Methane transport from subsurface reservoirs to shallow marine sediment is characterized by uniqu... more Methane transport from subsurface reservoirs to shallow marine sediment is characterized by unique biogeochemical interactions significant for ocean chemistry. Sulfate-Methane Transition Zone (SMTZ) is an important diagenetic front in the sediment column that quantitatively consumes the diffusive methane fluxes from deep methanogenic sources toward shallow marine sediments via sulfate-driven anaerobic oxidation of methane (AOM). Recent global compilation from diffusion-controlled marine settings suggests methane from below and sulfate from above fluxing into the SMTZ at an estimated rate of 3.8 and 5.3 Tmol year −1 , respectively, and wider estimate for methane flux ranges from 1 to 19 Tmol year −1 . AOM converts the methane carbon to dissolved inorganic carbon (DIC) at the SMTZ. Organoclastic sulfate reduction (OSR) and deep-DIC fluxes from methanogenic zones contribute additional DIC to the shallow sediments. Here, we provide a quantification of 8.7 Tmol year −1 DIC entering the methane-charged shallow sediments due to AOM, OSR, and the deep-DIC flux (range 6.4-10.2 Tmol year −1 ). Of this total DIC pool, an estimated 6.5 Tmol year −1 flows toward the water column (range: 3.2-9.2 Tmol year −1 ), and 1.7 Tmol year −1 enters the authigenic carbonate phases (range: 0.6-3.6 Tmol year −1 ). This summary highlights that carbonate authigenesis in settings dominated by diffusive methane fluxes is a significant component of marine carbon burial, comparable to ∼15% of carbonate accumulation on continental shelves and in the abyssal ocean, respectively. Further, the DIC outflux through the SMTZ is comparable to ∼20% of global riverine DIC flux to oceans. This DIC outflux will contribute alkalinity or CO 2 in different proportions to the water column, depending on the rates of authigenic carbonate precipitation and sulfide oxidation and will significantly impact ocean chemistry and potentially atmospheric CO 2 . Settings with substantial carbonate precipitation and sulfide oxidation at present are contributing CO 2 and thus to ocean acidification. Our synthesis emphasizes the importance of SMTZ as not only a methane sink but also an important diagenetic front for global DIC cycling. We further underscore the need to incorporate a DIC pump in methane-charged shallow marine sediments to models for coastal and geologic carbon cycling.
The greenhouse gas methane (CH 4) contributed to a warm climate that maintained liquid water and ... more The greenhouse gas methane (CH 4) contributed to a warm climate that maintained liquid water and sustained Earth's habitability in the Precambrian despite the faint young sun. The viability of methanogenesis (ME) in ferruginous environments, however, is debated, as iron reduction can potentially outcompete ME as a pathway of organic carbon remineralization (OCR). Here, we document that ME is a dominant OCR process in Brownie Lake, Minnesota (midwestern United States), which is a ferruginous (iron-rich, sulfate-poor) and meromictic (stratified with permanent anoxic bottom waters) system. We report ME accounting for ≥90% and >9% ± 7% of the anaerobic OCR in the water column and sediments, respectively, and an overall particulate organic carbon loading to CH 4 conversion efficiency of ≥18% ± 7% in the anoxic zone of Brownie Lake. Our results, along with previous reports from ferruginous systems, suggest that even under low primary productivity in Precambrian oceans, the efficient conversion of organic carbon would have enabled marine CH 4 to play a major role in early Earth's biogeochemical evolution.
Precipitation of methane-derived authigenic carbonates (MDAC) is an integral part of marine metha... more Precipitation of methane-derived authigenic carbonates (MDAC) is an integral part of marine methane production and consumption, but MDAC's relative significance to the global marine carbon cycle is not well understood. Here we provide a synthesis and perspective to highlight MDAC from a global marine carbon biogeochemistry viewpoint. MDAC formation is a result and archive of carbon-sulfur (C-S) coupling in the shallow sulfatic zone and carbon-silicon (C-Si) coupling in deeper methanic sediments. MDAC constitute a carbon sequestration of 3.93 Tmol C yr − 1 (range 2.34-5.8 Tmol C yr − 1) in the modern ocean and are the thirdlargest carbon burial mechanism in marine sediments. This burial compares to 29% (11-57%) organic carbon and 10% (6-23%) skeletal carbonate carbon burial along continental margins. MDAC formation is also an important sink for benthic alkalinity and, thereby, a potential contributor to bottom water acidification. Our understanding of the impact of MDAC on global biogeochemical cycles has evolved over the past five decades from what was traditionally considered a passive carbon sequestration mechanism in a seep-oasis setting to what is now considered a dynamic carbonate factory expanding from deep sediments to bottom waters-a factory that has been operational since the Precambrian. We present a strong case for the need to improve regional scale quantification of MDAC accumulation rates and associated carbonate biogeochemical parameters, leading to their incorporation in present and paleo-carbon budgets in the next phase of MDAC exploration.
