R-3 Repository :: Browsing by Author "Dickens, Gerald R."

Browsing by Author "Dickens, Gerald R."

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Assessing offsets between the δ13C of sedimentary components and the global exogenic carbon pool across early Paleogene carbon cycle perturbations
(Wiley, 2012) Sluijs, Appy; Dickens, Gerald R.
[1] Negative stable carbon isotope excursions (CIEs) across the Paleocene–Eocene thermal maximum (PETM; ~56 Ma) range between 2‰ and 7‰, even after discounting sections with truncated records. Individual carbon isotope records differ in shape and magnitude from variations in the global exogenic carbon cycle through changes in (1) the relative abundance of mixed components with different δ13C within a measured substrate, (2) isotope fractionation through physiological change, and (3) the isotope composition of the carbon source. All three factors likely influence many early Paleogene δ13C records, especially across the PETM and other hyperthermal events. We apply these concepts to late Paleocene–early Eocene (∼58–52 Ma) records from Lomonosov Ridge, Arctic Ocean. Linear regression analyses show correlations between the δ13C of total organic carbon (TOC) and two proxies for the relative contribution of terrestrial organic components to sediment TOC: the branched and isoprenoid tetraether index and palynomorphs. We use these correlations to subtract the terrestrial component from δ13CTOC and calculate marine organic matter δ13C. The results show that the magnitude of the CIE in δ13CTOC across the PETM is exaggerated relative to the magnitude of the CIE in δ13CMOM by ~3‰ due to increased contributions of terrestrial organic carbon during the event. Collectively, all carbon isotope records across the PETM and other major climate–carbon cycle perturbations in Earth's history are potentially biased through one or more of the above factors. Indeed, it is highly unlikely that any δ13C record shows the true shape and magnitude of the CIE for the global exogenic carbon cycle. For the PETM, we conclude that CIE in the exogenic carbon cycle is likely <4‰, but it will take additional analyses and modeling to obtain an accurate value for this CIE.
Barium cycling in shallow sediment above active mud volcanoes in the Gulf of Mexico
(2005) Castellini, D. Grace; Dickens, Gerald R.
Two mud volcanoes in the Gulf of Mexico were examined to understand barium cycling in shallow seafloor sediment at regions of intense methane expulsion. Due to anaerobic oxidation of methane and sulfate reduction, barium interacts with methane and sulfate, producing barite fronts at the sulfate-methane transition and barium-rich pore fluids underneath. Formation waters likely feed volcanoes and deliver large amounts of Ba2+ to the system from below. Locally elevated Ba2+ concentrations amplified cycling, producing sigmodial pore water profiles in the shallowest sediment. The expulsion of Ba-rich fluids directly to the water column concentrates barite in surficial sediments, which can also enhance cycling. Further, two types of barium-rich, carbonate nodules were recovered. Rocky nodules resemble buried carbonate crusts, diagenetically altered by pore fluids. Smooth nodules are likely preserved barite fronts marking past sulfate-methane transitions. Both types of nodules can serve as a modern analogue for barite deposits in the geologic record.
Carbon and oxygen isotopes of bulk carbonate in sediment deposited beneath the eastern equatorial Pacific over the last 8 million years
(Wiley, 2015) Reghellin, Daniele; Coxall, Helen K.; Dickens, Gerald R.; Backman, Jan
To improve the understanding and utility of bulk carbonate stable carbon and oxygen isotope measurements, we examine sediment from cores in the eastern equatorial Pacific that span the last 8 Ma. We measured δ13C and δ18O in 791 samples from Integrated Ocean Drilling Program Site U1338 and Deep Sea Drilling Project Site 573, both located close to the Pacific equator. In 100 samples, we measured δ13C and δ18O on isolated <63 µm and <38 µm fractions, which concentrates calcareous nannofossil carbonate and progressively excludes foraminiferal carbonate. Bulk carbonate δ13C and δ18O records are similar to published records from other sites drilled near the equator and seem to reflect mixed layer conditions, albeit with some important caveats involving the precipitation of calcite by coccolithophores. The comparatively lower δ13C and δ18O of the <63 µm and <38 µm fractions in sediments younger than 4.4 Ma is attributed to an increase in deep-dwelling planktic foraminifera material in bulk carbonate, shifting the bulk isotopic signals toward higher values. Bulk carbonate δ13C is similar over 2500 km along the Pacific equator, suggesting covarying concentrations and δ13C of dissolved inorganic carbon within surface waters since 8 Ma. Greater bulk sediment δ13C and δ18O, higher sedimentation rates, and low content of coarse material suggest intensified wind-driven upwelling and enhanced primary productivity along the Pacific equator between 8.0 and 4.4 Ma, although a full understanding of bulk carbonate records will require extensive future work.
Carbon cycle and climate fluctuations during the early Paleogene: Sedimentological characteristics and environmental ramifications
(2014-11-13) Slotnick, Benjamin; Dickens, Gerald R.; Lee, Cin-Ty A.; Anderson, John B.; Masiello, Carrie A.; Rudolf, Volker H.W.
