Distribution Of Total Organic Carbon Research Articles

A petrophysically driven seismic inversion method for the total organic carbon content of source rocks

Organic carbon content is the main index for evaluating organic matter abundance of source rocks, and it is still difficult to quantitatively predict total organic carbon (TOC) in source rocks based on seismic data. The study of seismic fluid identification driven by petrophysics can help to understand the fluid characteristics and distribution patterns of subsurface oil and gas reservoirs. First, this paper clarifies the sweet spot parameters (parameters characterizing hydrocarbon enrichment) and sensitive elastic parameters (a parameter characterizing the nature of an ideal elastic body) of source rocks through theoretical petrophysical modeling. Next, the paper establishes the relationship between sensitive elastic parameters and sweet spot parameters TOC and constructs a statistical petrophysical model that can characterize the relationship between the two on this basis. And then we construct the joint distribution of TOC and elastic impedance through the Bayesian theoretical framework to obtain the maximum posterior probability estimate as the final TOC inversion results of source rocks. Our method successfully predicts the spreading of high-quality source rocks in the Wen4 Section of Lufeng 13 Subsag, and the inversion results are within an uncertainty range of ±14 m for well data, which proves the reliability of the method. The prediction results show that the organic matter abundance of source rocks in the Wen4 Section is high, and the organic carbon content is generally higher than 2%, which provides a reliable basis for the further implementation of the resource scale of the depression and the clarification of the hydrocarbon-rich area, which provides technical support for the evaluation of the source rocks of the new depression in the new area.

Re-evaluation of the genetic mechanism of overpressure: Western slope zone of the Xihu Depression, East China Sea Shelf Basin

The accurate prediction of pore pressure relies on a comprehensive understanding of the genetic mechanism of overpressure and its impact on rock's physical properties. The western slope zone of the Xihu Depression in the East China Sea Shelf Basin serves as a valuable natural experimental site for re-evaluating the mechanism of overpressure formation due to significant differences in scholars' understanding of this phenomenon. This study uses a combination of multilogging analysis, the Bowers method, and density-velocity crossplotting to demonstrate that overpressure in the area is not attributed to disequilibrium compaction, combined with the geological background of overpressure generation. By considering the types of clay minerals, their diagenetic transformation, temperature, and current overpressure distribution, the main genetic mechanisms of overpressure are determined to be chemical compaction and fluid expansion. Fluid expansion is primarily associated with hydrocarbon generation, as evidenced by the correspondence between the overpressure top interface and chloroform asphalt “A,” hydrocarbon generation potential and TOC (total organic carbon) distribution. Furthermore, the initial scale generation of hydrocarbons is consistent with the current overpressure top interface. A mathematical model is developed to quantify the proportion of overpressure by considering the identified genetic mechanism and the vertical variation characteristics of mudrock porosity resulting from original components, mechanical compaction, and chemical compaction. The proposed “one point-two lines” model characterizes the contributions of fluid expansion and chemical compaction to overpressure formation during the intensive chemical compaction stage. The overpressure generated by hydrocarbon generation through fluid expansion likely induces formation fractures, leading to the episodic expulsion of oil and gas into reservoirs. This charging pattern of overpressure exhibits fast speed and low migration loss, which holds significant implications for forming large and medium-sized oil and gas fields with high enrichment.

Source Rock Evaluation from Rock to Seismic Data: An Integrated Machine-Learning-Based Work Flow and Application in the Brazilian Presalt (Santos Basin)

The capacity to predict the occurrence and quality of source rocks in a sedimentary basin is of great economic importance in the evaluation of conventional and non-conventional petroleum resources. Direct laboratory examinations of rock samples are the most accurate way to obtain their geochemical properties. However, rock information is usually sparse, and source rocks are often sampled at positions that may not be representative of the average organic content and quality of oil kitchens. This work proposes a work flow supported by machine learning methods (random forest, DBSCAN, and NGBoost) to automate the source rock characterization process to maximize the use of available data, expand data information, and reduce data analysis time. From the automated quality control of the input data through the extrapolation of laboratory measurements to continuous well logs of geochemical properties, culminating in the 3D estimation of these properties, we generate volumes of total organic carbon (TOC) by applying machine learning techniques. The proposed method provides more accurate predictions, reducing uncertainties in the characterization of source rocks and assisting in exploratory decision making. This methodology was applied in the presalt source rocks from Santos Basin (Brazil) and allowed us to quantify the TOC distribution, improving the interpretation of the main source rock interval top and base based only on seismic amplitude data. The result suggests higher TOC values in the northern and western grabens of the studied area and a higher charge risk in the eastern area.