Meromictic ferruginous (Fe-rich, anoxic, and permanently stratified) lakes offer a unique analog ... more Meromictic ferruginous (Fe-rich, anoxic, and permanently stratified) lakes offer a unique analog to evaluate the biogeochemical dynamics during the Earth's past when oceans were characterized by similar redox conditions. This study investigated carbon cycling within ferruginous and meromictic Brownie Lake in Minneapolis, MN. Previous works have reported an active methane cycling in this system with ebullitive losses of methane. Water column concentrations and stable carbon isotopes of methane (δ 13 C CH4), dissolved and particulate organic carbon (δ 13 C DOC , δ 13 C POC), and dissolved inorganic carbon (δ 13 C DIC), along with concertation profiles of major nutrients, anions, and cations were collected over several years. The increasing ammonium and DIC concentrations with depth below the chemocline (~4 m below the surface) indicate organic matter remineralization by microbial heterotrophy. However, the increasing δ 13 C DIC values and methane concentrations with depth suggest active methanogenesis below the chemocline. The DOC concentration profile below the chemocline did not show comparable variation with DIC and CH 4 concentrations, suggestive that only a portion of the organic carbon pool is available for methanogenesis and the existence of a sizeable recalcitrant DOC pool. A noticeable depletion of δ 13 C DIC , δ 13 C DOC , and δ 13 C POC along with enrichment in δ 13 C CH4 at 3m depth suggests strong aerobic methane oxidation above the chemocline impacting the organic and inorganic carbon pools. Estimated CH 4 storage yielded 33.46 g C m-2 , a very high amount when compared to other lakes with similar surface area (5 ha). Further, the correlation between dissolved Fe concentrations with concentrations of DOC, DIC, CH 4 , NH 4 , P, and δ 13 C DIC , and δ 13 C POC suggests an active role of iron in carbon cycling. Ongoing works aim to quantify the carbon sources and sinks in the system, and evaluate the proportion of exported POC mineralized by heterotrophy, converted to methane, or buried in sediments. These results will contribute to our evolving understanding of the role of methane biogeochemistry in the Precambrian ferruginous oceans.
Methane transport from subsurface reservoirs to shallow marine sediment is characterized by uniqu... more Methane transport from subsurface reservoirs to shallow marine sediment is characterized by unique biogeochemical interactions significant for ocean chemistry. Sulfate-Methane Transition Zone (SMTZ) is an important diagenetic front in the sediment column that quantitatively consumes the diffusive methane fluxes from deep methanogenic sources toward shallow marine sediments via sulfate-driven anaerobic oxidation of methane (AOM). Recent global compilation from diffusion-controlled marine settings suggests methane from below and sulfate from above fluxing into the SMTZ at an estimated rate of 3.8 and 5.3 Tmol year −1 , respectively, and wider estimate for methane flux ranges from 1 to 19 Tmol year −1. AOM converts the methane carbon to dissolved inorganic carbon (DIC) at the SMTZ. Organoclastic sulfate reduction (OSR) and deep-DIC fluxes from methanogenic zones contribute additional DIC to the shallow sediments. Here, we provide a quantification of 8.7 Tmol year −1 DIC entering the methane-charged shallow sediments due to AOM, OSR, and the deep-DIC flux (range 6.4-10.2 Tmol year −1). Of this total DIC pool, an estimated 6.5 Tmol year −1 flows toward the water column (range: 3.2-9.2 Tmol year −1), and 1.7 Tmol year −1 enters the authigenic carbonate phases (range: 0.6-3.6 Tmol year −1). This summary highlights that carbonate authigenesis in settings dominated by diffusive methane fluxes is a significant component of marine carbon burial, comparable to ∼15% of carbonate accumulation on continental shelves and in the abyssal ocean, respectively. Further, the DIC outflux through the SMTZ is comparable to ∼20% of global riverine DIC flux to oceans. This DIC outflux will contribute alkalinity or CO 2 in different proportions to the water column, depending on the rates of authigenic carbonate precipitation and sulfide oxidation and will significantly impact ocean chemistry and potentially atmospheric CO 2. Settings with substantial carbonate precipitation and sulfide oxidation at present are contributing CO 2 and thus to ocean acidification. Our synthesis emphasizes the importance of SMTZ as not only a methane sink but also an important diagenetic front for global DIC cycling. We further underscore the need to incorporate a DIC pump in methane-charged shallow marine sediments to models for coastal and geologic carbon cycling.
