Showing papers in "Geochemistry Geophysics Geosystems in 2019"

GlobSed: Updated total sediment thickness in the World’s oceans

Abstract: We present GlobSed, a new global 5‐arc‐minute total sediment thickness grid for the world's oceans and marginal seas. GlobSed covers a larger area than previously published global grids and incorporates updates for the NE Atlantic, Arctic, Southern Ocean, and Mediterranean regions, which results in a 29.7% increase in estimated total oceanic sediment volume. We use this new global grid and a revised global oceanic lithospheric age grid to assess the relationship between the total sediment thickness and age of the underlying oceanic lithosphere and its latitude. An analytical approximation model is used to mathematically describe sedimentation trends in major oceanic basins and to allow paleobathymetric reconstructions at any given geological time. This study provides a much‐needed update of the sediment thickness distribution of the world oceans and delivers a model for sedimentation rates on oceanic crust through time that agrees well with selected drill data used for comparison.

Effects of Improved 17O Correction on Interlaboratory Agreement in Clumped Isotope Calibrations, Estimates of Mineral-Specific Offsets, and Temperature Dependence of Acid Digestion Fractionation

Abstract: The clumped isotopic composition of carbonate-derived CO (denoted Δ) is a function of carbonate formation temperature and in natural samples can act as a recorder of paleoclimate, burial, or diagenetic conditions. The absolute abundance of heavy isotopes in the universal standards VPDB and VSMOW (defined by four parameters: R , R , R , and λ) impact calculated Δ values. Here, we investigate whether use of updated and more accurate values for these parameters can remove observed interlaboratory differences in the measured T-Δ relationship. Using the updated parameters, we reprocess 14 published calibration data sets measured in 11 different laboratories, representing many mineralogies, bulk compositions, sample types, reaction temperatures, and sample preparation and analysis methods. Exploiting this large composite data set (n = 1,253 sample replicates), we investigate the possibility for a “universal” clumped isotope calibration. We find that applying updated parameters improves the T-Δ relationship (reduces residuals) within most labs and improves overall agreement but does not eliminate all interlaboratory differences. We reaffirm earlier findings that different mineralogies do not require different calibration equations and that cleaning procedures, method of pressure baseline correction, and mass spectrometer type do not affect interlaboratory agreement. We also present new estimates of the temperature dependence of the acid digestion fractionation for Δ (Δ*), based on combining reprocessed data from four studies, and new theoretical equilibrium values to be used in calculation of the empirical transfer function. Overall, we have ruled out a number of possible causes of interlaboratory disagreement in the T-Δ relationship, but many more remain to be investigated.

The climate data toolbox for MATLAB

Abstract: Climate science is highly interdisciplinary by nature, so understanding interactions between Earth processes inherently warrants the use of analytical software that can operate across the disciplines of Earth science. Toward this end, we present the Climate Data Toolbox for MATLAB, which contains more than 100 functions that span the major climate-related disciplines of Earth science. The toolbox enables streamlined, entirely scriptable workflows that are intuitive to write and easy to share. Included are functions to evaluate uncertainty, perform matrix operations, calculate climate indices, and generate common data displays. Documentation is presented pedagogically, with thorough explanations of how each function works and tutorials showing how the toolbox can be used to replicate results of published studies. As a well-tested, well-documented platform for interdisciplinary collaborations, the Climate Data Toolbox for MATLAB aims to reduce time spent writing low-level code, let researchers focus on physics rather than coding and encourage more efficacious code sharing.

Magmatic Response to Subduction Initiation: Part 1. Fore‐arc Basalts of the Izu‐Bonin Arc From IODP Expedition 352