Vertical distribution and pollution assessment of TN, TP, and TOC in the sediment cores of cage farming areas in Dongshan Bay of southeast China

Cage farming is an important means of aquacultural production, while its potential environmental pollution needs to be further investigated. In this study, Dongshan Bay was taken as an example to investigate whether long-term cage farming in a semi-closed bay would cause environmental pollution via vertical distribution assessment. The four sediment cores (YB1, YB2, B1, and B2) were collected from two cage farming areas. Total nitrogen (TN), total organic carbon (TOC), and total phosphorus (TP) were measured. The results showed a negative correlation between TN and TOC and the sediment depth at four sampling sites, and TP was also negatively correlated with the depth at three sites. The average TN (1,405.8, 1,413.8, 1,115.7, and 936.1 mgkg−1) and TP (1,206, 1,141.6, 1,064.6, and 932.8 mgkg−1) values of the four sites were markedly higher than the safety level, with the indexes STN, STP, FF, and ON of the four sites indicating moderate to severe pollution of nitrogen and phosphorus. Particularly, the YB1 and YB2 sites in the area with lower current speed have more severe pollution. The C/N ratio uncovered that the organic matter (OM) might mainly derived from the phytoplankton and nonfibrous plants as a result of excessive fish feed and feces. Collectively, the results indicated that long-term (more than 10 years) cage farming activity in a semi-closed bay such as Dongshan Bay had a negative impact on the environmental quality. Despite limitations in sample size and the absence of stable isotopic analyses, this study enhances our understanding of environmental changes and endogenous pollution risks in shallow marine aquaculture areas. Moreover, it suggests practical approaches such as implementing alternative farming and fallowing periods, should be conducted to mitigate the pollution.

Pacific Water impacts the burial of black and total organic carbon on the Chukchi Sea shelf, Arctic Ocean

The Arctic shelf is an important carbon reservoir, especially the Chukchi Sea, which plays a vital role in the biogeochemical processes of polar carbon cycles. In this study, we measured the contents of black carbon (BC) and total organic carbon (TOC) in a sediment core of the Chukchi Sea shelf, to evaluate the controlling factors of distribution and burial of BC and TOC. Our results show that the distribution of TOC and BC was strongly influenced by the Pacific Water (PW) inflow, resulting in nearly synchronous change since the early Holocene. The early Holocene (∼9.9 to 8.0 cal kyr BP) was characterized by the minimum of TOC, total nitrogen (TN), and BC, and the most depleted δ13CTOC and δ13CBC values throughout the Holocene, which indicate low primary productivity and low BC supplies under the weak PW flux. The carbon (TOC and BC) burial on the Chukchi Sea shelf was significantly enhanced during the middle Holocene (8.0–4.0 cal kyr BP) due to the rapid increase in PW flux. Since 4.0 cal kyr BP, PW inflow is stable and active, which enables sea ice extent to modulate the stratigraphic variations of BC. In particular, an increase in summer Arctic sea ice during this period may enhance ice-rafted debris and terrestrial carbon supply by promoting erosion of coastal peat soils and permafrost. Our results suggest that PW inflow is the first-order factor controlling Arctic carbon burial during the Holocene, while insolation-induced sea ice increase started playing a role when the PW inflow is active and stable during the late Holocene.