Precipitation of methane-derived authigenic carbonates (MDAC) is an integral part of marine metha... more Precipitation of methane-derived authigenic carbonates (MDAC) is an integral part of marine methane production and consumption, but MDAC's relative significance to the global marine carbon cycle is not well understood. Here we provide a synthesis and perspective to highlight MDAC from a global marine carbon biogeochemistry viewpoint. MDAC formation is a result and archive of carbon-sulfur (C-S) coupling in the shallow sulfatic zone and carbon-silicon (C-Si) coupling in deeper methanic sediments. MDAC constitute a carbon sequestration of 3.93 Tmol C yr − 1 (range 2.34-5.8 Tmol C yr − 1) in the modern ocean and are the thirdlargest carbon burial mechanism in marine sediments. This burial compares to 29% (11-57%) organic carbon and 10% (6-23%) skeletal carbonate carbon burial along continental margins. MDAC formation is also an important sink for benthic alkalinity and, thereby, a potential contributor to bottom water acidification. Our understanding of the impact of MDAC on global biogeochemical cycles has evolved over the past five decades from what was traditionally considered a passive carbon sequestration mechanism in a seep-oasis setting to what is now considered a dynamic carbonate factory expanding from deep sediments to bottom waters-a factory that has been operational since the Precambrian. We present a strong case for the need to improve regional scale quantification of MDAC accumulation rates and associated carbonate biogeochemical parameters, leading to their incorporation in present and paleo-carbon budgets in the next phase of MDAC exploration.
Precipitation of methane-derived authigenic carbonates (MDAC) is an integral part of marine metha... more Precipitation of methane-derived authigenic carbonates (MDAC) is an integral part of marine methane production and consumption, but MDAC's relative significance to the global marine carbon cycle is not well understood. Here we provide a synthesis and perspective to highlight MDAC from a global marine carbon biogeochemistry viewpoint. MDAC formation is a result and archive of carbon-sulfur (C-S) coupling in the shallow sulfatic zone and carbon-silicon (C-Si) coupling in deeper methanic sediments. MDAC constitute a carbon sequestration of 3.93 Tmol C yr − 1 (range 2.34-5.8 Tmol C yr − 1) in the modern ocean and are the thirdlargest carbon burial mechanism in marine sediments. This burial compares to 29% (11-57%) organic carbon and 10% (6-23%) skeletal carbonate carbon burial along continental margins. MDAC formation is also an important sink for benthic alkalinity and, thereby, a potential contributor to bottom water acidification. Our understanding of the impact of MDAC on global biogeochemical cycles has evolved over the past five decades from what was traditionally considered a passive carbon sequestration mechanism in a seep-oasis setting to what is now considered a dynamic carbonate factory expanding from deep sediments to bottom waters-a factory that has been operational since the Precambrian. We present a strong case for the need to improve regional scale quantification of MDAC accumulation rates and associated carbonate biogeochemical parameters, leading to their incorporation in present and paleo-carbon budgets in the next phase of MDAC exploration.
Four adjacent lakes (Arco, Budd, Deming, and Josephine) within Itasca State Park in Minnesota, US... more Four adjacent lakes (Arco, Budd, Deming, and Josephine) within Itasca State Park in Minnesota, USA are reported to be meromictic in the scientific literature. However, seasonally persistent chemoclines have never been documented. We collected seasonal profiles of temperature and specific conductance and placed temperature sensor chains in two lakes for ~ 1 year to explore whether these lakes remain stratified through seasonal mixing events, and what factors contribute to their stability. The results indicate that all lakes are predominantly thermally stratified and are prone to mixing in isothermal periods during spring and fall. Despite brief, semi-annual erosion of thermal stratification, Deming Lake showed no signs of complete mixing from 2006 to 2009 and 2019-2022. Geochemical data indicate that water in Budd Lake, the most dilute lake, is predominantly sourced from precipitation. The water in the other three lakes is calcium-magnesium bicarbonate type, reflecting a source of wa...
Meromictic ferruginous (Fe-rich, anoxic, and permanently stratified) lakes offer a unique analog ... more Meromictic ferruginous (Fe-rich, anoxic, and permanently stratified) lakes offer a unique analog to evaluate the biogeochemical dynamics during the Earth's past when oceans were characterized by similar redox conditions. This study investigated carbon cycling within ferruginous and meromictic Brownie Lake in Minneapolis, MN. Previous works have reported an active methane cycling in this system with ebullitive losses of methane. Water column concentrations and stable carbon isotopes of methane (δ 13 C CH4), dissolved and particulate organic carbon (δ 13 C DOC , δ 13 C POC), and dissolved inorganic carbon (δ 13 C DIC), along with concertation profiles of major nutrients, anions, and cations were collected over several years. The increasing ammonium and DIC concentrations with depth below the chemocline (~4 m below the surface) indicate organic matter remineralization by microbial heterotrophy. However, the increasing δ 13 C DIC values and methane concentrations with depth suggest active methanogenesis below the chemocline. The DOC concentration profile below the chemocline did not show comparable variation with DIC and CH 4 concentrations, suggestive that only a portion of the organic carbon pool is available for methanogenesis and the existence of a sizeable recalcitrant DOC pool. A noticeable depletion of δ 13 C DIC , δ 13 C DOC , and δ 13 C POC along with enrichment in δ 13 C CH4 at 3m depth suggests strong aerobic methane oxidation above the chemocline impacting the organic and inorganic carbon pools. Estimated CH 4 storage yielded 33.46 g C m-2 , a very high amount when compared to other lakes with similar surface area (5 ha). Further, the correlation between dissolved Fe concentrations with concentrations of DOC, DIC, CH 4 , NH 4 , P, and δ 13 C DIC , and δ 13 C POC suggests an active role of iron in carbon cycling. Ongoing works aim to quantify the carbon sources and sinks in the system, and evaluate the proportion of exported POC mineralized by heterotrophy, converted to methane, or buried in sediments. These results will contribute to our evolving understanding of the role of methane biogeochemistry in the Precambrian ferruginous oceans.