Abstract: The Izu‐Bonin‐Mariana (IBM) fore arc preserves igneous rock assemblages that formed during subduction initiation circa 52 Ma. International Ocean Discovery Program (IODP) Expedition 352 cored four sites in the fore arc near the Ogasawara Plateau in order to document the magmatic response to subduction initiation and the physical, petrologic, and chemical stratigraphy of a nascent subduction zone. Two of these sites (U1440 and U1441) are underlain by fore‐arc basalt (FAB). FABs have mid‐ocean ridge basalt (MORB)‐like compositions, however, FAB are consistently lower in the high‐field strength elements (TiO2, P2O5, Zr) and Ni compared to MORB, with Na2O at the low end of the MORB field and FeO* at the high end. Almost all FABs are light rare earth element depleted, with low total REE, and have low ratios of highly incompatible to less incompatible elements (Ti/V, Zr/Y, Ce/Yb, and Zr/Sm) relative to MORB. Chemostratigraphic trends in Hole U1440B are consistent with the uppermost lavas forming off axis, whereas the lower lavas formed beneath a spreading center axis. Axial magma of U1440B becomes more fractionated upsection; overlying off‐axis magmas return to more primitive compositions. Melt models require a two‐stage process, with early garnet field melts extracted prior to later spinel field melts, with up to 23% melting to form the most depleted compositions. Mantle equilibration temperatures are higher than normal MORB (1,400 °C–1,480 °C) at relatively low pressures (1–2 GPa), which may reflect an influence of the Manus plume during subduction initiation. Our data support previous models of FAB origin by decompression melting but imply a source more depleted than normal MORB source mantle.

Carbon Fluxes and Primary Magma CO2 Contents Along the Global Mid-Ocean Ridge System

Abstract: The concentration of carbon in primary mid-ocean ridge basalts (MORBs), and the associated fluxes of CO2 outgassed at ocean ridges, is examined through new data obtained by secondary ion mass spectrometry (SIMS) on 753 globally distributed MORB glasses. MORB glasses are typically 80–90% degassed of CO2. We thus use the limited range in CO2/Ba (81.3 ± 23) and CO2/Rb (991 ± 129), derived from undegassed MORB and MORB melt inclusions, to estimate primary CO2 concentrations for ridges that have Ba and/or Rb data. When combined with quality-controlled volatile-element data from the literature (n = 2,446), these data constrain a range of primary CO2 abundances that vary from 104 ppm to 1.90 wt%. Segment-scale data reveal a range in MORBmagma flux varying by a factor of 440 (from 6.8 × 10 to 3.0 × 10 m/year) and an integrated global MORB magma flux of 16.5 ± 1.6 km/year. When combined with CO2/Ba and CO2/Rb-derived primary magma CO2 abundances, the calculated segment-scale CO2 fluxes vary by more than 3 orders of magnitude (3.3 × 10 to 4.0 × 10 mol/year) and sum to an integrated global MORB CO2 flux of 1:32þ0:77 0:85 × 10 12 mol/year. Variations in ridge CO2 fluxes have a muted effect on global climate; however, because the vast majority of CO2 degassed at ridges is dissolved into seawater and enters the marine bicarbonate cycle. MORB degassing would thus only contribute to long-term variations in climate via degassing directly into the atmosphere in shallow-water areas or where the ridge system is exposed above sea level. Plain Language Summary Estimated CO2 contents of primary mid-ocean ridge basalts (MORB), calculated on a segment-by-segment basis, vary from 104 ppm to 1.9 wt%. CO2-enriched MORB are present in all ocean basins, in particular, in the Atlantic Ocean basin, which is younger and more likely to contain admixed material from recent subduction compared to the much older Pacific Ocean basin. CO2 fluxes at individual ridge segments vary by 3 orders of magnitude due primarily to large variability in primary CO2 content. This study provides the most detailed and accurate estimate to date of the integrated total flux of CO2 from mid-ocean ridges of 1:32 þ0:77 0:85 × 10 12 mol/year.

Pacific‐Panthalassic reconstructions: Overview, errata and the way forward

Abstract: We have devised a new absolute Late Jurassic‐Cretaceous Pacific plate model using a fixed hot spot approach coupled with paleomagnetic data from Pacific large igneous provinces (LIPs) while simultaneously minimizing plate velocity and net lithosphere rotation (NR). This study was motivated because published Pacific plate models for the 83.5‐ to 150‐Ma time interval are variably flawed, and their use affects modeling of the entire Pacific‐Panthalassic Ocean and interpretation of its margin evolution. These flaws could be corrected, but the revised models would imply unrealistically high plate velocities and NR. We have developed three new Pacific realm models with varying degrees of complexity, but we focus on the one that we consider most realistic. This model reproduces many of the Pacific volcanic paths, modeled paleomagnetic latitudes fit well with direct observations, plate velocities and NR resulting from the model are low, and all reconstructed Pacific LIPs align along the surface‐projected margin of the Pacific large low shear wave velocity province. The emplacement of the Shatsky Rise LIP at ~144 Ma probably caused a major plate boundary reorganization as indicated by a major jump of the Pacific‐Izanagi‐Farallon triple junction and a noteworthy change of the Pacific‐Izanagi seafloor spreading direction at around chron M20 time.