The early Paleogene was marked by extensive changes related to Earth surface temperature, carbon cycling, and the hydrological cycle. This included at least two, and probably more, geologically brief (~200-k.yr.) intervals of extreme warming, the Paleocene-Eocene thermal maximum (PETM) and the Eocene thermal maximum-2 (ETM-2) along with a moderately-long (~1.5-2 M.yr.) period of warmth (i.e.; Early Eocene Climatic Optimum [EECO]). This was preceded by a moderately-long (~2 M.yr.) period of cool conditions (i.e. Paleocene Carbon Isotope Maximum [PCIM]) and followed by the initiation of long-term cooling through the Cenozoic. The long-term rise in warmth and numerous short-term “hyperthermal” events, marked by pronounced negative carbon isotope excursions and clay-rich horizons (Zachos et al., 2005; Nicolo et al., 2007), have been linked to massive injections of 13C-depleted carbon into the ocean-atmosphere system and intense global climate change, but the exact character of the hyperthermals is not well-recognized. To better constrain and understand their occurrences, well-resolved and high-resolution records across the entire interval of interest is necessary. Preceding studies demonstrated major fluctuations in carbon cycling and terrestrial weathering (e.g.; Nicolo et al., 2007) during the latest Paleocene and earliest Eocene as well as a significant drop of dissolved oxygen concentrations during the PETM onset (Nicolo et al., 2010). However, causes, environmental impact, and relationships relating carbon release and capture to terrigenous runoff and surface temperatures throughout the exogenic system, particularly in spatial and temporal contexts, remain unclear. The southern Pacific region (Clarence Valley) and Indian Ocean (Ninetyeast Ridge) represent large realms of our planet’s ocean system that have been largely overlooked in part due to limited Paleogene core availability from Integrated Ocean Drilling Program (IODP). There is a need to better improve records related to carbon in the exogenic system throughout the Cenozoic, and in particular, the Paleogene. As of now, records lack in these regions. Clarence Valley in eastern Marlborough, New Zealand trends southwest for ~80 km between the Seaward and Inland Kaikouras. Along the northwest margin is a succession of streams, including Mead and Branch Streams, which have incised and exposed an uplifted and rotated block consisting of Amuri Limestone, a calcareous-rich formation within the Muzzle Group. These outer shelf and upper continental slope strata originally accumulated as terrigenous detrital clay minerals, biogenic silica, and biogenic carbonate. This was situated contiguous to a neritic carbonate platform along a passive margin of proto-New Zealand at ~55-50°S latitude. Ninetyeast Ridge, one of the longest near-linear features on Earth with a length of ~4600 km (from ~10ºN to ~31ºS), is located in the Indian Ocean. Numerous Deep Sea Drilling Program (DSDP) and Ocean Drilling Program (ODP) sites have been drilled adjacent to or along the ridge crest. Three sites in particular drilled during DSDP Leg 22 (i.e.; Sites 213, 214, and 215) include calcareous-rich material from much of the Paleogene. These sediments can provide additional constraints to further our understanding of Indian Ocean paleoceanographic changes during the Paleogene. The lack of data for the Paleogene has been an issue for quite some time now. As such, the Paleogene remains a specific interval of time that has the potential to be much better understood by integrating current views of carbon cycling to new and wellresolved data-sets. Here I analyzed sequences exposed in Clarence Valley and sections from Ninetyeast Ridge DSDP Sites to address these issues. These records were integrated into a global context to relate short-term (<100 k.yr.) and long-term (> 1 m.yr) changes. I generated lithologic and carbon isotopic records to evaluate the entire period of interest, including the PCIM, specific hyperthermals, EECO, and the Middle Eocene Climatic Optimum (MECO). Integration of each section revealed similarities, including long-term trends with similar carbon isotopic baselines and short-term events. Expanded marl-rich units concurrent to lower δ13C, specifically across CIEs, generally characterized marginal sedimentation whereas condensed intervals largely spanned deep-water settings, resulting from carbonate dissolution. Together, these studies indicate carbon addition and removal mechanisms repeatedly spanned much of the Paleogene, causing calcite compensation depth (CCD) fluctuations, related to lithologic, and carbon isotopic changes.
Constraints on Ocean Acidification Associated with Rapid and Massive Carbon Injections of the Early Paleogene: The Geological Record at Ocean Drilling Program Site 1215, Equatorial Pacific Ocean
(2012) Leon-Rodriguez, Lizette; Dickens, Gerald R.
