DIC ∕ NH4+ ratios in pore waters and the stable carbon isotope composition of remineralized DIC indicated that the degraded organic matter on the Siberian shelf and slope was a mixture of marine and terrestrial organic matter. Based on dual end-member calculations, the terrestrial organic carbon contribution varied between 32 and 36 %, with a higher contribution in the Laptev Sea than in the East Siberian Sea. Extrapolation of the measured degradation rates using isotope end-member apportionment over the outer shelf of the Laptev and East Siberian seas suggests that about 16 Tg C yr−1 is respired in the outer shelf seafloor sediment. Of the organic matter buried below the oxygen penetration depth, between 0.6 and 1.3 Tg C yr−1 is degraded by anaerobic processes, with a terrestrial organic carbon contribution ranging between 0.3 and 0.5 Tg yr−1.