Content and Spatial Distribution of Carbon in Shuangtaizi Estuary, China

Contents of total organic carbon (TOC), total inorganic carbon (TIC) and total carbon (TC) in water and sediment in Shuangtaizi estuary in China were studied. The results showed that the TIC and TC contents in water were high, and the TOC content was very low. In sediment, the TOC and TC contents were high, while the TIC was very low. TOC, TIC and TC had obvious spatial distribution characteristics in water and sediment in the area. The distribution of TOC and TC in sediment was similar, which was increase along with the distance to the mouth. The distribution of TIC and TC in water was similar. The TIC was mainly distributed at the outer mouth in surface water, while at the inner in deep water was little. The terrigenous input was the main source of carbon in the area. Carbon sequestration in the long range would convert the inorganic carbon to organic carbon.

Seismic prediction of shale reservoir quality parameters: A case study of the Longmaxi–Wufeng formation in the WY area

Shale is a crucial natural gas resource, attracting global exploration and development interest. China has abundant shale gas resources that will drive future oil and gas exploration advances by increasing reserves and production. The WY shale gas field is the most productive and has the greatest potential for exploration and development. This study analyzed high-quality shale logging response characteristics and drilling logging, seismic, and analytical test data in the WY area to establish a rock physical model of seismic attribute parameters and shale reservoir quality parameters. Seismic elastic parameters were converted into indicators that directly reflect shale reservoir quality, such as total organic carbon (TOC), high-quality reservoir thickness, porosity, brittleness index, and crack development strength. Corresponding regression equations were established to predict quality parameters.The results showed that shale reservoir quality parameters have a good correlation with seismic parameters. The TOC distribution ranged from 2% to 5% in the study area and was generally high in the north but low in the south. The high-quality shale reserve had a thickness of over 40 meters, and except for the northwest region, the porosity was nearly over 4%. The overall brittleness of the study area was favorable, and the brittleness index was over 35%, which is suitable for network fractures formation in subsequent fracturing operations. The anisotropy of shale in S1l1I was small, and the overall fractures were underdeveloped in the study area. Drilling verifications showed that the prediction results of the quality parameters of high-quality shale reservoirs were consistent with actual drilling test results with high reliability. This study provides guidance for comprehensive prediction of sweet spots and subsequent fracturing and well location deployment.In summary, this study provides valuable insights into shale gas exploration and development in the WY area by establishing a rock physical model, predicting quality parameters, and offering guidance for fracturing and well location deployment.

Spatial Distribution and Sources of Organic Matter in the Surface Sediments of Fuxian Lake, SW China

Sedimentary organic matter is an important component of the metabolism of a lake’s ecosystem, and it is generally derived from both the watershed and the primary productivity of a lake. Understanding the sources of organic matter in lakes and lake trophic status is important when evaluating the quality of lake ecosystems. We summarize the spatial distribution of total nitrogen (TN), total organic carbon (TOC), TOC/TN (C/N) molar ratios, and organic carbon isotope (δ13Corg) of the surface sediments of Fuxian Lake, Yunnan–Guizhou Plateau, Southwest China, which is the second deepest freshwater oligotrophic lake in China. The results show that the distributions of TN, TOC, C/N, and δ13Corg of the surface sediments are spatially heterogeneous, which is also the case for the trophic conditions of the lake. Compared with the adjacent eutrophic lakes and typical lakes in other areas with strong human activities, the content of organic matter is at a low level. Meanwhile, the autochthonous organic carbon in the surface sediments was characterized by lower δ13Corg (−25.3~−28.5) and C/N (8.7~12.9), suggesting that the biological carbon pump effect plays a significant part in the stability of carbon sinks by coupling with carbonate weathering. Our results emphasize the importance of the carbon sink of coupled carbonate weathering and aquatic photosynthesis in the evolution of the carbon cycle in lakes. Although modern monitoring shows that Fuxian Lake is an oligotrophic lake, there are potential risks of organic nitrogen pollution with respect to surface sediments, especially in northern and southern shallow-water areas. The organic pollution of lakes can be reduced by controlling the discharge of wastewater and reducing the nutrient loading of agricultural runoff.