Massive amounts of 13 C-depleted carbon rapidly entered the ocean more than once during the early Paleogene, providing a geological framework for understanding future perturbations in carbon cycling, including ocean acidification. To assess the number of events and their impact on deep-sea carbonate accumulation, I have studied carbonate ooze units of the upper Paleocene-lower Eocene, which were deposited on a subsiding flank of the East Pacific Rise (ODP Site 1215). From this record several proxies were used to ascertain changes in carbonate dissolution: carbonate content, foraminiferal test fragmentation, and planktic/benthic foraminiferal ratio. Based on these analyses, 1 observe that carbonate preservation generally increased from the late Paleocene (56 Ma) through the early Eocene (51.5 Ma), after which it became poor to negligible. This trend was punctuated by four short-term intervals characterized by carbonate dissolution and pronounced negative d 18 O and d 13 C excursions. It is inferred that these were anomalously warm periods (hyperthermals) caused by massive and relative fast 13 C-depleted carbon injections. These correspond to the PETM (∼55.5 Ma), H1/ETM-2 (∼53.7 Ma), I1 (∼53.2 Ma), and K/X (∼52.5 Ma) events. I also calculated carbonate, planktic, and benthic foraminiferal mass accumulation rates for the Site 1215. These were used to comprehensively examine the history of carbonate accumulation in the equatorial Pacific Ocean throughout the early Paleogene. I deduce that in the long-term (>10 5 yr) the lysocline and calcite compensation depth (CCD) generally deepened between 55.4 and 51.5 Ma; but rapidly (≤10 5 yr) shoaled and subsequently overcompensated during and after the four intervals of massive carbon injection. Planktic foraminiferal assemblages found in the record of Site 1215 follow a predicted pattern for selective dissolution. Species of Acarinina are preferentially preserved over Morozovella, which are preferentially preserved over Subbotina, Igorina and Globanomalina. A tiny and previously overlooked species, Praetenuitella antica n.sp, is formally described in this manuscript. This species is also resistant to dissolution. The findings of this study provide firm constraints to model the short and long-term carbon cycle dynamics during the early Paleogene
The DeepMIP contribution to PMIP4: methodologies for selection, compilation and analysis of latest Paleocene and early Eocene climate proxy data, incorporating version 0.1 of the DeepMIP database
(Copernicus Publications, 2019) Hollis, Christopher J.; Dunkley Jones, Tom; Anagnostou, Eleni; Bijl, Peter K.; Cramwinckel, Margot J.; Cui, Ying; Dickens, Gerald R.; Edgar, Kirsty M.; Eley, Yvette; Evans, David; Foster, Gavin L.; Frieling, Joost; Inglis, Gordon N.; Kennedy, Elizabeth M.; Kozdon, Reinhard; Lauretano, Vittoria; Lear, Caroline H.; Littler, Kate; Lourens, Lucas; Meckler, A. Nele; Naafs, B. David A.; Pälike, Heiko; Pancost, Richard D.; Pearson, Paul N.; Röhl, Ursula; Royer, Dana L.; Salzmann, Ulrich; Schubert, Brian A.; Seebeck, Hannu; Sluijs, Appy; Speijer, Robert P.; Stassen, Peter; Tierney, Jessica; Tripati, Aradhna; Wade, Bridget; Westerhold, Thomas; Witkowski, Caitlyn; Zachos, James C.; Zhang, Yi Ge; Huber, Matthew; Lunt, Daniel J.
The early Eocene (56 to 48 million years ago) is inferred to have been the most recent time that Earth's atmospheric CO2 concentrations exceeded 1000 ppm. Global mean temperatures were also substantially warmer than those of the present day. As such, the study of early Eocene climate provides insight into how a super-warm Earth system behaves and offers an opportunity to evaluate climate models under conditions of high greenhouse gas forcing. The Deep Time Model Intercomparison Project (DeepMIP) is a systematic model–model and model–data intercomparison of three early Paleogene time slices: latest Paleocene, Paleocene–Eocene thermal maximum (PETM) and early Eocene climatic optimum (EECO). A previous article outlined the model experimental design for climate model simulations. In this article, we outline the methodologies to be used for the compilation and analysis of climate proxy data, primarily proxies for temperature and CO2. This paper establishes the protocols for a concerted and coordinated effort to compile the climate proxy records across a wide geographic range. The resulting climate “atlas” will be used to constrain and evaluate climate models for the three selected time intervals and provide insights into the mechanisms that control these warm climate states. We provide version 0.1 of this database, in anticipation that this will be expanded in subsequent publications.
Eocene (46–44 Ma) Onset of Australia‐Pacific Plate Motion in the Southwest Pacific Inferred From Stratigraphy in New Caledonia and New Zealand
(Wiley, 2020) Dallanave, Edoardo; Maurizot, Pierre; Agnini, Claudia; Sutherland, Rupert; Hollis, Christopher J.; Collot, Julien; Dickens, Gerald R.; Bachtadse, Valerian; Strogen, Dominic; Morgans, Hugh E.G.
The Pacific plate circuit went through a complex reorganization during the early to middle Eocene, approximately coinciding with the onset of subduction along the western Pacific margin. However, the timing and dynamics of this change in the southwest Pacific and evolution of subduction beneath the Tonga‐Kermadec Arc are not fully resolved. We present magneto‐biostratigraphic data from an early to middle Eocene sedimentary section exposed in the Koumac‐Gomen area, New Caledonia, which is an emerged portion of the Norfolk Ridge. The 260 m‐thick succession contains a transition from pelagic micrite to terrigenous‐rich calciturbidite that is observed regionally in New Caledonia and which is interpreted to represent a shift from sedimentation on a stable submarine plateau to slope formation developed under a convergent tectonic regime. The stratigraphic contact between pelagic micrite and overlying calciturbidite is not exposed, but our magnetic polarity‐based chronology constrains the age of transition to 46–44 Ma, in agreement with the 45.3 Ma age recently obtained from the Noumea area in southern New Caledonia. We integrate records from New Caledonia with recent magnetostratigraphic data from South Island, New Zealand, where marked variations in terrigenous input occurred during the early and middle Eocene. Synchronous sedimentary changes in the southwest Pacific occurred at the same time as onset of rapid seafloor spreading south of Australia and New Zealand. We infer that the underlying cause of stratigraphic change was inception of slip at a new configuration of the Australia‐Pacific plate boundary, which evolved into the Tonga‐Kermadec subduction system.
Evidence for methanogenesis on slope sites during the late Paleocene and early Eocene: Carbonate concretions from the Dukla Nappe, outer Carpathians, Poland
(2009) Silver, Andrew C.; Dickens, Gerald R.
