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  • 1.
    Bhatnagar, Guarav
    et al.
    Rice University.
    Chatterjee, Sayantan
    Rice University.
    Chapman, Walter G.
    Rice University.
    Dugan, Brandon
    Rice University.
    Dickens, Gerald R.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hirasaki, George J.
    Rice University.
    Analytical theory relating the depth of the sulfate‐methane transition to gas hydrate distribution and saturation2011In: Geochemistry Geophysics Geosystems, ISSN 1525-2027, E-ISSN 1525-2027, Vol. 12, no 3, p. 1-21Article in journal (Refereed)
    Abstract [en]

    [1] We develop a theory that relates gas hydrate saturation in marine sediments to the depth of the sulfate‐ methane transition (SMT) zone below the seafloor using steady state, analytical expressions. These expres- sions are valid for systems in which all methane transported into the gas hydrate stability zone (GHSZ) comes from deeper external sources (i.e., advective systems). This advective constraint causes anaerobic oxidation of methane to be the only sulfate sink, allowing us to link SMT depth to net methane flux. We also develop analytical expressions that define the gas hydrate saturation profile based on SMT depth and site‐specific parameters such as sedimentation rate, methane solubility, and porosity. We evaluate our analytical model at four drill sites along the Cascadia Margin where methane sources from depth dominate. With our model, we calculate average gas hydrate saturations across GHSZ and the top occurrence of gas hydrate at these sites as 0.4% and 120 mbsf (Site 889), 1.9% and 70 mbsf (Site U1325), 4.7% and 40 mbsf (Site U1326), and 0% (Site U1329), mbsf being meters below seafloor. These values compare favorably with average saturations and top occurrences computed from resistivity log and chloride data. The analyt- ical expressions thus provide a fast and convenient method to calculate gas hydrate saturation and first‐ order occurrence at a given geologic setting where vertically upward advection dominates the methane flux. 

  • 2.
    Borthwick, Verity E.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Schmidt, S.
    Piazolo, Sandra
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Gundlach, C.
    Quantification of mineral behavior in four dimensions: grain boundary and substructure dynamics in salt2012In: Geochemistry Geophysics Geosystems, ISSN 1525-2027, E-ISSN 1525-2027, Vol. 13, p. Q05005-Article in journal (Refereed)
    Abstract [en]

    Here we present the first four dimensional (time and three dimensional space resolved) experiment on a strongly deformed geological material. Results show that even complicated microstructures with large continuous and discontinuous changes in crystallographic orientation can be resolved quantitatively. The details that can be resolved are unprecedented and therefore the presented technique promises to become influential in a wide range of geoscientific investigations. Grain and subgrain scale processes are fundamental to mineral deformation and associated Earth Dynamics, and time resolved observation of these processes is vital for establishing an in-depth understanding of the latter. However, until recently, in situ experiments were restricted to observations of two dimensional surfaces. We compared experimental results from two dynamic, in situ annealing experiments on a single halite crystal; a 2D experiment conducted inside the scanning electron microscope and a 3D X-ray diffraction experiment. This allowed us to evaluate the possible effects of the free surface on grain and subgrain processes. The extent to which surface effects cause experimental artifacts in 2D studies has long been questioned. Our study shows that, although the nature of recovery processes are the same, the area swept by subgrain boundaries is up to 5 times larger in the volume than observed on the surface. We suggest this discrepancy is due to enhanced drag force on subgrain boundaries by thermal surface grooving. Our results show that while it is problematic to derive absolute mobilities from 2D experiments, derived relative mobilities between boundaries with different misorientation angles can be used.