This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Methane transport from subsurface reservoirs to shallow marine sediment is characterized by uniqu... more Methane transport from subsurface reservoirs to shallow marine sediment is characterized by unique biogeochemical interactions significant for ocean chemistry. Sulfate-Methane Transition Zone (SMTZ) is an important diagenetic front in the sediment column that quantitatively consumes the diffusive methane fluxes from deep methanogenic sources toward shallow marine sediments via sulfate-driven anaerobic oxidation of methane (AOM). Recent global compilation from diffusion-controlled marine settings suggests methane from below and sulfate from above fluxing into the SMTZ at an estimated rate of 3.8 and 5.3 Tmol year −1 , respectively, and wider estimate for methane flux ranges from 1 to 19 Tmol year −1. AOM converts the methane carbon to dissolved inorganic carbon (DIC) at the SMTZ. Organoclastic sulfate reduction (OSR) and deep-DIC fluxes from methanogenic zones contribute additional DIC to the shallow sediments. Here, we provide a quantification of 8.7 Tmol year −1 DIC entering the methane-charged shallow sediments due to AOM, OSR, and the deep-DIC flux (range 6.4-10.2 Tmol year −1). Of this total DIC pool, an estimated 6.5 Tmol year −1 flows toward the water column (range: 3.2-9.2 Tmol year −1), and 1.7 Tmol year −1 enters the authigenic carbonate phases (range: 0.6-3.6 Tmol year −1). This summary highlights that carbonate authigenesis in settings dominated by diffusive methane fluxes is a significant component of marine carbon burial, comparable to ∼15% of carbonate accumulation on continental shelves and in the abyssal ocean, respectively. Further, the DIC outflux through the SMTZ is comparable to ∼20% of global riverine DIC flux to oceans. This DIC outflux will contribute alkalinity or CO 2 in different proportions to the water column, depending on the rates of authigenic carbonate precipitation and sulfide oxidation and will significantly impact ocean chemistry and potentially atmospheric CO 2. Settings with substantial carbonate precipitation and sulfide oxidation at present are contributing CO 2 and thus to ocean acidification. Our synthesis emphasizes the importance of SMTZ as not only a methane sink but also an important diagenetic front for global DIC cycling. We further underscore the need to incorporate a DIC pump in methane-charged shallow marine sediments to models for coastal and geologic carbon cycling.
Offshore hydrocarbon accumulations in the Gulf of Mexico (GoM) are often accompanied by natural s... more Offshore hydrocarbon accumulations in the Gulf of Mexico (GoM) are often accompanied by natural seepage of oil and gas from subsurface reservoirs into shallow sediments and the water column. This study investigated the temporal patterns and carbon-sulfur (C-S) coupling associated with authigenic carbonate samples recovered from surface sediments of a crude oil seepage site in southern GoM (Chapopote asphalt volcano, Bay of Campeche) using radioactive U-Th dates, and stable C, O, and S isotopes. The results were compared with data from multiple seep sites in the northern GoM where methane seepage is dominant along with non-methane hydrocarbons (ethane , propane, crude oil, etc.). U-Th age-dating of Chapopote seep carbonate samples yielded ages of 13.5 ka to 4.6 ka before present (BP), suggesting that Chapopote asphalt seepage has been ongoing for thousands of years. The results are also consistent with previous studies from the northern GoM that hypothesize that seeps along the GoM continental slope were active during the last deglaciation. δ13CCaCO3 and δ18OCaCO3 values from authigenic carbonates at Chapopote indicated a mixed contribution of methane and non-methane hydrocarbons to the dissolved inorganic carbon (DIC) pool, consistent with previous results. Comparison of δ13CCaCO3 vs. δ34SCRS (CRS = chromium reducible sulfur) from carbonate samples showed noticeable differences at the Chapopote seep site (average δ13CCaCO3 -25‰ VDPB, δ34SCRS -27‰ VCDT) relative to the methane seep-dominated samples from the northern GoM (average δ13CCaCO3 < -40‰ VDPB, δ34SCRS >0‰ VCDT). Our results point toward distinguishable differences in the paired δ13CDIC and δ34Ssulfide signatures produced via the diagenetic processes of sulfate-driven anaerobic oxidation of methane versus non-methane hydrocarbons. These results potentially provide an important proxy for identification of such diagenetic processes within the sedimentary records.