The late Paleocene to early Eocene was a prolonged period of global warming punctuated by abrupt intervals of rapid temperature rise. These hyperthermal events, especially including the Paleocene-Eocene thermal maximum, are characterized by negative carbon isotope excursions (CIEs), which signify massive input of 13C-depleted carbon. A widely-discussed mechanism to explain such carbon injection is destabilization and degassing of methane hydrates in marine sediment. Although large amounts of methane hydrate deposits should have existed during the late Paleocene and early Eocene, evidence has been limited. This study documents late Paleocene and early Eocene siderite-dominated carbonate concretions hosted in turbidites of the Dukla Nappe, Outer Carpathians, Poland. These concretions have delta13C ranges attributable to formation in methanogenic environments. Furthermore, grain-to-grain relationships and preserved sedimentary fabrics indicate authigenic formation prior to compaction. Given that they were deposited in sufficient water depth to host stable hydrates despite elevated ocean temperatures, these concretions provide supporting evidence of active methanogenesis and the accumulation of methane hydrates during the late Paleocene and early Eocene.
Fluid relationships in the Northern Gulf of Mexico using dissolved ion concentrations and strontium isotopes
(2008) Hubbard, L. Ashley; Dugan, Brandon; Dickens, Gerald R.
Pore fluids from the slope in the Gulf of Mexico demonstrate specific ion enrichment and a range in concentration. Dissolved metal and halogen fluid concentrations and strontium isotope ratios from ten sites on the Gulf of Mexico (GoM) continental slope were compared to identify the variations in chemistry. Geochemical discrepancies are interpreted as coming from chloride sources, fluid mixing and fluid diagenesis at depth. We have adapted a grouping scheme developed by Fu and Aharon (1998) in order to highlight seep fluid relationships, including seep fluid from six additional locations. Chloride sources were assessed based on bromide to chloride and sodium to chloride trends and strontium isotope ratios. Chloride source end members include connate seawater, dissolved salt, and ancient evaporated seawater. Out of the sites examined, three sites are classified as having a chloride signal dominated by salt dissolution. Bromide to chloride ratios fall between 0.23 x 10 -3 and seawater (1.5 x 10 -3 ). Sodium to chloride ratios fall between 1.16 and seawater (0.85) and strontium ratios have a large distribution (0.707911-0.709220). Evidence of subaerially evaporated seawater is preserved in fluids from at least two or three sites. Bromide to chloride ratios fall between 2.47 x 10 -3 and seawater. Sodium to chloride ratios fall between 0.75 and seawater and strontium ratios demonstrate a narrow range of values in the least altered fluids (0.708662-0.709172). The majority of saline vent fluids demonstrate mixing between chloride source end members and a wide range of dissolved ion concentrations. Diverse ion enrichment behavior clearly indicates that the processes controlling GoM seep fluid chemistries are complicated and often site dependent resulting from differences in flux, early brine generation and fluid/sediment interactions.
Gas hydrate dissociation prolongs acidification of the Anthropocene oceans
(Wiley, 2015) Boudreau, Bernard P.; Luo, Yiming; Meysman, Filip J.R.; Middelburg, Jack J.; Dickens, Gerald R.
Anthropogenic warming of the oceans can release methane (CH4) currently stored in sediments as gas hydrates. This CH4 will be oxidized to CO2, thus increasing the acidification of the oceans. We employ a biogeochemical model of the multimillennial carbon cycle to determine the evolution of the oceanic dissolved carbonate system over the next 13 kyr in response to CO2 from gas hydrates, combined with a reasonable scenario for long-term anthropogenic CO2 emissions. Hydrate-derived CO2 will appreciably delay the neutralization of ocean acidity and the return to preindustrial-like conditions. This finding is the same with CH4 release and oxidation in either the deep ocean or the atmosphere. A change in CaCO3 export, coupled to CH4 release, would intensify the transient rise of the carbonate compensation depth, without producing any changes to the long-term evolution of the carbonate system. Overall, gas hydrate destabilization implies a moderate additional perturbation to the carbonate system of the Anthropocene oceans.