Nitrogen Oxide Concentrations in Natural Waters on Early Earth

Abstract: A key challenge in origins-of-life studies is estimating the abundances of species relevant to the chemical pathways proposed to have contributed to the emergence of life on early Earth. Dissolved nitrogen oxide anions (NO$_{X}^{-}$), in particular nitrate (NO$_{3}^{-}$) and nitrite (NO$_{2}^{-}$), have been invoked in diverse origins-of-life chemistry, from the oligomerization of RNA to the emergence of protometabolism. Recent work has calculated the supply of NO$_{X}^{-}$ from the prebiotic atmosphere to the ocean, and reported steady-state [NO$_{X}^{-}$] to be high across all plausible parameter space. These findings rest on the assumption that NO$_{X}^{-}$ is stable in natural waters unless processed at a hydrothermal vent. Here, we show that NO$_{X}^{-}$ is unstable in the reducing environment of early Earth. Sinks due to UV photolysis and reactions with reduced iron (Fe$^{2+}$) suppress [NO$_{X}^{-}$] by several orders of magnitude relative to past predictions. For pH$=6.5-8$ and $T=0-50^\circ$C, we find that it is most probable that NO$_{X}^{-}$]$<1~\mu$M in the prebiotic ocean. On the other hand, prebiotic ponds with favorable drainage characteristics may have sustained [NO$_{X}^{-}$]$\geq 1~\mu$M. As on modern Earth, most NO$_{X}^{-}$ on prebiotic Earth should have been present as NO$_{3}^{-}$, due to its much greater stability. These findings inform the kind of prebiotic chemistries that would have been possible on early Earth. We discuss the implications for proposed prebiotic chemistries, and highlight the need for further studies of NO$_{X}^{-}$ kinetics to reduce the considerable uncertainties in predicting [NO$_{X}^{-}$] on early Earth.

Mantle Dynamics of Western Pacific and East Asia: New Insights from P Wave Anisotropic Tomography

Abstract: Seismic anisotropy records past and present tectonic deformations and provides important constraints for understanding the structure and dynamics of the Earth's interior. In this work, we use tremendous amounts of high‐quality Pwave arrival times from local and regional earthquakes to determine a high‐resolution tomographic model of 3‐D P wave azimuthal anisotropy down to 1,000‐km depth beneath East Asia. Our results show that trench‐parallel fast‐velocity directions (FVDs) are visible in the shallow portion of the subducting Pacific slab (<80 km), whereas the deeper portion of the Pacific slab mainly exhibits trench‐normal FVDs, except for the stagnant slab in the mantle transition zone (MTZ) where obvious NE‐SW FVDs are revealed. The FVDs in the subslab mantle change from a subduction‐parallel trend at depths of 80–400 km to a subduction‐normal trend in the MTZ. Large‐scale low‐velocity anomalies are revealed beneath the Philippine Sea plate where the FVD is NE‐SW. The FVDs along the Izu‐Bonin arc and in a slab gap exhibit a striking anticlockwise toroidal trend. All these features may reflect complex 3‐D flows in the mantle wedge due to tearing and dehydration processes of the subducting Pacific slab. The subducting Pacific slab is split at ~300‐km depth under the Bonin arc and then penetrates into the lower mantle, whereas under East Asia the Pacific slab becomes stagnant in the MTZ and reaches the North‐South Gravity Lineament in China. The intraplate volcanoes in East Asia are caused by hot and wet upwelling flows in the big mantle wedge above the stagnant Pacific slab.

A Mantle Plume Origin for the Scandinavian Dyke Complex: A “Piercing Point” for 615 Ma Plate Reconstruction of Baltica?

Abstract: The origin of Large Igneous Provinces (LIPs) associated with continental breakup and the reconstruction of continents older than ca. 320 million years (pre-Pangea) are contentious research ...