Bioturbation in sediment cores from the Clarion-Clipperton Zone in the northeast Pacific: Evidence from excess 210Pb

Radioactivity levels of 210Pb and 226Ra were measured to evaluate the bioturbation coefficients and mixing depths in one sediment core collected from the polymetallic nodule area inside the Clarion-Clipperton Zone (CCZ), a contract area of the China Ocean Mineral Resources Association (COMR) in the eastern Pacific Ocean. With a steady-state diffusion mode, the bioturbation coefficient and mixing depth of the sediment core were estimated to be 81.5 cm2/a and 51.3 cm, respectively, higher than that of the sediment core from the previous results from abyssal or global areas, which may be attributed to the distribution of total organic carbon (TOC) abundance. In addition, the transport flux of TOC from the surface layer to the deep layer were evaluated to be 1.15 mmol/(cm2·a), resulting in 83,200 tons of carbon buried annually into the sediment in the CCZ, indicating that a potential carbon sink will be disturbed with future exploration.

Organic Compounds and Antibiotic-Resistant Bacteria Behavior in Greywater Treated by a Constructed Wetland.

Laundry greywater is considered as an alternative source of non-potable water, as it is discharged in approximately 70% of homes. Because this water contains compounds such as biodegradable and recalcitrant organic matter, surfactants, and microbiological compounds, it must be treated prior to reuse. Therefore, the objective of this study was to assess the behavior of organic matter and antibiotic-resistant bacteria (ARB) in greywater treated by a constructed wetland (CW). The results show that the organic matter removal efficiencies were 67.19%, 50.15%, and 63.57% for biological oxygen demand (BOD5), chemical oxygen demand (COD) and total organic carbon (TOC), respectively; these efficiencies were not significant (p > 0.05). In addition, the CW allows the distribution of TOC and ionic compounds in the fractions below 1000 Da to increase by 5.03% and 13.05%, respectively. Meanwhile, the treatment of microbiological compounds generated non-significant removals (p > 0.05), along with increases in bacteria resistant to the antibiotics ciprofloxacin (CIP) and ceftriaxone (CTX) of 36.34%, and 40.79%, respectively. In addition, a strong association between ARB to CIP, CTX, cationic and non-ionic surfactants was determined, indicating the role of surfactants in ARB selection. It is suggested that disinfection systems should be employed prior to the reuse of the treated water.

Soil dissolved organic carbon leaching characteristics of the coastal saline seepweed wetlands in the Wuleidaowan national wetland

As the most active component of soil TOC (Total Organic Carbon), DOC (Dissolved Organic Carbon) is either adsorbed by the soil and used by plants or transported by soil water into the water column, where it participates in the broader carbon cycle. As a substantial "mobile carbon pool" in soils, its leaching process is an essential pathway for the loss of soil TOC. This paper analysed the investigated the leaching characteristics of soil DOC by simulating leaching in an indoor soil column based on the spatial distribution of soil TOC and DOC contents in order to reveal the leaching and migration characteristics of soil DOC at different depths and its main influencing factors. The DOC content of soil leachate varies somewhat under conditions such as PH and soil phosphorous content. However, the overall trend is that the DOC content of soil leachate generally increases with the depth of the soil layer. The DOC content in the soil leachate gradually decreased with increasing leaching time. In the absence of exogenous carbon recharge, the loss of TOC from the upper layers of the soil was significantly more remarkable due to the prolonged leaching. In addition, soil TOC loss declined after leaching and decreased significantly with increasing soil depth. Because the soil DOC is adsorbed and trapped in other layers during leaching, the DOC in the upper layer was always lost during the leaching test. The experiments used highly concentrated DOC solutions as drench solutions. According to the analysis of the intensity of the fluorescence centres of each soil layer in the three-dimensional fluorescence pattern of the drench solution of each soil layer, the ability of different soil layers to adsorb and retain organic matter of different fractions differs. In addition, the selected soil layers planted with Saline seepweed show a general decreasing trend in the ability to adsorb and retain organic matter as the depth increases.