High-resolution and high-precision correlation of dark and light layers in the Quaternary hemipelagic sediments of the Japan Sea recovered during IODP Expedition 346
(Springer, 2018) Tada, Ryuji; Irino, Tomohisa; Ikehara, Ken; Karasuda, Akinori; Sugisaki, Saiko; Xuan, Chuang; Sagawa, Takuya; Itaki, Takuya; Kubota, Yoshimi; Lu, Song; Seki, Arisa; Murray, Richard W.; Alvarez-Zarikian, Carlos; Anderson, William T. Jr.; Bassetti, Maria-Angela; Brace, Bobbi J.; Clemens, Steven C.; Gurgel, Marcio H. da Costa; Dickens, Gerald R.; Dunlea, Ann G.; Gallagher, Stephen J.; Giosan, Liviu; Henderson, Andrew C.G.; Holbourn, Ann E.; Kinsley, Christopher W.; Lee, Gwang Soo; Lee, Kyung Eun; Lofi, Johanna; Lopes, Christina I.C.D.; Saavedra-Pellitero, Mariem; Peterson, Larry C.; Singh, Raj K.; Toucanne, Samuel; Wan, Shiming; Zheng, Hongbo; Ziegler, Martin
The Quaternary hemipelagic sediments of the Japan Sea are characterized by centimeter- to decimeter-scale alternation of dark and light clay to silty clay, which are bio-siliceous and/or bio-calcareous to a various degree. Each of the dark and light layers are considered as deposited synchronously throughout the deeper (> 500 m) part of the sea. However, attempts for correlation and age estimation of individual layers are limited to the upper few tens of meters. In addition, the exact timing of the depositional onset of these dark and light layers and its synchronicity throughout the deeper part of the sea have not been explored previously, although the onset timing was roughly estimated as ~ 1.5 Ma based on the result of Ocean Drilling Program legs 127/128. Consequently, it is not certain exactly when their deposition started, whether deposition of dark and light layers was synchronous and whether they are correlatable also in the earlier part of their depositional history. The Quaternary hemipelagic sediments of the Japan Sea were drilled at seven sites during Integrated Ocean Drilling Program Expedition 346 in 2013. Alternation of dark and light layers was recovered at six sites whose water depths are > ~ 900 m, and continuous composite columns were constructed at each site. Here, we report our effort to correlate individual dark layers and estimate their ages based on a newly constructed age model at Site U1424 using the best available paleomagnetic datum and marker tephras. The age model is further tuned to LR04 δ18O curve using gamma ray attenuation density (GRA) since it reflects diatom contents that are higher during interglacial high-stands. The constructed age model for Site U1424 is projected to other sites using correlation of dark layers to form a high-resolution and high-precision paleo-observatory network that allows to reconstruct changes in material fluxes with high spatio-temporal resolutions.
The impact of lithologic heterogeneity and focused fluid flow upon gas hydrate distribution in marine sediments
(American Geophysical Union, 2014) Chatterjee, Sayantan; Bhatnagar, Gaurav; Dugan, Brandon; Dickens, Gerald R.; Chapman, Walter G.; Hirasaki, George J.
Gas hydrate and free gas accumulation in heterogeneous marine sediment is simulated using a two-dimensional (2-D) numerical model that accounts for mass transfer over geological timescales. The model extends a previously documented one-dimensional (1-D) model such that lateral variations in permeability (k) become important. Various simulations quantitatively demonstrate how focused fluid flow through high-permeability zones affects local hydrate accumulation and saturation. Simulations that approximate a vertical fracture network isolated in a lower permeability shale (kfracture >> kshale) show that focused fluid flow through the gas hydrate stability zone (GHSZ) produces higher saturations of gas hydrate (25–70%) and free gas (30–60%) within the fracture network compared to surrounding shale. Simulations with a dipping, high-permeability sand layer also result in elevated saturations of gas hydrate (60%) and free gas (40%) within the sand because of focused fluid flow through the GHSZ. Increased fluid flux, a deep methane source, or both together increase the effect of flow focusing upon hydrate and free gas distribution and enhance hydrate and free gas concentrations along the high-permeability zones. Permeability anisotropy, with a vertical to horizontal permeability ratio on the order of 10−2, enhances transport of methane-charged fluid to high-permeability conduits. As a result, gas hydrate concentrations are enhanced within these high-permeability zones. The dip angle of these high-permeability structures affects hydrate distribution because the vertical component of fluid flux dominates focusing effects. Hydrate and free gas saturations can be characterized by a local Peclet number (localized, vertical, focused, and advective flux relative to diffusion) relative to the methane solubility gradient, somewhat analogous to such characterization in 1-D systems. Even in lithologically complex systems, local hydrate and free gas saturations might be characterized by basic parameters (local flux and diffusivity).
Large-Amplitude Variations in Carbon Cycling and Terrestrial Weathering during the Latest Paleocene and Earliest Eocene: The Record at Mead Stream, New Zealand
(University of Chicago Press, 2012) Slotnick, Benjamin S.; Dickens, Gerald R.; Nicolo, Micah J.; Hollis, Christopher J.; Crampton, James S.; Zachos, James C.; Sluijs, Appy
The late Paleocene to early Eocene was marked by major changes in Earth surface temperature and carbon cycling. This included at least two, and probably more, geologically brief (<200-k.yr.) intervals of extreme warming, the Paleocene-Eocene thermal maximum (PETM) and the Eocene thermal maximum-2 (ETM-2). The long-term rise in warmth and short-term “hyperthermal” events have been linked to massive injections of 13C-depleted carbon into the ocean-atmosphere system and intense global climate change. However, the causes, environmental impact, and relationships remain uncertain because detailed and coupled proxy records do not extend across the entire interval of interest; we are still recognizing the exact character of the hyperthermals and developing models to explain their occurrence. Here we present lithologic and carbon isotope records for a 200-m-thick sequence of latest Paleocene– earliest Eocene upper slope limestone exposed along Mead Stream, New Zealand. New carbon isotope and lithologic analyses combined with previous work on this expanded section shows that the PETM and ETM-2, the suspected H-2, I-1, I-2, and K/X hyperthermals, and several other horizons are marked by pronounced negative carbon isotope excursions and clay-rich horizons. Generally, the late Paleocene–early Eocene lithologic and δ¹³C records at Mead Stream are similar to records recovered from deep-sea sites, with an important exception: lows in δ¹³C and carbonate content consistently span intervals of relatively high sedimentation (terrigenous dilution) rather than intervals of relatively low sedimentation (carbonate dissolution). These findings indicate that, over ~6 m.yr., there was a series of short-termclimate perturbations, each characterized by massive input of carbon and greater continental weathering. The suspected link involves global warming, elevated greenhouse-gas concentrations, and enhanced seasonal precipitation.