Late Eocene Southern Ocean cooling and invigoration of circulation preconditioned Antarctica for full-scale glaciation

Abstract: During the Eocene‐Oligocene Transition (EOT; 34–33.5 Ma), Antarctic ice sheets relatively rapidly expanded, leading to the first continent‐scale glaciation of the Cenozoic. Declining atmospheric CO2 concentrations and associated feedbacks have been invoked as underlying mechanisms, but the role of the quasi‐coeval opening of Southern Ocean gateways (Tasman Gateway and Drake Passage) and resulting changes in ocean circulation is as yet poorly understood. Definitive field evidence from EOT sedimentary successions from the Antarctic margin and the Southern Ocean is lacking, also because the few available sequences are often incomplete and poorly dated, hampering detailed paleoceanographic and paleoclimatic analysis. Here we use organic dinoflagellate cysts (dinocysts) to date and correlate critical Southern Ocean EOT successions. We demonstrate that widespread winnowed glauconite‐rich lithological units were deposited ubiquitously and simultaneously in relatively shallow‐marine environments at various Southern Ocean localities, starting in the late Eocene (~35.7 Ma). Based on organic biomarker paleothermometry and quantitative dinocyst distribution patterns, we analyze Southern Ocean paleoceanographic change across the EOT. We obtain strong indications for invigorated surface and bottom water circulation at sites affected by polar westward‐flowing wind‐driven currents, including a westward‐flowing Antarctic Countercurrent, starting at about 35.7 Ma. The mechanism for this oceanographic invigoration remains poorly understood. The circum‐Antarctic expression of the phenomenon suggests that, rather than triggered by tectonic deepening of the Tasman Gateway, progressive pre‐EOT atmospheric cooling played an important role. At localities affected by the Antarctic Countercurrent, sea surface productivity increased and simultaneously circum‐Antarctic surface waters cooled. We surmise that combined, these processes contributed to preconditioning the Antarctic continent for glaciation.

Geo-fO(2): integrated software for analysis of magmatic oxygen fugacity

Abstract: Oxygen fugacity (fO(2)) is a fundamental thermodynamic property governing redox potential in solid Earth systems. Analysis of magmatic fO(2) aids our understanding of the valence state and solubility of multivalent elements during magma evolution. Specialized software, Geo-fO(2), was developed for calculating magmatic fO(2) on the basis of oxybarometers and thermobarometers for common minerals (amphibole, zircon, and biotite) in intermediate-silicic magmas. With user-friendly interfaces, it is easy to input files (.csv or Excel files), output data in Excel files, and plot results as binary diagrams that can be saved as vector graphics and modified using image-processing software.

Seismic Evidence for a Mantle Transition Zone Origin of the Wudalianchi and Halaha Volcanoes in Northeast China

Abstract: There exists much debate about origins of cretaceous-present volcanism in northeast (NE) China. Here we present high-resolution seismic images of the upper mantle beneath NE China by inverting P-wave travel-time data recorded by two dense linear arrays. The inclusion of the new data set has greatly improved sampling of the upper mantle beneath the study region, providing tight constraint on the seismic structure under the intraplate Wudalianchi and Halaha volcanoes. Local-scale low P-wave velocity (low-Vp) anomalies are revealed in the shallow mantle beneath the two volcanoes, whereas a large-scale high-Vp zone is imaged in the mantle transition zone (MTZ). These new results suggest that the two volcanoes, though located at different sites above the stagnant Pacific slab in the MTZ, are likely related to the deep subduction and dehydration of the Pacific slab, possibly through hot and wet upwellings in the big mantle wedge (BMW) beneath Wudalianchi and through deeper hydrous upwelling related to slab avalanche beneath Halaha. Our results also reveal other striking features, such as high-Vp structures resting atop the 410 km discontinuity beneath the Great Xing’an Range and the Songliao Basin, which are attributed to detached continental lithosphere. The delamination most likely occurred in the Cretaceous, which induced widespread magmatism in NE China.

Relationships Between Olivine CPO and Deformation Parameters in Naturally Deformed Rocks and Implications for Mantle Seismic Anisotropy

Abstract: We analyze peridotites from a wide range of tectonic settings to investigate relationships between olivine lattice preferred orientation (LPO) and deformation conditions in naturally deformed rocks...