Developing ecological risk assessment of metals released from sediment based on sediment quality guidelines linking with the properties: A case study for Kaohsiung Harbor

This study aimed to introduce sediment properties (total organic carbon (TOC), acid-volatile sulfides (AVS), particle size distribution) into sediment quality guideline-based risk quotients to assess the potential toxicity of metals (Ni, Cu, Zn, Cr, Cd, and Pb) released from sediments. Sediment was collected at three times points in 20 sampling sites in Kaohsiung Harbor. The Microtox® toxicity test was used to assess the sediment toxicity and the relationship between sediment toxicity and risk quotient estimated based on the metal concentration was constructed. To improve the toxicity prediction and modify the risk quotient according to the sediment properties, stepwise multiple linear regression (MLR) models that have been tested over wide ranges of TOC, AVS, and particle size distribution to determine the key sediment properties. Common multimetal indices, including the pollution load index, modified degree of contamination index, Nemerow pollution index, potential ecological risk index, and total toxic risk index, were compared with sediment toxicity to evaluate the degrees of correlation. By modifying the relationship between metal toxicity and the risk quotient by including TOC and AVS, the prediction showed that sediments in Kaohsiung Harbor were generally of slight acute toxicity to acute toxicity to organisms, with sampling sites near an industrial zone showing a higher probability of high acute toxicity. In particular, the acute risk of adverse effects on aquatic organisms from sediments in the Salt River estuary was significantly higher than that at other sites, which was consistent with the results of assessment based on the multimetal indices. This study suggests that the MLR-based approach may facilitate the adoption of updated site-specific metals standards that more accurately account for the parameters affecting metal bioavailability than metal concentration standard alone.

Change of tillage system affects the soil carbon pools characters, reduces carbon emissions and improves maize yield in the Loess Plateau

Understanding the response of soil aggregates and soil carbon pools to different tillage practices is an important way to illustrate the role of conservation tillage in climate change mitigation. Therefore, a long-term tillage experiment was conducted from 2007 to 2020 with four tillage practices: 1) The rotational tillage practice consisting of no-tillage in the first year, conventional tillage in the second year, subsoiling in the third year and a cycle of three years (NCS); 2) no-tillage (NT); 3) subsoiling (ST); and 4) conventional tillage (CT) to explore the relationship among soil aggregates, soil carbon pools, soil carbon sequestration and emissions. After long-term experiment, NCS and NT improved the soil total organic carbon (TOC) stock by 14.4% and 9.0% compared with CT in the 0–40 cm soil depth. Meanwhile, NCS, NT, ST decreased carbon emissions by 18.6∼36.6% during the observation period, when compared with CT. Partial least squares path model revealed that soil aggregates structures and soil carbon pools stability were the main causes of soil carbon sequestration and emissions affected by conservation tillage. NCS, NT and ST increased soil aggregates proportion, mean weight diameter, geometric mean diameter and aggregates associated carbon compared with CT. Meanwhile, NCS and NT improved the carbon pools management index (CPMI) by 11.1% and 16.5%, respectively. The increased stability of soil aggregates and the improvement of soil carbon pools reduced soil carbon emissions and increased soil carbon sequestration. However, NT and ST significantly increased the TOC stratification ratios (SR) by 38.8% and 30.1% in the 0–10 and 10–20 cm soil depth compared with NCS, which was not conducive to the vertical distribution of TOC. In additional, NCS, NT and ST increased yield by 16.7∼22.6% compared with CT. Collectively, based on the combined effects of environmental protection, production, and soil physicochemical property improvement, we recommend NCS (with highest comprehensive index: 6.76) as a sustainable tillage practice that can improve yield with ecological effects.

Organic carbon dynamics in the continental shelf waters of the eastern Arabian Sea.