  • 3. Böttner, Christoph
    et al.
    Berndt, Christian
    Reinardy, Benedict T.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Geersen, Jacob
    Karstens, Jens
    Bull, Jonathan M.
    Callow, Ben J.
    Lichtschlag, Anna
    Schmidt, Mark
    Elger, Judith
    Schramm, Bettina
    Haeckel, Matthias
    Pockmarks in the Witch Ground Basin, Central North Sea2019In: Geochemistry Geophysics Geosystems, ISSN 1525-2027, E-ISSN 1525-2027, Vol. 20, no 4, p. 1698-1719Article in journal (Refereed)
    Abstract [en]

    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 2-D and 3-D 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 1,500 pockmarks of two distinct morphological classes spread over an area of 225 km(2). 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. Plain Language Summary Marine sediments host large amounts of methane (CH4), which is a potent greenhouse gas. The amount of methane released into the atmosphere is, however, largely unknown making it difficult to implement this methane source in climate models. Here we use geophysical, geochemical, and sedimentological data to map the distribution of fluid escape structures in the central North Sea. More than 1,500 pockmarks, which are circular to semicircular depressions of the seafloor, indicate fluid flow from the subsurface. There are two distinct morphological classes of pockmarks corresponding to at least two different fluid sources. Class 1 pockmarks are large, show active venting, and are located above vertical fluid conduits in the subsurface, which feed fluids from deeper strata. Class 2 pockmarks, which comprise 99.5% of all pockmarks, are smaller and limited to the soft sediments directly below the seafloor. Older pockmarks in the subsurface document distinct phases of pockmark formation, likely triggered by external forces after the retreat of ice in the North Sea. The amount of methane released from natural geological sources based on pockmark numbers may be wrong as these do not take into account the origin and composition of released fluids.

  • 4. Geiger, Harri
    et al.
    Mattsson, Tobias
    Deegan, Frances M.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Uppsala University, Sweden.
    Troll, Valentin R.
    Burchardt, Steffi
    Gudmundsson, Olafur
    Tryggvason, Ari
    Krumbholz, Michael
    Harris, Chris
    Magma plumbing for the 2014-2015 Holuhraun eruption, Iceland2016In: Geochemistry Geophysics Geosystems, ISSN 1525-2027, E-ISSN 1525-2027, Vol. 17, no 8, p. 2953-2968Article in journal (Refereed)
    Abstract [en]

    The 2014-2015 Holuhraun eruption on Iceland was located within the Askja fissure swarm but was accompanied by caldera subsidence in the Baroarbunga central volcano 45 km to the southwest. Geophysical monitoring of the eruption identified a seismic swarm that migrated from Baroarbunga to the Holuhraun eruption site over the course of two weeks. In order to better understand this lateral connection between Baroarbunga and Holuhraun, we present mineral textures and compositions, mineral-melt-equilibrium calculations, whole rock and trace element data, and oxygen isotope ratios for selected Holuhraun samples. The Holuhraun lavas are compositionally similar to recorded historical eruptions from the Baroarbunga volcanic system but are distinct from the historical eruption products of the nearby Askja system. Thermobarometry calculations indicate a polybaric magma plumbing system for the Holuhraun eruption, wherein clinopyroxene and plagioclase crystallized at average depths of approximate to 17 km and approximate to 5 km, respectively. Crystal resorption textures and oxygen isotope variations imply that this multilevel plumbing system facilitated magma mixing and assimilation of low-O-18 Icelandic crust prior to eruption. In conjunction with the existing geophysical evidence for lateral migration, our results support a model of initial vertical magma ascent within the Baroarbunga plumbing system followed by lateral transport of aggregated magma batches within the upper crust to the Holuhraun eruption site.

  • 5.
    O'Regan, Matt
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Preto, Pedro
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stranne, Christian
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Koshurnikov, Andrey
    Surface heat flow measurements from the East Siberian continental slope and southern Lomonosov Ridge, Arctic Ocean2016In: Geochemistry Geophysics Geosystems, ISSN 1525-2027, E-ISSN 1525-2027, Vol. 17, no 5, p. 1608-1622Article in journal (Refereed)
    Abstract [en]

