Chris Mark

Earth's last half-billion years exhibit alternating greenhouse and icehouse climate states, but we do not know why. Global climate state reflects atmospheric CO2 partial pressure, a product of volcanic emission and silicate weathering drawdown, modulated by the biosphere. Global climate simulations on geological timescales show temporal correlation with tectonic collisions. However, collision simultaneously terminates arc volcanism and exposes fresh rock for weathering: these processes are hard to deconvolve.Recent studies have provocatively hypothesised that Earth’s climate state is set by ophiolite obduction in tropical collisions. Here, rapid weathering of reactive rocks efficiently consumes CO2. This hypothesis yields a testable prediction: that the sedimentary archive of ancient tropical orogens will contain ophiolitic detrital minerals at the onset of icehouse periods. Here, we use automated heavy mineral analysis of key sedimentary sections from the New Guinea and Taconic-Grampian orogens to identify and quantify ophiolite detritus. We integrate these data with detrital geochronology and bulk geochemistry to assess ophiolitic weathering contribution to CO2 drawdown.

Fluvial stromatolites are widespread in upper Paleocene–Eocene successions of the South Pyrenean Foreland Basin (SPFB, Spain) yet their geochronological and paleoenvironmental potential remains largely unexplored. Recently documented stromatolites from the Escanilla Formation provide a unique opportunity to investigate continental environmental change across the Middle Eocene Climatic Optimum (MECO), a prolonged global warming event that is well constrained in marine archives but remains poorly dated and resolved in terrestrial records.Although existing magnetostratigraphic frameworks provide broad temporal control, in-situ U–Pb dating of fluvial stromatolites using LA-MC-ICPMS yields absolute age constraints that refine the stratigraphic placement of the MECO within the Escanilla Formation and improve chronologic resolution across the SPFB. These age anchors allow direct integration of continental sedimentary records with global climate archives.High-resolution stable-isotope analyses (δ¹³C and δ¹⁸O) performed using SIMS across individual stromatolite growth bands capture seasonal variability in hydrological and thermal conditions. Variations in δ¹⁸O are interpreted to reflect changes in water temperature and/or evaporative conditions, while δ¹³C variability records shift in dissolved inorganic carbon sources and microbial activity within fluvial systems. Together, these data provide insight into seasonality, hydrology, and climate sensitivity of continental environments during a major Eocene warming event.This study demonstrates that fluvial stromatolites represent robust, datable continental archives capable of resolving both absolute timing and high-frequency environmental variability during past greenhouse climate perturbations.

The Atlantic Meridional Overturning Circulation (AMOC) has shown signs of decline over the last two decades. Climate models project that a continued slowdown of the AMOC will increase precipitation over parts of northern Europe, particularly in the Irish-British Isles1, with potential impacts on agriculture and related systems. However, the ability of climate models to predict when such changes might occur remains limited, calling for the use of paleoclimate archives. Here, we present a stalagmite‑based paleoclimate record from the west coast of Ireland spanning 11.1–7.7 ka (b2k). Combined Sr/Ca and stable isotope data indicate a sudden increase in precipitation at ~8.6 ka, coincident with the collapse of the Hudson Bay Ice Saddle (HBIS)2 and a reduction in eastern North Atlantic bottom and surface currents3,4. We interpret this hydroclimatic shift as a response to the slowdown of the AMOC caused by the HBIS freshwater discharge, indicating a minimum time lag (of decadal scale) between ocean circulation disruption and atmospheric response. Due to enhanced thermal and pressure gradients over the North Atlantic, a weakened AMOC can favour positive North Atlantic Oscillation (NAO+) conditions, which typically bring wetter and stormier weather over northern Europe. We therefore associate the ~8.6 ka precipitation increase with the development of NAO+ conditions in the region, which aligns with existing work5. In addition, our record evidences sustained precipitation throughout the '8.2 ka' cooling anomaly, suggesting that, regardless of temperature direction, heightened precipitation is a persistent consequence of AMOC reduction in northwest Europe.1: Jackson et al. (2015) Climate Dynamics, 45. 2: Lochte et al. (2019) Nature Communications, 10, 586. 3: Ellison et al. (2006) Science, 312. 4: Thornalley et al. (2009) Nature, 457. 5: Smith et al. (2016) Scientific Reports, 6, 24745.