The seasonal and spatial distribution of total organic carbon (TOC) is presented for the coastal waters of the eastern Arabian Sea, which experiences seasonal suboxia during the late southwest monsoon (SWM). This study reveals that high TOC was observed off Kochi as compared to Goa and Mangalore transects, and may be attributed to stronger upwelling along the Kerala coast. This is also supported by the excess carbon due to upwelling during the late SWM that varied from 37 μM(Goa), 39 μM(Mangalore), to 51μM (Kochi). Our seasonal data from 2014 to 2020 at the Goa transect indicates that high TOC is seen during late SWM to fall inter monsoon (FIM) and between the late northeast monsoon (NEM) to the early spring inter monsoon (SIM). The high TOC concentrations and C/N ratios observed during the FIM are a combination of high primary production, the buildup of remnant organic matter from the previous season (due to prevailing low oxygen conditions), accumulation of refractory organic carbon, and release from diatoms (especially Chaetoceros sp.). Inter-annual variations indicate that phytoplankton blooms resulted in higher TOC concentrations, especially during the year 2020. Based on a comparison with an Elnino-Southern Oscillation (ENSO) year (2015), we can infer that the partitioning of carbon may increase from particulate to dissolved phase in future warming scenarios.

Spatial–temporal distribution of total organic carbon and its transportation in the Jiulong River Estuary

Spatial and temporal distributions of total organic carbon (TOC) in the Jiulong River Estuary (JRE) were determined using data collected during three cruises in summer 2010, autumn 2010, and spring 2011. The TOC concentration influencing factors were identified, and the export fluxes were calculated. TOC concentration ranges were 0.73–4.17 mg/L in summer, 0.90–5.32 mg/L in autumn, and 1.78–8.03 mg/L in spring, respectively. TOC concentrations of the surface water and nearshore area were higher than those of the bottom water and offshore area, respectively, and the maximum TOC content occurred in the JRE upper reaches. The TOC concentration decreased with increasing salinity and exhibited a significant positive correlation with petroleum and dissolved inorganic nitrogen (DIN), indicating the influence of terrestrial input. A weak relationship between TOC and chlorophyll-a indicated that phytoplankton was not the dominant source of TOC. TOC fluxes discharged into the JRE were 50.39 × 103 t/a in 2010 and 46.08 × 103 t/a in 2011, and those transported into the sea were 38.46 × 103 t/a in 2010 and 33.66 × 103 t/a in 2011, respectively, accounting for approximately 75% of the total estuary fluxes. This study elucidates the biogeochemical processes of estuarine organic carbon and provides a quantitative basis for the land–sea integration of carbon dioxide emission reduction and sink increase projects.

Paleogeographic and sedimentary evolution of Meso–Neoproterozoic strata in the Ordos Basin, western North China Craton

Exploration for deep and ultra–deep hydrocarbon resources plays a significant role in meeting the world's increasing energy demand. Meso–Neoproterozoic strata are considered as a possible target interval for hydrocarbon exploration in deep formations. In this regard, bitumen and potential source rocks have been identified in the Meso–Neoproterozoic strata of the Ordos Basin, western North China Craton, yet the distribution and paleogeographic evolution of these strata remain unclear. In this study, recent drilling data were combined with seismic data and fieldwork to examine the paleo environment of these Meso–Neoproterozoic strata. The results indicate that the Meso–Neoproterozoic strata are mostly distributed in the southwestern part of the basin and grow thinner from the southwest to the northeast. These sediments developed in an intracontinental rift setting with four NE–trending rift troughs having formed during the Changcheng Period. Sedimentary facies during this period were mainly fluvial–deltaic, littoral–neritic marine (quartzitic), and transitional facies. In the Jixian Period, the basin was mainly a marginal depression with a narrow sedimentation range in which carbonate tidal flats and dolomites with stromatolites and siliceous interlayers developed. During the Sinian Period, the basin was a marginal depression characterized by deposition of glacial moraine conglomerate along the southwestern margin. The Changcheng System neritic facies developed in the rift margin and northern margin of the basin controlling the distribution of total organic carbon of argillaceous source rock. The determination of the distribution, characteristics, and paleogeographic evolution of Meso–Neoproterozoic sediments is conductive to the prediction of source rock distribution, and is important for hydrocarbon exploration in the Ordos Basin.