Methane transport from subsurface reservoirs to shallow marine sediment is characterized by uniqu... more Methane transport from subsurface reservoirs to shallow marine sediment is characterized by unique biogeochemical interactions significant for ocean chemistry. Sulfate-Methane Transition Zone (SMTZ) is an important diagenetic front in the sediment column that quantitatively consumes the diffusive methane fluxes from deep methanogenic sources toward shallow marine sediments via sulfate-driven anaerobic oxidation of methane (AOM). Recent global compilation from diffusion-controlled marine settings suggests methane from below and sulfate from above fluxing into the SMTZ at an estimated rate of 3.8 and 5.3 Tmol year −1 , respectively, and wider estimate for methane flux ranges from 1 to 19 Tmol year −1 . AOM converts the methane carbon to dissolved inorganic carbon (DIC) at the SMTZ. Organoclastic sulfate reduction (OSR) and deep-DIC fluxes from methanogenic zones contribute additional DIC to the shallow sediments. Here, we provide a quantification of 8.7 Tmol year −1 DIC entering the methane-charged shallow sediments due to AOM, OSR, and the deep-DIC flux (range 6.4-10.2 Tmol year −1 ). Of this total DIC pool, an estimated 6.5 Tmol year −1 flows toward the water column (range: 3.2-9.2 Tmol year −1 ), and 1.7 Tmol year −1 enters the authigenic carbonate phases (range: 0.6-3.6 Tmol year −1 ). This summary highlights that carbonate authigenesis in settings dominated by diffusive methane fluxes is a significant component of marine carbon burial, comparable to ∼15% of carbonate accumulation on continental shelves and in the abyssal ocean, respectively. Further, the DIC outflux through the SMTZ is comparable to ∼20% of global riverine DIC flux to oceans. This DIC outflux will contribute alkalinity or CO 2 in different proportions to the water column, depending on the rates of authigenic carbonate precipitation and sulfide oxidation and will significantly impact ocean chemistry and potentially atmospheric CO 2 . Settings with substantial carbonate precipitation and sulfide oxidation at present are contributing CO 2 and thus to ocean acidification. Our synthesis emphasizes the importance of SMTZ as not only a methane sink but also an important diagenetic front for global DIC cycling. We further underscore the need to incorporate a DIC pump in methane-charged shallow marine sediments to models for coastal and geologic carbon cycling.
The greenhouse gas methane (CH 4) contributed to a warm climate that maintained liquid water and ... more The greenhouse gas methane (CH 4) contributed to a warm climate that maintained liquid water and sustained Earth's habitability in the Precambrian despite the faint young sun. The viability of methanogenesis (ME) in ferruginous environments, however, is debated, as iron reduction can potentially outcompete ME as a pathway of organic carbon remineralization (OCR). Here, we document that ME is a dominant OCR process in Brownie Lake, Minnesota (midwestern United States), which is a ferruginous (iron-rich, sulfate-poor) and meromictic (stratified with permanent anoxic bottom waters) system. We report ME accounting for ≥90% and >9% ± 7% of the anaerobic OCR in the water column and sediments, respectively, and an overall particulate organic carbon loading to CH 4 conversion efficiency of ≥18% ± 7% in the anoxic zone of Brownie Lake. Our results, along with previous reports from ferruginous systems, suggest that even under low primary productivity in Precambrian oceans, the efficient conversion of organic carbon would have enabled marine CH 4 to play a major role in early Earth's biogeochemical evolution.
Precipitation of methane-derived authigenic carbonates (MDAC) is an integral part of marine metha... more Precipitation of methane-derived authigenic carbonates (MDAC) is an integral part of marine methane production and consumption, but MDAC&#39;s relative significance to the global marine carbon cycle is not well understood. Here we provide a synthesis and perspective to highlight MDAC from a global marine carbon biogeochemistry viewpoint. MDAC formation is a result and archive of carbon-sulfur (C-S) coupling in the shallow sulfatic zone and carbon-silicon (C-Si) coupling in deeper methanic sediments. MDAC constitute a carbon sequestration of 3.93 Tmol C yr − 1 (range 2.34-5.8 Tmol C yr − 1) in the modern ocean and are the thirdlargest carbon burial mechanism in marine sediments. This burial compares to 29% (11-57%) organic carbon and 10% (6-23%) skeletal carbonate carbon burial along continental margins. MDAC formation is also an important sink for benthic alkalinity and, thereby, a potential contributor to bottom water acidification. Our understanding of the impact of MDAC on global biogeochemical cycles has evolved over the past five decades from what was traditionally considered a passive carbon sequestration mechanism in a seep-oasis setting to what is now considered a dynamic carbonate factory expanding from deep sediments to bottom waters-a factory that has been operational since the Precambrian. We present a strong case for the need to improve regional scale quantification of MDAC accumulation rates and associated carbonate biogeochemical parameters, leading to their incorporation in present and paleo-carbon budgets in the next phase of MDAC exploration.