Understanding how the minerals in reservoir rocks respond to CO₂ injection is vital for the success and safety of Carbon Capture and Storage (CCS) projects. Feldspars are the most common mineral in the Earth's crust and act as primary framework grains in sandstones. Compared to quartz, feldspars are mechanically weak and chemically reactive. Dissolved feldspars can re-precipitate as clays, which in CCS reservoirs could impact fluid-flow. While caprock mineral stability is well studied, reservoir mineral reactivity, particularly of feldspars, remains understudied. To address this knowledge gap, we present microstructural and geochemical data from batch experiments that reacted CO₂-enriched fluids with feldspar-bearing sandstone sampled from the Captain Sandstone Member, the primary reservoir for the Acorn CCS Project (UK). Experiments were conducted in a hydrostatic pressure vessel at 70 MPa confining pressure, 50 MPa pore pressure, and temperatures ranging from 80 to 550 °C, using CO₂-enriched water to simulate reservoir conditions. Pre- and post-reaction samples were analysed using XRD, SEM-EDS, and XCT to assess microstructural and mineralogical changes. Results show that CO₂:feldspar interactions differ significantly from control experiments involving water alone. At reservoir-relevant temperatures (80 °C), incongruent dissolution of K-feldspar weakened grains which led to microfracturing. At 250 °C, CO₂ fluids caused total dissolution of calcite grains and cement and selective leaching of calcium from oligoclase, enriching the pore fluid with Ca²⁺. Above 400 °C, coupled dissolution–precipitation processes were observed, including congruent K-feldspar dissolution, secondary porosity development, and localised precipitation of Ca-aluminosilicates and K-bearing phases around dissolving K-feldspars. These transformations could alter reservoir flow pathways and induce mechanical risks, i.e. destabilising nearby faults or initiating reservoir collapse. Given feldspars' prevalence in crustal rocks and CCS sandstone reservoirs, their reactive behaviour under in-situ conditions and in the presence of aggressive fluids demands greater attention.

Understanding how the minerals in reservoir rocks respond to CO₂ injection is vital for the success and safety of Carbon Capture and Storage (CCS) projects. Feldspars are the most common mineral in the Earth’s crust and act as primary framework grains in sandstones. Compared to quartz, feldspars are mechanically weak and chemically reactive. Dissolved feldspars can re-precipitate as clays, which in CCS reservoirs could impact fluid-flow. While caprock mineral stability is well studied, reservoir mineral reactivity, particularly of feldspars, remains understudied. To address this knowledge gap, we present microstructural and geochemical data from batch experiments that reacted CO₂-enriched fluids with feldspar-bearing sandstone sampled from the Captain Sandstone Member, the primary reservoir for the Acorn CCS Project (UK). Experiments were conducted in a hydrostatic pressure vessel at 70 MPa confining pressure, 50 MPa pore pressure, and temperatures ranging from 80 °C to 550 °C, using CO₂-enriched water to simulate reservoir conditions. Pre- and post-reaction samples were analysed using XRD, SEM-EDS, and XCT to assess microstructural and mineralogical changes. Results show that CO₂:feldspar interactions differ significantly from control experiments involving water alone. At reservoir-relevant temperatures (80 °C), incongruent dissolution of K-feldspar weakened grains which led to microfracturing. At 250 °C, CO₂ fluids caused total dissolution of calcite grains and cement and selective leaching of calcium from oligoclase, enriching the pore fluid with Ca²⁺. Above 400 °C, coupled dissolution–precipitation processes were observed, including congruent K-feldspar dissolution, secondary porosity development, and localised precipitation of Ca-aluminosilicates and K-bearing phases around dissolving K-feldspars. These transformations could alter reservoir flow pathways and induce mechanical risks, i.e. destabilising nearby faults or initiating reservoir collapse. Given feldspars’ prevalence in crustal rocks and CCS sandstone reservoirs, their reactive behaviour under in-situ conditions and in the presence of aggressive fluids demands greater attention.