Dynamics of Outgassing and Plume Transport Revealed by Proximal Unmanned Aerial System (UAS) Measurements at Volcán Villarrica, Chile

Abstract: Volcanic gas emissions are intimately linked to the dynamics of magma ascent and outgassing, and, on geological timescales, constitute an important source of volatiles to the Earth's atmosphere. Measurements of gas composition and flux are therefore critical to both volcano monitoring and to determining the contribution of volcanoes to global geochemical cycles. However, significant gaps remain in our global inventories of volcanic emissions, (particularly for CO2, which requires proximal sampling of a concentrated plume) for those volcanoes where the near‐vent region is hazardous or inaccessible. Unmanned Aerial Systems (UAS) provide a robust and effective solution to proximal sampling of dense volcanic plumes in extreme volcanic environments. Here, we present gas compositional data acquired using a gas sensor payload aboard a UAS flown at Volcan Villarrica, Chile. We compare UAS‐derived gas timeseries to simultaneous crater rim multi‐GAS data and UV camera imagery to investigate early plume evolution. SO2 concentrations measured in the young proximal plume exhibit periodic variations that are well‐correlated with the concentrations of other species. By combining molar gas ratios (CO2/SO2 = 1.48–1.68, H2O/SO2 = 67–75 and H2O/CO2 = 45–51) with the SO2 flux (142 ± 17 t/day) from UV camera images, we derive CO2 and H2O fluxes of ~150 t/day and ~2850 t/day, respectively. We observe good agreement between time‐averaged molar gas ratios obtained from simultaneous UAS‐ and ground‐based multi‐GAS acquisitions. However, the UAS measurements made in the young, less diluted plume reveal additional short‐term periodic structure that reflects active degassing through discrete, audible gas exhalations.

Late Miocene Intensified Tectonic Uplift and Climatic Aridification on the Northeastern Tibetan Plateau: Evidence From Clay Mineralogical and Geochemical Records in the Xining Basin

Abstract: The uplift of the Tibetan Plateau (TP) during the late Cenozoic is thought to be one of crucial factors controlling Asian climate. However, the complex interaction between tectonics and climate change is still unclear. Here we present the first record of clay mineralogy and elemental geochemistry covering ~12.7–4.8 Ma in a fluvial‐lacustrine sequence in the Xining Basin. Geochemical provenance proxies (Th/Sc, Zr/Sc, and Cr/Zr) in the <2‐μm fraction show a significant provenance change at ~8.8 Ma. Silicate‐based weathering indexes (CIA, CIW, and PIA) displayed coeval changes with provenance but discrepant changes with regional climate. Since the clay mineralogy exhibits significant change at ~7.8 Ma uncorrelated with modifications in provenance, it can be employed to reveal regional climate change. The rise in illite and associated decrease in the sum of smectite and illite/smectite mixed layers reflect gradual and slow aridification since ~12.7 Ma with intensified drying since ~7.8 Ma until approaching the modern climate status. Our results, together with other regional climatic and tectonic records, clearly illustrate that accelerated uplift of the northeastern TP since ~8–9 Ma has mainly modulated the regional erosion, weathering, transportation, and sedimentation and amplified the global cooling and drying trend toward the regional climate of modern conditions. Our study suggests that in the tectonically active northeastern TP, a comprehensive mineralogical and geochemical investigation of the fine‐grained fraction of the basin sediments could retrieve the interactions between tectonics and climate behind the complex change in exhumed lithology and sedimentary routing systems.

Magma Chamber Growth During Intercaldera Periods: Insights From Thermo‐Mechanical Modeling With Applications to Laguna del Maule, Campi Flegrei, Santorini, and Aso

Abstract: Crustal magma chambers can grow to be hundreds to thousands of cubic kilometers, potentially feeding catastrophic caldera‐forming eruptions. Smaller‐volume chambers are expected to erupt frequently and freeze quickly; a major outstanding question is how magma chambers ever grow to the sizes required to sustain the largest eruptions on Earth. We use a thermo‐mechanical model to investigate the primary factors that govern the extrusive:intrusive ratio in a chamber, and how this relates to eruption frequency, eruption size, and long‐term chamber growth. The model consists of three fundamental timescales: the magma injection timescale τin, the cooling timescale τcool, and the timescale for viscous relaxation of the crust τrelax. We estimate these timescales using geologic and geophysical data from four volcanoes (Laguna del Maule, Campi Flegrei, Santorini, Aso) to compare them with the model. In each of these systems, τin is much shorter than τcool and slightly shorter than τrelax, conditions that in the model are associated with efficient chamber growth and simultaneous eruption. In addition, the model suggests that the magma chambers underlying these volcanoes are growing at rates between ~10‐4‐10‐2 km3/yr, speeding up over time as the chamber volume increases. We find scaling relationships for eruption frequency and size that suggest that as chambers grow and volatiles exsolve, eruption frequency decreases but eruption size increases. These scaling relationships provide a good match to the eruptive history from the natural systems, suggesting the relationships can be used to constrain chamber growth rates and volatile saturation state from the eruptive history alone.