Meromictic ferruginous (Fe-rich, anoxic, and permanently stratified) lakes offer a unique analog ... more Meromictic ferruginous (Fe-rich, anoxic, and permanently stratified) lakes offer a unique analog to evaluate the biogeochemical dynamics during the Earth's past when oceans were characterized by similar redox conditions. This study investigated carbon cycling within ferruginous and meromictic Brownie Lake in Minneapolis, MN. Previous works have reported an active methane cycling in this system with ebullitive losses of methane. Water column concentrations and stable carbon isotopes of methane (δ 13 C CH4), dissolved and particulate organic carbon (δ 13 C DOC , δ 13 C POC), and dissolved inorganic carbon (δ 13 C DIC), along with concertation profiles of major nutrients, anions, and cations were collected over several years. The increasing ammonium and DIC concentrations with depth below the chemocline (~4 m below the surface) indicate organic matter remineralization by microbial heterotrophy. However, the increasing δ 13 C DIC values and methane concentrations with depth suggest active methanogenesis below the chemocline. The DOC concentration profile below the chemocline did not show comparable variation with DIC and CH 4 concentrations, suggestive that only a portion of the organic carbon pool is available for methanogenesis and the existence of a sizeable recalcitrant DOC pool. A noticeable depletion of δ 13 C DIC , δ 13 C DOC , and δ 13 C POC along with enrichment in δ 13 C CH4 at 3m depth suggests strong aerobic methane oxidation above the chemocline impacting the organic and inorganic carbon pools. Estimated CH 4 storage yielded 33.46 g C m-2 , a very high amount when compared to other lakes with similar surface area (5 ha). Further, the correlation between dissolved Fe concentrations with concentrations of DOC, DIC, CH 4 , NH 4 , P, and δ 13 C DIC , and δ 13 C POC suggests an active role of iron in carbon cycling. Ongoing works aim to quantify the carbon sources and sinks in the system, and evaluate the proportion of exported POC mineralized by heterotrophy, converted to methane, or buried in sediments. These results will contribute to our evolving understanding of the role of methane biogeochemistry in the Precambrian ferruginous oceans.
Methane transport from subsurface reservoirs to shallow marine sediment is characterized by uniqu... more Methane transport from subsurface reservoirs to shallow marine sediment is characterized by unique biogeochemical interactions significant for ocean chemistry. Sulfate-Methane Transition Zone (SMTZ) is an important diagenetic front in the sediment column that quantitatively consumes the diffusive methane fluxes from deep methanogenic sources toward shallow marine sediments via sulfate-driven anaerobic oxidation of methane (AOM). Recent global compilation from diffusion-controlled marine settings suggests methane from below and sulfate from above fluxing into the SMTZ at an estimated rate of 3.8 and 5.3 Tmol year −1 , respectively, and wider estimate for methane flux ranges from 1 to 19 Tmol year −1. AOM converts the methane carbon to dissolved inorganic carbon (DIC) at the SMTZ. Organoclastic sulfate reduction (OSR) and deep-DIC fluxes from methanogenic zones contribute additional DIC to the shallow sediments. Here, we provide a quantification of 8.7 Tmol year −1 DIC entering the methane-charged shallow sediments due to AOM, OSR, and the deep-DIC flux (range 6.4-10.2 Tmol year −1). Of this total DIC pool, an estimated 6.5 Tmol year −1 flows toward the water column (range: 3.2-9.2 Tmol year −1), and 1.7 Tmol year −1 enters the authigenic carbonate phases (range: 0.6-3.6 Tmol year −1). This summary highlights that carbonate authigenesis in settings dominated by diffusive methane fluxes is a significant component of marine carbon burial, comparable to ∼15% of carbonate accumulation on continental shelves and in the abyssal ocean, respectively. Further, the DIC outflux through the SMTZ is comparable to ∼20% of global riverine DIC flux to oceans. This DIC outflux will contribute alkalinity or CO 2 in different proportions to the water column, depending on the rates of authigenic carbonate precipitation and sulfide oxidation and will significantly impact ocean chemistry and potentially atmospheric CO 2. Settings with substantial carbonate precipitation and sulfide oxidation at present are contributing CO 2 and thus to ocean acidification. Our synthesis emphasizes the importance of SMTZ as not only a methane sink but also an important diagenetic front for global DIC cycling. We further underscore the need to incorporate a DIC pump in methane-charged shallow marine sediments to models for coastal and geologic carbon cycling.