Understanding how the minerals in reservoir rocks respond to CO₂ injection is vital for the success and safety of Carbon Capture and Storage (CCS) projects. Feldspars are the most common mineral in the Earth’s crust and act as primary framework grains in sandstones. Compared to quartz, feldspars are mechanically weak and chemically reactive. Dissolved feldspars can re-precipitate as clays, which in CCS reservoirs could impact fluid-flow. While caprock mineral stability is well studied, reservoir mineral reactivity, particularly of feldspars, remains understudied. To address this knowledge gap, we present microstructural and geochemical data from batch experiments that reacted CO₂-enriched fluids with feldspar-bearing sandstone sampled from the Captain Sandstone Member, the primary reservoir for the Acorn CCS Project (UK). Experiments were conducted in a hydrostatic pressure vessel at 70 MPa confining pressure, 50 MPa pore pressure, and temperatures ranging from 80 °C to 550 °C, using CO₂-enriched water to simulate reservoir conditions. Pre- and post-reaction samples were analysed using XRD, SEM-EDS, and XCT to assess microstructural and mineralogical changes. Results show that CO₂:feldspar interactions differ significantly from control experiments involving water alone. At reservoir-relevant temperatures (80 °C), incongruent dissolution of K-feldspar weakened grains which led to microfracturing. At 250 °C, CO₂ fluids caused total dissolution of calcite grains and cement and selective leaching of calcium from oligoclase, enriching the pore fluid with Ca²⁺. Above 400 °C, coupled dissolution–precipitation processes were observed, including congruent K-feldspar dissolution, secondary porosity development, and localised precipitation of Ca-aluminosilicates and K-bearing phases around dissolving K-feldspars. These transformations could alter reservoir flow pathways and induce mechanical risks, i.e. destabilising nearby faults or initiating reservoir collapse. Given feldspars’ prevalence in crustal rocks and CCS sandstone reservoirs, their reactive behaviour under in-situ conditions and in the presence of aggressive fluids demands greater attention.

Understanding how the minerals in reservoir rocks respond to CO₂ injection is vital for the success and safety of Carbon Capture and Storage (CCS) projects. Feldspars are the most common mineral in the Earth’s crust and act as primary framework grains in sandstones. Compared to quartz, feldspars are mechanically weak and chemically reactive. Dissolved feldspars can re-precipitate as clays, which in CCS reservoirs could impact fluid-flow. While caprock mineral stability is well studied, reservoir mineral reactivity, particularly of feldspars, remains understudied. To address this knowledge gap, we present microstructural and geochemical data from batch experiments that reacted CO₂-enriched fluids with feldspar-bearing sandstone sampled from the Captain Sandstone Member, the primary reservoir for the Acorn CCS Project (UK). Experiments were conducted in a hydrostatic pressure vessel at 70 MPa confining pressure, 50 MPa pore pressure, and temperatures ranging from 80 °C to 550 °C, using CO₂-enriched water to simulate reservoir conditions. Pre- and post-reaction samples were analysed using XRD, SEM-EDS, and XCT to assess microstructural and mineralogical changes. Results show that CO₂:feldspar interactions differ significantly from control experiments involving water alone. At reservoir-relevant temperatures (80 °C), incongruent dissolution of K-feldspar weakened grains which led to microfracturing. At 250 °C, CO₂ fluids caused total dissolution of calcite grains and cement and selective leaching of calcium from oligoclase, enriching the pore fluid with Ca²⁺. Above 400 °C, coupled dissolution–precipitation processes were observed, including congruent K-feldspar dissolution, secondary porosity development, and localised precipitation of Ca-aluminosilicates and K-bearing phases around dissolving K-feldspars. These transformations could alter reservoir flow pathways and induce mechanical risks, i.e. destabilising nearby faults or initiating reservoir collapse. Given feldspars’ prevalence in crustal rocks and CCS sandstone reservoirs, their reactive behaviour under in-situ conditions and in the presence of aggressive fluids demands greater attention.

Understanding how the minerals in reservoir rocks respond to CO₂ injection is vital for the success and safety of Carbon Capture and Storage (CCS) projects. Feldspars are the most common mineral in the Earth’s crust and act as primary framework grains in sandstones. Compared to quartz, feldspars are mechanically weak and chemically reactive. Dissolved feldspars can re-precipitate as clays, which in CCS reservoirs could impact fluid-flow. While caprock mineral stability is well studied, reservoir mineral reactivity, particularly of feldspars, remains understudied. To address this knowledge gap, we present microstructural and geochemical data from batch experiments that reacted CO₂-enriched fluids with feldspar-bearing sandstone sampled from the Captain Sandstone Member, the primary reservoir for the Acorn CCS Project (UK). Experiments were conducted in a hydrostatic pressure vessel at 70 MPa confining pressure, 50 MPa pore pressure, and temperatures ranging from 80 °C to 550 °C, using CO₂-enriched water to simulate reservoir conditions. Pre- and post-reaction samples were analysed using XRD, SEM-EDS, and XCT to assess microstructural and mineralogical changes. Results show that CO₂:feldspar interactions differ significantly from control experiments involving water alone. At reservoir-relevant temperatures (80 °C), incongruent dissolution of K-feldspar weakened grains which led to microfracturing. At 250 °C, CO₂ fluids caused total dissolution of calcite grains and cement and selective leaching of calcium from oligoclase, enriching the pore fluid with Ca²⁺. Above 400 °C, coupled dissolution–precipitation processes were observed, including congruent K-feldspar dissolution, secondary porosity development, and localised precipitation of Ca-aluminosilicates and K-bearing phases around dissolving K-feldspars. These transformations could alter reservoir flow pathways and induce mechanical risks, i.e. destabilising nearby faults or initiating reservoir collapse. Given feldspars’ prevalence in crustal rocks and CCS sandstone reservoirs, their reactive behaviour under in-situ conditions and in the presence of aggressive fluids demands greater attention.