Characterization of marine shale in Western Hubei Province based on unmanned aerial vehicle oblique photographic data

The marine shale in the Sinian Doushantuo Formation of Qinglinkou outcrop section is well developed, but the current characterization methods for outcrops are unsatisfactory. In this paper, the data of outcrop in the field study area were collected by Unmanned Aerial Vehicle, then processed and interpreted by oblique photography technology combined with manual investigation. Subsequently, we established a quantitative geological knowledge database of the shale formations and carried out the typical section of anatomy analysis. The results showed that the high-precision image information captured by unmanned aerial vehicle oblique photography technology can be well coupled with a three-dimensional coordinate system. The three-dimensional digital model was used to characterize the lithologic assemblage, thickness and distribution characteristics of the target reservoir. Based on this digital model, we established the three-dimensional lithology and the total organic carbon models of the outcrop area. The spatial distribution characteristics of interbedding between marine dolomite and shale in the outcrop area were displayed, and the distribution of total organic carbon was revealed under lithological constraints. The models are beneficial for the analysis and prediction of the lithology and total organic carbon, which is of great significance to the understanding of shale gas sweet spots. Cited as: Yin, S., Feng, K., Nie, X., Chen, Q., Liu, Y., Wang, P. Characterization of marine shale in Western Hubei Province based on unmanned aerial vehicle oblique photographic data. Advances in Geo-Energy Research, 2022, 6(3): 252-263. https://doi.org/10.46690/ager.2022.03.08

A century of human-induced environmental changes and the combined roles of nutrients and land use in Lake Victoria catchment on eutrophication

Lake Victoria, a lifeline for millions of people in East Africa, is affected by anthropogenic activities resulting in eutrophication and impacting the aquatic life and water quality. Therefore, understanding the ongoing changes in the catchment is critical for its restoration. In this context, catchment and lake sediments are important archives in tracing nutrient inputs and their dominant sources to establish causality with human activities and productivity shifts. In this study, we determine the 1) changes in concentrations of total organic carbon (TOC), black carbon (BC), total nitrogen (TN), C/N ratio, and phosphorous (P) fractions in catchment sediments and the open lake, 2) distribution of diatom population in the lake, and 3) land use and land cover changes in the catchment. The distribution of TOC, BC, TN, C/N, and P correlate while showing spatial and temporal variations. In particular, the steady increase in BC confirms atmospheric inputs from anthropogenic activities in the catchment. However, lake sediments show more variations than catchment-derived sediments in geochemical trends. Notably, the catchment has undergone dramatic land use changes since the 1960s (post-independence). This change is most evident in satellite records from 1985 to 2014, which indicate accelerated human activities. For example, urban growth (666–1022%) and agricultural expansion (23–48%) increased sharply at the expense of a decline in forest cover, grassland, and woodlands in the catchment. Cities like Kisumu and Homa Bay expanded, coinciding with rapid population growth and urbanization. Consequently, nutrient inputs have increased since the 1960s, and this change corresponds with the divergence of diatom communities in the lake. In addition, the transition to Nitzschia and cyanobacteria mark increasing cultural eutrophication in the lake. The geochemical trends and statistical data support our inference(s) and provide insights into urban development and agriculture practices, which propelled increased nutrients from the catchment and productivity shifts in the lake.

Millennium timescale carbon stability in an Andisol: How persistent are organo-metal complexes?