Precipitation of methane-derived authigenic carbonates (MDAC) is an integral part of marine metha... more Precipitation of methane-derived authigenic carbonates (MDAC) is an integral part of marine methane production and consumption, but MDAC&#39;s relative significance to the global marine carbon cycle is not well understood. Here we provide a synthesis and perspective to highlight MDAC from a global marine carbon biogeochemistry viewpoint. MDAC formation is a result and archive of carbon-sulfur (C-S) coupling in the shallow sulfatic zone and carbon-silicon (C-Si) coupling in deeper methanic sediments. MDAC constitute a carbon sequestration of 3.93 Tmol C yr − 1 (range 2.34-5.8 Tmol C yr − 1) in the modern ocean and are the thirdlargest carbon burial mechanism in marine sediments. This burial compares to 29% (11-57%) organic carbon and 10% (6-23%) skeletal carbonate carbon burial along continental margins. MDAC formation is also an important sink for benthic alkalinity and, thereby, a potential contributor to bottom water acidification. Our understanding of the impact of MDAC on global biogeochemical cycles has evolved over the past five decades from what was traditionally considered a passive carbon sequestration mechanism in a seep-oasis setting to what is now considered a dynamic carbonate factory expanding from deep sediments to bottom waters-a factory that has been operational since the Precambrian. We present a strong case for the need to improve regional scale quantification of MDAC accumulation rates and associated carbonate biogeochemical parameters, leading to their incorporation in present and paleo-carbon budgets in the next phase of MDAC exploration.
Precipitation of methane-derived authigenic carbonates (MDAC) is an integral part of marine metha... more Precipitation of methane-derived authigenic carbonates (MDAC) is an integral part of marine methane production and consumption, but MDAC's relative significance to the global marine carbon cycle is not well understood. Here we provide a synthesis and perspective to highlight MDAC from a global marine carbon biogeochemistry viewpoint. MDAC formation is a result and archive of carbon-sulfur (C-S) coupling in the shallow sulfatic zone and carbon-silicon (C-Si) coupling in deeper methanic sediments. MDAC constitute a carbon sequestration of 3.93 Tmol C yr − 1 (range 2.34-5.8 Tmol C yr − 1) in the modern ocean and are the thirdlargest carbon burial mechanism in marine sediments. This burial compares to 29% (11-57%) organic carbon and 10% (6-23%) skeletal carbonate carbon burial along continental margins. MDAC formation is also an important sink for benthic alkalinity and, thereby, a potential contributor to bottom water acidification. Our understanding of the impact of MDAC on global biogeochemical cycles has evolved over the past five decades from what was traditionally considered a passive carbon sequestration mechanism in a seep-oasis setting to what is now considered a dynamic carbonate factory expanding from deep sediments to bottom waters-a factory that has been operational since the Precambrian. We present a strong case for the need to improve regional scale quantification of MDAC accumulation rates and associated carbonate biogeochemical parameters, leading to their incorporation in present and paleo-carbon budgets in the next phase of MDAC exploration.
Four adjacent lakes (Arco, Budd, Deming, and Josephine) within Itasca State Park in Minnesota, US... more Four adjacent lakes (Arco, Budd, Deming, and Josephine) within Itasca State Park in Minnesota, USA are reported to be meromictic in the scientific literature. However, seasonally persistent chemoclines have never been documented. We collected seasonal profiles of temperature and specific conductance and placed temperature sensor chains in two lakes for ~ 1 year to explore whether these lakes remain stratified through seasonal mixing events, and what factors contribute to their stability. The results indicate that all lakes are predominantly thermally stratified and are prone to mixing in isothermal periods during spring and fall. Despite brief, semi-annual erosion of thermal stratification, Deming Lake showed no signs of complete mixing from 2006 to 2009 and 2019-2022. Geochemical data indicate that water in Budd Lake, the most dilute lake, is predominantly sourced from precipitation. The water in the other three lakes is calcium-magnesium bicarbonate type, reflecting a source of wa...
Meromictic ferruginous (Fe-rich, anoxic, and permanently stratified) lakes offer a unique analog ... more Meromictic ferruginous (Fe-rich, anoxic, and permanently stratified) lakes offer a unique analog to evaluate the biogeochemical dynamics during the Earth's past when oceans were characterized by similar redox conditions. This study investigated carbon cycling within ferruginous and meromictic Brownie Lake in Minneapolis, MN. Previous works have reported an active methane cycling in this system with ebullitive losses of methane. Water column concentrations and stable carbon isotopes of methane (δ 13 C CH4), dissolved and particulate organic carbon (δ 13 C DOC , δ 13 C POC), and dissolved inorganic carbon (δ 13 C DIC), along with concertation profiles of major nutrients, anions, and cations were collected over several years. The increasing ammonium and DIC concentrations with depth below the chemocline (~4 m below the surface) indicate organic matter remineralization by microbial heterotrophy. However, the increasing δ 13 C DIC values and methane concentrations with depth suggest active methanogenesis below the chemocline. The DOC concentration profile below the chemocline did not show comparable variation with DIC and CH 4 concentrations, suggestive that only a portion of the organic carbon pool is available for methanogenesis and the existence of a sizeable recalcitrant DOC pool. A noticeable depletion of δ 13 C DIC , δ 13 C DOC , and δ 13 C POC along with enrichment in δ 13 C CH4 at 3m depth suggests strong aerobic methane oxidation above the chemocline impacting the organic and inorganic carbon pools. Estimated CH 4 storage yielded 33.46 g C m-2 , a very high amount when compared to other lakes with similar surface area (5 ha). Further, the correlation between dissolved Fe concentrations with concentrations of DOC, DIC, CH 4 , NH 4 , P, and δ 13 C DIC , and δ 13 C POC suggests an active role of iron in carbon cycling. Ongoing works aim to quantify the carbon sources and sinks in the system, and evaluate the proportion of exported POC mineralized by heterotrophy, converted to methane, or buried in sediments. These results will contribute to our evolving understanding of the role of methane biogeochemistry in the Precambrian ferruginous oceans.