Understanding how the minerals in reservoir rocks respond to CO₂ injection is vital for the success and safety of Carbon Capture and Storage (CCS) projects. Feldspars are the most common mineral in the Earth’s crust and act as primary framework grains in sandstones. Compared to quartz, feldspars are mechanically weak and chemically reactive. Dissolved feldspars can re-precipitate as clays, which in CCS reservoirs could impact fluid-flow. While caprock mineral stability is well studied, reservoir mineral reactivity, particularly of feldspars, remains understudied. To address this knowledge gap, we present microstructural and geochemical data from batch experiments that reacted CO₂-enriched fluids with feldspar-bearing sandstone sampled from the Captain Sandstone Member, the primary reservoir for the Acorn CCS Project (UK). Experiments were conducted in a hydrostatic pressure vessel at 70 MPa confining pressure, 50 MPa pore pressure, and temperatures ranging from 80 °C to 550 °C, using CO₂-enriched water to simulate reservoir conditions. Pre- and post-reaction samples were analysed using XRD, SEM-EDS, and XCT to assess microstructural and mineralogical changes. Results show that CO₂:feldspar interactions differ significantly from control experiments involving water alone. At reservoir-relevant temperatures (80 °C), incongruent dissolution of K-feldspar weakened grains which led to microfracturing. At 250 °C, CO₂ fluids caused total dissolution of calcite grains and cement and selective leaching of calcium from oligoclase, enriching the pore fluid with Ca²⁺. Above 400 °C, coupled dissolution–precipitation processes were observed, including congruent K-feldspar dissolution, secondary porosity development, and localised precipitation of Ca-aluminosilicates and K-bearing phases around dissolving K-feldspars. These transformations could alter reservoir flow pathways and induce mechanical risks, i.e. destabilising nearby faults or initiating reservoir collapse. Given feldspars’ prevalence in crustal rocks and CCS sandstone reservoirs, their reactive behaviour under in-situ conditions and in the presence of aggressive fluids demands greater attention.

Understanding how the minerals in reservoir rocks respond to CO₂ injection is vital for the success and safety of Carbon Capture and Storage (CCS) projects. Feldspars are the most common mineral in the Earth’s crust and act as primary framework grains in sandstones. Compared to quartz, feldspars are mechanically weak and chemically reactive. Dissolved feldspars can re-precipitate as clays, which in CCS reservoirs could impact fluid-flow. While caprock mineral stability is well studied, reservoir mineral reactivity, particularly of feldspars, remains understudied. To address this knowledge gap, we present microstructural and geochemical data from batch experiments that reacted CO₂-enriched fluids with feldspar-bearing sandstone sampled from the Captain Sandstone Member, the primary reservoir for the Acorn CCS Project (UK). Experiments were conducted in a hydrostatic pressure vessel at 70 MPa confining pressure, 50 MPa pore pressure, and temperatures ranging from 80 °C to 550 °C, using CO₂-enriched water to simulate reservoir conditions. Pre- and post-reaction samples were analysed using XRD, SEM-EDS, and XCT to assess microstructural and mineralogical changes. Results show that CO₂:feldspar interactions differ significantly from control experiments involving water alone. At reservoir-relevant temperatures (80 °C), incongruent dissolution of K-feldspar weakened grains which led to microfracturing. At 250 °C, CO₂ fluids caused total dissolution of calcite grains and cement and selective leaching of calcium from oligoclase, enriching the pore fluid with Ca²⁺. Above 400 °C, coupled dissolution–precipitation processes were observed, including congruent K-feldspar dissolution, secondary porosity development, and localised precipitation of Ca-aluminosilicates and K-bearing phases around dissolving K-feldspars. These transformations could alter reservoir flow pathways and induce mechanical risks, i.e. destabilising nearby faults or initiating reservoir collapse. Given feldspars’ prevalence in crustal rocks and CCS sandstone reservoirs, their reactive behaviour under in-situ conditions and in the presence of aggressive fluids demands greater attention.