    Surface heat flow data in the Arctic Ocean are needed to assess hydrocarbon and methane hydrate distributions, and provide constraints into the tectonic origins and nature of underlying crust. However, across broad areas of the Arctic, few published measurements exist. This is true for the outer continental shelf and slope of the East Siberian Sea, and the adjoining deep water ridges and basins. Here we present 21 new surface heat flow measurements from this region of the Arctic Ocean. On the Southern Lomonosov Ridge, the average measured heat flow, uncorrected for effects of sedimentation and topography, is 574 mW/m(2) (n=4). On the outer continental shelf and slope of the East Siberian Sea (ESS), the average is 5710 mW/m(2) (n=16). An anomalously high heat flow of 20328 mW/m(2) was measured at a single station in the Herald Canyon. With the exception of this high heat flow, the new data from the ESS are consistent with predictions for thermally equilibrated lithosphere of continental origin that was last affected by thermotectonic processes in the Cretaceous to early Cenozoic. Variability within the data likely arises from differences in radiogenic heat production within the continental crust and overlying sediments. This can be further explored by comparing the data with geophysical constraints on sediment and crustal thicknesses.

  • 6. Pontbriand, Claire W.
    et al.
    Soule, S. Adam
    Sohn, Robert A.
    Humphris, Susan E.
    Kunz, Clayton
    Singh, Hanumant
    Nakamura, Ko-ichi
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Shank, Timothy
    Effusive and explosive volcanism on the ultraslow-spreading Gakkel Ridge, 85 degrees E2012In: Geochemistry Geophysics Geosystems, ISSN 1525-2027, E-ISSN 1525-2027, Vol. 13, p. Q10005-Article in journal (Refereed)
    Abstract [en]

    We use high-definition seafloor digital imagery and multibeam bathymetric data acquired during the 2007 Arctic Gakkel Vents Expedition (AGAVE) to evaluate the volcanic characteristics of the 85 degrees E segment of the ultraslow spreading Gakkel Ridge (9 mm yr(-1) full rate). Our seafloor imagery reveals that the axial valley is covered by numerous, small-volume (order similar to 1000 m(3)) lava flows displaying a range of ages and morphologies as well as unconsolidated volcaniclastic deposits with thicknesses up to 10 cm. The valley floor contains two prominent volcanic lineaments made up of axis-parallel ridges and small, cratered volcanic cones. The lava flows appear to have erupted from a number of distinct source vents within the similar to 12-15 km-wide axial valley. Only a few of these flows are fresh enough to have potentially erupted during the 1999 seismic swarm at this site, and these are associated with the Oden and Loke volcanic cones. We model the widespread volcaniclastic deposits we observed on the seafloor as having been generated by the explosive discharge of CO2 that accumulated in (possibly deep) crustal melt reservoirs. The energy released during explosive discharge, combined with the buoyant rise of hot fluid, lofted fragmented clasts of rapidly cooling magma into the water column, and they subsequently settled onto the seafloor as fall deposits surrounding the source vent.

  • 7. Rogerson, Mike
    et al.
    Colmenero-Hidalgo, Elena
    Levine, R.C.
    Rohling, E.J.
    Bigg, G.R.
    Schönfeld, Joachim
    Cacho, I
    Sierro, F.J.
    Völker, Antje
    Löwemark, Ludvig
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Reguera, M.I.
    de Abreu, L.
    Garrick, K.
    Enhanced Mediterranean-Atlantic exchange during Atlantic freshening phases2010In: Geochemistry Geophysics Geosystems, ISSN 1525-2027, E-ISSN 1525-2027, Vol. 11, no 8, p. Q08013-Article in journal (Refereed)
    Abstract [en]

    The Atlantic-Mediterranean exchange of water at Gibraltar represents a significant heat and freshwater sink for the North Atlantic and is a major control on the heat, salt and freshwater budgets of the Mediterranean Sea. Consequently, an understanding of the response of the exchange system to external changes is vital to a full comprehension of the hydrographic responses in both ocean basins. Here, we use a synthesis of empirical (oxygen isotope, planktonic foraminiferal assemblage) and modeling (analytical and general circulation) approaches to investigate the response of the Gibraltar Exchange system to Atlantic freshening during Heinrich Stadials (HSs). HSs display relatively flat W–E surface hydrographic gradients more comparable to the Late Holocene than the Last Glacial Maximum. This is significant, as it implies a similar state of surface circulation during these periods and a different state during the Last Glacial Maximum. During HS1, the gradient may have collapsed altogether, implying very strong water column stratification and a single thermal and δ18Owater condition in surface water extending from southern Portugal to the eastern Alboran Sea. Together, these observations imply that inflow of Atlantic water into the Mediterranean was significantly increased during HS periods compared to background glacial conditions. Modeling efforts confirm that this is a predictable consequence of freshening North Atlantic surface water with iceberg meltwater and indicate that the enhanced exchange condition would last until the cessation of anomalous freshwater supply into to the northern North Atlantic. The close coupling of dynamics at Gibraltar Exchange with the Atlantic freshwater system provides an explanation for observations of increased Mediterranean Outflow activity during HS periods and also during the last deglaciation. This coupling is also significant to global ocean dynamics, as it causes density enhancement of the Atlantic water column via the Gibraltar Exchange to be inversely related to North Atlantic surface salinity. Consequently, Mediterranean enhancement of the Atlantic Meridional Overturning Circulation will be greatest when the overturning itself is at its weakest, a potentially critical negative feedback to Atlantic buoyancy change during times of ice sheet collapse.