How Do Variably Striking Faults Reactivate During Rifting? Insights From Southern Malawi

Abstract: Crustal extension is commonly thought to be accommodated by faults that strike orthogonal and obliquely to the regional trend of the minimum compressive stress (σ3). Activation of oblique faults can, however, be conceptually problematic as under Andersonian faulting, it requires preexisting crustal weaknesses, high fluid pressures, and/or stress rotations. Furthermore, measurements of incremental fault displacements, which are typically used to identify oblique faulting, do not necessarily reflect regional stresses. Here, we assess oblique faulting by calculating the stress ratio (σ3/σ1, where σ1 is the maximum compressive stress), slip tendency, and effective coefficient of friction (μs′) required to reactivate variably striking normal faults under different trends of σ3. We apply this analysis to NW and NNE striking active faults at the southern end of the Malawi Rift, where NE‐SW, ENE‐WSW, E‐W, and SE‐NW σ3 trends have previously been proposed. A uniform σ3 trend is inferred for this region as recent joints sets do not rotate along the rift. With a NE‐SW trending σ3, NW‐striking faults are well oriented, however, NNE‐striking faults require μs′ 0.55. These σ3 trends are also comparable to a focal mechanism stress inversion, regional joint orientations, and previously reported geodetically derived extension directions. We therefore conclude that unlike typical models of oblique rifting, the southern Malawi Rift consists of faults that all strike slightly oblique to σ3.

Generation of Earth's Early Continents From a Relatively Cool Archean Mantle

Abstract: Several lines of evidence suggest that the Archean (4.0–2.5 Ga) mantle was hotter than today’s potential temperature (TP ) of 1350 ýC. However, the magnitude of such dierence is poorly constrained, with TP estimation spanning from 1500 ýC to 1600 ýC during the Meso-Archean (3.2-2.8 Ga). Such dierences have major implications for the interpreted mechanisms of continental crust generation on the early Earth, as their ecacy is highly sensitive to the TP . Here, we integrate petrological modeling with thermo-mechanical simulations to understand the dynamics of crust formation during Archean. Our results predict that partial melting of primitive oceanic crust produces felsic melts with geochemical signatures matching those observed in Archean cratons from a mantle TP as low as 1450 ýC thanks to lithospheric-scale Rayleigh–Taylor-type instabilities. These simulations also infer the occurrence of intraplate deformation events that allow an ecient transport of crustal material into the mantle, hydrating it.

Trans-Dimensional Surface Reconstruction With Different Classes of Parameterization

Abstract: Portions of this software development were funded by AuScope through the National Collaborative Research Infrastructure Strategy (NCRIS), an Australian Federal Government programme. R. H. and T. B. are funded by the European Unions Horizon 2020 research and innovation programme under grant agreement 716542.

Pockmarks in the Witch Ground Basin, Central North Sea

Abstract: Marine sediments host large amounts of methane (CH4), which is a potent greenhouse gas. Quantitative estimates for methane release from marine sediments are scarce, and a poorly constrained temporal variability leads to large uncertainties in methane emission scenarios. Here, we use 2D and 3D seismic reflection, multibeam bathymetric, geochemical and sedimentological data to (I) map and describe pockmarks in the Witch Ground Basin (central North Sea), (II) characterize associated sedimentological and fluid migration structures, and (III) analyze the related methane release. More than 1500 pockmarks of two distinct morphological classes spread over an area of 225 km2. The two classes form independently from another and are corresponding to at least two different sources of fluids. Class 1 pockmarks are large in size (> 6 m deep, > 250 m long, and > 75 m wide), show active venting, and are located above vertical fluid conduits that hydraulically connect the seafloor with deep methane sources. Class 2 pockmarks, which comprise 99.5 % of all pockmarks, are smaller (0.9‐3.1 m deep, 26‐140 m long, and 14‐57 m wide) and are limited to the soft, fine‐grained sediments of the Witch Ground Formation and possibly sourced by compaction‐related dewatering. Buried pockmarks within the Witch Ground Formation document distinct phases of pockmark formation, likely triggered by external forces related to environmental changes after deglaciation. Thus, greenhouse gas emissions from pockmark fields cannot be based on pockmark numbers and present‐day fluxes but require an analysis of the pockmark forming processes through geological time.