This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Methane transport from subsurface reservoirs to shallow marine sediment is characterized by uniqu... more Methane transport from subsurface reservoirs to shallow marine sediment is characterized by unique biogeochemical interactions significant for ocean chemistry. Sulfate-Methane Transition Zone (SMTZ) is an important diagenetic front in the sediment column that quantitatively consumes the diffusive methane fluxes from deep methanogenic sources toward shallow marine sediments via sulfate-driven anaerobic oxidation of methane (AOM). Recent global compilation from diffusion-controlled marine settings suggests methane from below and sulfate from above fluxing into the SMTZ at an estimated rate of 3.8 and 5.3 Tmol year −1 , respectively, and wider estimate for methane flux ranges from 1 to 19 Tmol year −1. AOM converts the methane carbon to dissolved inorganic carbon (DIC) at the SMTZ. Organoclastic sulfate reduction (OSR) and deep-DIC fluxes from methanogenic zones contribute additional DIC to the shallow sediments. Here, we provide a quantification of 8.7 Tmol year −1 DIC entering the methane-charged shallow sediments due to AOM, OSR, and the deep-DIC flux (range 6.4-10.2 Tmol year −1). Of this total DIC pool, an estimated 6.5 Tmol year −1 flows toward the water column (range: 3.2-9.2 Tmol year −1), and 1.7 Tmol year −1 enters the authigenic carbonate phases (range: 0.6-3.6 Tmol year −1). This summary highlights that carbonate authigenesis in settings dominated by diffusive methane fluxes is a significant component of marine carbon burial, comparable to ∼15% of carbonate accumulation on continental shelves and in the abyssal ocean, respectively. Further, the DIC outflux through the SMTZ is comparable to ∼20% of global riverine DIC flux to oceans. This DIC outflux will contribute alkalinity or CO 2 in different proportions to the water column, depending on the rates of authigenic carbonate precipitation and sulfide oxidation and will significantly impact ocean chemistry and potentially atmospheric CO 2. Settings with substantial carbonate precipitation and sulfide oxidation at present are contributing CO 2 and thus to ocean acidification. Our synthesis emphasizes the importance of SMTZ as not only a methane sink but also an important diagenetic front for global DIC cycling. We further underscore the need to incorporate a DIC pump in methane-charged shallow marine sediments to models for coastal and geologic carbon cycling.
Offshore hydrocarbon accumulations in the Gulf of Mexico (GoM) are often accompanied by natural s... more Offshore hydrocarbon accumulations in the Gulf of Mexico (GoM) are often accompanied by natural seepage of oil and gas from subsurface reservoirs into shallow sediments and the water column. This study investigated the temporal patterns and carbon-sulfur (C-S) coupling associated with authigenic carbonate samples recovered from surface sediments of a crude oil seepage site in southern GoM (Chapopote asphalt volcano, Bay of Campeche) using radioactive U-Th dates, and stable C, O, and S isotopes. The results were compared with data from multiple seep sites in the northern GoM where methane seepage is dominant along with non-methane hydrocarbons (ethane , propane, crude oil, etc.). U-Th age-dating of Chapopote seep carbonate samples yielded ages of 13.5 ka to 4.6 ka before present (BP), suggesting that Chapopote asphalt seepage has been ongoing for thousands of years. The results are also consistent with previous studies from the northern GoM that hypothesize that seeps along the GoM continental slope were active during the last deglaciation. δ13CCaCO3 and δ18OCaCO3 values from authigenic carbonates at Chapopote indicated a mixed contribution of methane and non-methane hydrocarbons to the dissolved inorganic carbon (DIC) pool, consistent with previous results. Comparison of δ13CCaCO3 vs. δ34SCRS (CRS = chromium reducible sulfur) from carbonate samples showed noticeable differences at the Chapopote seep site (average δ13CCaCO3 -25‰ VDPB, δ34SCRS -27‰ VCDT) relative to the methane seep-dominated samples from the northern GoM (average δ13CCaCO3 < -40‰ VDPB, δ34SCRS >0‰ VCDT). Our results point toward distinguishable differences in the paired δ13CDIC and δ34Ssulfide signatures produced via the diagenetic processes of sulfate-driven anaerobic oxidation of methane versus non-methane hydrocarbons. These results potentially provide an important proxy for identification of such diagenetic processes within the sedimentary records.
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