Significant linear correlation between organic carbon (OC) and reactive aluminum and iron (metal) are often observed across a wide range of acidic soils including Andisols, and often explained by the formation of stable organo-metal complexes. However, the chemical variation and the long-term stability of the complexes remain unclear. We thus compared two buried A horizons (the burial period of 0.5 and 4.6 kyr, based on tephra chronology) to examine the changes in OC chemistry as well as its association with the metal phases extractable by sodium pyrophosphate (PP) and acid oxalate (OX). Both horizons were subdivided into six layers, and then the layer with the highest OC in each horizon was subjected to particle-size fractionation after maximum dispersion. 14C ages of the bulk and the size fractions were consistent with the tephra-based burial ages. Younger 14C age was shown for the organo-mineral fractions of finer sizes in both horizons, in accord with our previous study using a surface Andisol. PP-extractable metal concentration (metalPP) showed a highly significant linear correlation with OC across all the layers and the fractions from both horizons, whereas OX-extractable metal showed the correlation only within the younger buried horizon. Solid-state 13C NMR spectroscopy showed a stronger metalPP correlation with alkyl C than with aromatic C especially among the older horizon layers and the 14C age variation among the size fractions in this horizon was not related to aromaticity. Significant decline in the proportion of O-alkyl C and concurrent increase in that of aromatic C from the younger to older horizon may suggest preferential degradation of O-alkyl C under the buried condition. Over the burial time, however, the correlation between metalPP and aromatic C weakened whereas metalPP-alkyl C correlation increased. These results imply a shift in the dominant organic ligand in the organo-metal complexes. Furthermore, the shift in the distribution of total OC and metalPP from 0.2–2 μm fraction to <0.2 μm fraction suggested a physical weakening of microaggregates where organo-metal complexes likely acted as glue. The observed changes imply reduced OC stability, which may lead to the eventual disappearance of buried A horizons. The current study demonstrated that, while organo-metal complexes appeared persistent based on the OC-metalPP correlation, the changes in the nature of OC-metal association including the partial breakdown of microaggregates took place, in the millennium timescale.

Temporal and spatial differences between predicted and measured organic carbon in South Atlantic sediments: Constraints to organic facies modelling

The evaluation of total organic carbon (TOC) content from wells is a routine practice for assessing source rock potential, and empirical relationships to estimate TOC content have been proposed and used in organic facies modelling. However, no critical evaluations address how well the available TOC-predictive equations estimate the TOC content in recent and past sediments of the South Atlantic. In order to investigate the performance of the available TOC-predictive equations in this study, we compiled a large South Atlantic dataset composed of new and published modern TOC, dry bulk density, and sedimentation rate with the associated satellite-derived net primary productivity. We repeat this exercise for another time interval using the same input parameters but estimating net primary productivity using transfer functions. Our results indicate that all equations overestimate the modern TOC distribution, mainly at the continental margins and at the edges of the subtropical gyre. However, we must point out that this is partly due to our primary productivity values used as inputs, which exhibit higher values than the ones used in the calibration datasets of the TOC-predictive equations. In addition, the equations perform better, in terms of correlation, in the eastern compared to the western South Atlantic. We observe that in the west, the equation of Müller and Suess (1979) exhibited the highest correlation between predicted and measured TOC values, while in the east, the “Definition” equation using the carbon flux equation of Antia et al. (2001) and the burial efficiency equation of Betts and Holland (1991) showed the best results. In addition, we indicate that lateral transport of organic carbon at the margins might be influencing the correlation negatively. Overestimations the TOC content were also apparent for the downcore data, although with reduced magnitude, and correlation between predicted and measured TOC values was lower compared to the modern dataset. This can be explained by more sparse data in the glacial dataset and a strong lateral transport during this interval of low sea level. The reasons behind the different performances of equations are explained by the way they incorporate the input parameters into their formulations, which is especially relevant for primary productivity, sedimentation rates, and the inclusion of processes such as the carbon flux. We highlight that carbon flux equations should also be used with caution in very shallow and high productive systems, since they can produce unrealistic results. Our compilation indicates that the use of a single equation is not adequate to resolve the broad spatial TOC distribution in marine environments and that organic facies modelling software should consider regional aspects in order to produce adequate quantitative estimations of TOC content. Additionally, we used our dataset to investigate the optimal environmental conditions linked to organic-rich sediments in the South Atlantic. Our results show that high productivity, shallow depths, and optimal sedimentation conditions are required for the occurrence of organic-rich sediments. This can be viewed as important factors preconditioning the formation of black shales in the South Atlantic. Nevertheless, we highlight that orbitally-driven changes in hydroclimate and its impact in weathering-derived nutrient input probably played a major role on boosting productivity and preservation of marine organic carbon during sedimentation of high-TOC black shales in the South Atlantic.