Late Quaternary sediment accumulations and foraminiferal populations on the slopes of Gladden Basin (offshore Belize) and southern Ashmore Trough (Gulf of Papua) mixed siliciclastic-carbonate systems
(2007) Carson, Brooke Elizabeth; Droxler, Andre W.; Dickens, Gerald R.
The Belize margin, in the western Caribbean Sea, and Ashmore Trough, in the western Gulf of Papua, represent modern tropical mixed siliciclastic-carbonate depositional systems where significant masses of both river born terrigenous siliciclastics and neritic/pelagic carbonates accumulate at variables rates over space and time. This study examines variations in sedimentolgic and micropaleontologic parameters relative to late Quaternary sea level, climate, and paleoenvironment. This is accomplished through the evaluation of carbonate and siliciclastic accumulations, as well as planktic foraminiferal populations, of a 37.7 m giant piston core (MD02-2532) acquired from the slope of Gladden Basin adjacent to the Belize Barrier Reef, as well as benthic foraminiferal populations of two shorter (11.3 m) piston cores (MV-74 and MV-07/06) acquired on the slopes of Ashmore Trough, adjacent to the northern most extent of the Great Barrier Reef. Neritic carbonate fluxes to the slopes of Gladden Basin are largely regulated by sea level and consistent with well-established highstand shedding depositional concepts. Over the last ∼850 ka, neritic carbonate production (and export to the adjacent slopes) switches on when sea level floods the neritic carbonate regions and switches off when sea level falls and neritic carbonate regions are exposed. Siliciclastic accumulations are also controlled primarily by eustatic sea level fluctuations, with additional influences from local and regional variations in physiography, climate, and/or ocean currents. Planktic foraminiferal taxa of Gladden Basin are typical of tropical to subtropical populations and display significant variations in their downcore relative abundances, suggesting notable changes in surface water masses and oceanographic parameters over the last ∼630 ka. Temperature and salinity, often associated with glacial or interglacial intervals, appear to predominately influence the planktic foraminiferal populations. In Ashmore Trough, benthic foraminiferal relative abundances and multivariate analyses indicate three distinct assemblages whose proportions change over the last ∼83 ka. These assemblages signify distinct paleoenvironmental settings driven by organic carbon flux and sediment supply, as well as changes in sea level. Analysis of these late Quaternary mixed systems provides better understanding of their preservation in the rock record, particularly relative to sea level and sequence stratigraphic concepts.
Late Quaternary sediment dispersal and accumulation on slopes of the Great Barrier Reef mixed siliciclastic-carbonate depositional system, Gulf of Papua, Papua New Guinea and North Queensland Margin, Australia
(2007) Francis, Jason Michael; Dickens, Gerald R.; Droxler, Andre W.
The Great Barrier Reef (GBR) margin, located on the continental margin between Papua New Guinea and northeast Australia, is the largest extant example of a tropical mixed siliciclastic-carbonate depositional system. It is constructed by the combined input of terrigenous siliciclastic sediment delivered through riverine transport and biogenous carbonate sediment from neritic and pelagic production. This study investigates late Quaternary changes in sediment dispersal and accumulation on the slopes of this margin. Sedimentation across the GBR mixed system also serves as an important analog for understanding deposition on other extant and ancient systems and provides insight into global change, geochemical cycling, and resource management. Several concepts (e.g., reciprocal sedimentation, coeval sedimentation) have been proposed to explain spatial and temporal variations in siliciclastic and carbonate components. While these concepts are frequently used to evaluate ancient tropical mixed systems, they are rarely assessed in the Quaternary, an interval where the magnitude and timing of sea level are relatively well-constrained, and precise dating techniques can be used. These studies of the GBR mixed system integrate a full suite of data including core, seismic, and multi-beam bathymetry to gain a quantitative understanding of the GBR system and to evaluate reciprocal sedimentation concepts. Results indicate that slopes along the GBR margin have a complex depositional history. Sea level, climate, and margin physiography are all important depositional controls affecting timing, location, and mechanism of sediment dispersal. Reciprocal sedimentation can be used to predict carbonate accumulation. However, this approach must be combined with a firm understanding of sedimentary controls and processes to accurately predict siliciclastic accumulation along this margin.
Marine Carbon and Sulfur Geochemical Cycling: Reassessing Sulfate Reduction and Anaerobic Oxidation of Methane
(2017-11-28) Miller, Clint Matthew; Dickens, Gerald R.; Lee, Cin-Ty A.
Anaerobic oxidation of methane (AOM) likely represents a globally important process impacting chemical cycling of C and S across Earth’s surface. Although ubiquitous along continental margins, AOM remains underappreciated, as clear from its absence in most models for global C and S cycling. To some degree, this is because of longstanding ideas regarding the decomposition of particulate organic carbon, which assume organoclastic sulfate reduction (OSR) is the engine of carbon oxidation. Perhaps, more importantly, many sites purportedly dominated by AOM lack high-resolution pore water data, a full suite of pertinent sedimentary analyses, or both. Here, we collect and describe published seafloor organic carbon data, identify more than 400 locations with AOM in shallow sediments globally, and measure dissolved and solid concentrations of phases containing C and S at three locations to test whether AOM can drive most mass fluxes of these elements. The sites, located on slopes east of Peru and west of Japan, are under different oceanographic conditions and at different water depths (330-5086 m), but display commonalities in pore water profiles consistent with AOM. Dissolved SO42-, HS-, Ca2+, Mg2+, and Sr2+ fluxes display strong inflections at the sulfate-methane transition (SMT), while dissolved Mn2+ and Fe2+, though strongly correlated, are little affected at the SMT. Solid sulfides and perhaps S-bound organic carbon begin precipitating within 0.1 m below the seafloor (mbsf), culminating in sediment S contents of 1.18 wt. %. By contrast, authigenic carbonates begin to precipitate about 2 mbsf. Calculated sulfur mass accumulation rates (1200-4400 mol S m-2 ky-1), and authigenic carbonate precipitation rates (7.1-12.0 mol CaCO3 m-2 ky-1) are comparable to OSR published values. Inverse modeling of pore water profiles produce depth profiles with SMTs between 6.3 and 9.6 m thick, conversion of Ca2+, Mg2+, Sr2+ from solute to solid, and C and S mass balance. Additionally, modeled Fe-AOM and Mn-AOM rates are at least three orders of magnitude less than SO42-. These results show that AOM removes similar amounts of C and S from the ocean as OSR. Globally, we find AOM occurs on all continental margins at water depths up to 5500 m, indicating high CH4 flux is a common occurrence in today’s ocean. If this has also been the case in the geologic past, elemental flux models of Earth’s history may need to be revisited.
Modeling Fluid Flow Effects on Shallow Pore Water Chemistry and Methane Hydrate Distribution in Heterogeneous Marine Sediment
(2012-09-05) Chatterjee, Sayantan; Hirasaki, George J.; Chapman, Walter G.; Zygourakis, Kyriacos; Dickens, Gerald R.; Dugan, Brandon
The depth of the sulfate-methane transition (SMT) above gas hydrate systems is a direct proxy to interpret upward methane flux and hydrate saturation. However, two competing reaction pathways can potentially form the SMT. Moreover, the pore water profiles across the SMT in shallow sediment show broad variability leading to different interpretations for how carbon, including CH4, cycles within gas-charged sediment sequences over time. The amount and distribution of marine gas hydrate impacts the chemistry of several other dissolved pore water species such as the dissolved inorganic carbon (DIC). A one-dimensional (1-D) numerical model is developed to account for downhole changes in pore water constituents, and transient and steady-state profiles are generated for three distinct hydrate settings. The model explains how an upward flux of CH4 consumes most SO42- at a shallow SMT implying that anaerobic oxidation of methane (AOM) is the dominant SO42- reduction pathway, and how a large flux of 13C-enriched DIC enters the SMT from depth impacting chemical changes across the SMT. Crucially, neither the concentration nor the d13C of DIC can be used to interpret the chemical reaction causing the SMT. The overall thesis objective is to develop generalized models building on this 1-D framework to understand the primary controls on gas hydrate occurrence. Existing 1-D models can provide first-order insights on hydrate occurrence, but do not capture the complexity and heterogeneity observed in natural gas hydrate systems. In this study, a two-dimensional (2-D) model is developed to simulate multiphase flow through porous media to account for heterogeneous lithologic structures (e.g., fractures, sand layers) and to show how focused fluid flow within these structures governs local hydrate accumulation. These simulations emphasize the importance of local, vertical, fluid flux on local hydrate accumulation and distribution. Through analysis of the fluid fluxes in 2-D systems, it is shown that a local Peclet number characterizes the local hydrate and free gas saturations, just as the Peclet number characterizes hydrate saturations in 1-D, homogeneous systems. Effects of salinity on phase equilibrium and co-existence of hydrate and gas phases can also be investigated using these models. Finally, infinite slope stability analysis assesses the model to identify for potential subsea slope failure and associated risks due to hydrate formation and free gas accumulation. These generalized models can be adapted to specific field examples to evaluate the amount and distribution of hydrate and free gas and to identify conditions favorable for economic gas production.
Multiple early Eocene hyperthermal events: Their lithologic expressions and environmental consequences
(2009) Nicolo, Micah John; Dickens, Gerald R.
A gradual rise in Earth's surface temperature marks a transition from the late Paleocene to the early Eocene ca. 58-51 Ma. Paleocene/Eocene boundary (∼55.5 Ma) sediments deposited in the midst of this slow warming ubiquitously reveal evidence for a massive isotopically light carbon injection and an associated rapid but transient global warming event, or hyperthermal, that has been termed the Paleocene Eocene Thermal Maximum (PETM) and attributed to a carbon injection from multiple potential sources. The PETM has gained importance over the past two decades as a potential geologic analog to the modern anthropogenic carbon injection and climate change. However significant questions surrounding the nature of the carbon injection at the onset of the PETM remain. The Clarence River valley, located in the Marlborough region, South Island, New Zealand, contains a series of outcrops of lithified late Paleocene to early Eocene sediments originally deposited on a paleo-slope margin. Within these sections, the Lower Limestone Member of the Amuri Limestone Formation records the interval of interest. A Lower Limestone prominent recessed unit consisting of multiple marl-rich beds and recording a pronounced negative carbon isotopic excursion (CIE) marks the PETM at sections that have been bisected by tributaries to the Clarence River, including Mead Stream and Dee Stream. Here I detail and discuss Clarence valley Lower Limestone sections and relate these records to global trends with an emphasis on adding constraints to the PETM carbon injection. Specifically, I document the lithologic and carbon isotopic expression of the PETM and two younger paired sets of early Eocene events that, similar to the Mead Stream and Dee Stream PETM sections, reveal negative CIEs and expanded marl-rich units coincident to identical CIEs and condensed carbonate dissolution horizons in deep-sea sections. I further quantify the abundance of bioturbating macrofauna trace fossils through the PETM at both Mead Stream and Dee Stream and argue that New Zealand margin intermediate waters became hypoxic precisely coincident to the PETM carbon injection. In concert, these findings suggest a PETM carbon addition mechanism capable of both diminishing intermediate water dissolved oxygen and of repeated early Eocene injections.
Planktic foraminiferal response to early Eocene carbon cycle perturbations in the southeast Atlantic Ocean (ODP Site 1263)
(Elsevier, 2017) Luciani, Valeria; D'Onofrio, Roberta; Dickens, Gerald R.; Wade, Bridget S.
At low latitude locations in the northern hemisphere, striking changes in the relative abundances and diversity of the two dominant planktic foraminifera genera, Morozovella and Acarinina, are known to have occurred close to the Early Eocene Climatic Optimum (EECO; ~ 49–53 Ma). Lower Eocene carbonate-rich sediments at Ocean Drilling Program (ODP) Site 1263 were deposited on a bathymetric high (Walvis Ridge) at ~ 40° S, and afford an opportunity to examine such planktic foraminiferal assemblage changes in a temperate southern hemisphere setting. We present here quantified counts of early Eocene planktic foraminiferal assemblages from Hole 1263B, along with bulk sediment stable isotope analyses and proxy measurements for carbonate dissolution. The bulk sediment δ13C record at Site 1263 resembles similar records generated elsewhere, such that known and inferred hyperthermal events can be readily identified. Although some carbonate dissolution has occurred, the well-preserved planktic foraminiferal assemblages mostly represent primary changes in environmental conditions. Our results document the permanent decrease in Morozovella abundance and increase in Acarinina abundance at the beginning of the EECO, although this switch occurred ~ 165 kyr after that at low-latitude northern hemisphere locations. This suggests that unfavourable environmental conditions for morozovellids at the start of the EECO, such as sustained passage of a temperature threshold or other changes in surface waters, occurred at lower latitudes first. The remarkable turnover from Morozovella to Acarinina was widely geographically widespread, although the causal mechanism remains elusive. In addition, at Site 1263, we document the virtual disappearance within the EECO of the biserial chiloguembelinids, commonly considered as inhabiting intermediate water depths, and a reduction in abundance of the thermocline-dwelling subbotinids. We interpret these changes as signals of subsurface water properties, perhaps warming, and the associated contraction of ecological niches.
Pore water geochemistry along continental slopes north of the East Siberian Sea: inference of low methane concentrations
(European Geosciences Union, 2017) Miller, Clint M.; Dickens, Gerald R.; Jakobsson, Martin; Johansson, Carina; Koshurnikov, Andrey; O’Regan, Matt; Muschitiello, Francesco; Stranne, Christian; Mörth, Carl-Magnus
Continental slopes north of the East Siberian Sea potentially hold large amounts of methane (CH4) in sediments as gas hydrate and free gas. Although release of this CH4 to the ocean and atmosphere has become a topic of discussion, the region remains sparingly explored. Here we present pore water chemistry results from 32 sediment cores taken during Leg 2 of the 2014 joint Swedish–Russian–US Arctic Ocean Investigation of Climate–Cryosphere–Carbon Interactions (SWERUS-C3) expedition. The cores come from depth transects across the slope and rise extending between the Mendeleev and the Lomonosov ridges, north of Wrangel Island and the New Siberian Islands, respectively. Upward CH4 flux towards the seafloor, as inferred from profiles of dissolved sulfate (SO42−), alkalinity, and the δ13C of dissolved inorganic carbon (DIC), is negligible at all stations east of 143° E longitude. In the upper 8 m of these cores, downward SO42− flux never exceeds 6.2 mol m−2 kyr−1, the upward alkalinity flux never exceeds 6.8 mol m−2 kyr−1, and δ13C composition of DIC (δ13C-DIC) only moderately decreases with depth (−3.6 ‰ m−1 on average). Moreover, upon addition of Zn acetate to pore water samples, ZnS did not precipitate, indicating a lack of dissolved H2S. Phosphate, ammonium, and metal profiles reveal that metal oxide reduction by organic carbon dominates the geochemical environment and supports very low organic carbon turnover rates. A single core on the Lomonosov Ridge differs, as diffusive fluxes for SO42− and alkalinity were 13.9 and 11.3 mol m−2 kyr−1, respectively, the δ13C-DIC gradient was 5.6 ‰ m−1, and Mn2+ reduction terminated within 1.3 m of the seafloor. These are among the first pore water results generated from this vast climatically sensitive region, and they imply that abundant CH4, including gas hydrates, do not characterize the East Siberian Sea slope or rise along the investigated depth transects. This contradicts previous modeling and discussions, which due to the lack of data are almost entirely based on assumption.

Browsing by Author "Dickens, Gerald R."

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