  • 8. Schouten, Stefan
    et al.
    Hopmans, Ellen C.
    Rosell-Mele, Antoni
    Pearson, Ann
    Adam, Pierre
    Bauersachs, Thorsten
    Bard, Edouard
    Bernasconi, Stefano M.
    Bianchi, Thomas S.
    Brocks, Jochen J.
    Carlson, Laura Truxal
    Castaneda, Isla S.
    Derenne, Sylvie
    Selver, Ayca Dogrul
    Dutta, Koushik
    Eglinton, Timothy
    Fosse, Celine
    Galy, Valier
    Grice, Kliti
    Hinrichs, Kai-Uwe
    Huang, Yongsong
    Huguet, Arnaud
    Huguet, Carme
    Hurley, Sarah
    Ingalls, Anitra
    Jia, Guodong
    Keely, Brendan
    Knappy, Chris
    Kondo, Miyuki
    Krishnan, Srinath
    Lincoln, Sara
    Lipp, Julius
    Mangelsdorf, Kai
    Martinez-Garcia, Alfredo
    Menot, Guillemette
    Mets, Anchelique
    Mollenhauer, Gesine
    Ohkouchi, Naohiko
    Ossebaar, Jort
    Pagani, Mark
    Pancost, Richard D.
    Pearson, Emma J.
    Peterse, Francien
    Reichart, Gert-Jan
    Schaeffer, Philippe
    Schmitt, Gaby
    Schwark, Lorenz
    Shah, Sunita R.
    Smith, Richard W.
    Smittenberg, Rienk H.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Summons, Roger E.
    Takano, Yoshinori
    Talbot, Helen M.
    Taylor, Kyle W. R.
    Tarozo, Rafael
    Uchida, Masao
    van Dongen, Bart E.
    Van Mooy, Benjamin A. S.
    Wang, Jinxiang
    Warren, Courtney
    Weijers, Johan W. H.
    Werne, Josef P.
    Woltering, Martijn
    Xie, Shucheng
    Yamamoto, Masanobu
    Yang, Huan
    Zhang, Chuanlun L.
    Zhang, Yige
    Zhao, Meixun
    Damste, Jaap S. Sinninghe
    An interlaboratory study of TEX86 and BIT analysis of sediments, extracts, and standard mixtures2013In: Geochemistry Geophysics Geosystems, ISSN 1525-2027, E-ISSN 1525-2027, Vol. 14, no 12, p. 5263-5285Article in journal (Refereed)
    Abstract [en]

    Two commonly used proxies based on the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) are the TEX86 (TetraEther indeX of 86 carbon atoms) paleothermometer for sea surface temperature reconstructions and the BIT (Branched Isoprenoid Tetraether) index for reconstructing soil organic matter input to the ocean. An initial round-robin study of two sediment extracts, in which 15 laboratories participated, showed relatively consistent TEX86 values (reproducibility +/- 3-4 degrees C when translated to temperature) but a large spread in BIT measurements (reproducibility +/- 0.41 on a scale of 0-1). Here we report results of a second round-robin study with 35 laboratories in which three sediments, one sediment extract, and two mixtures of pure, isolated GDGTs were analyzed. The results for TEX86 and BIT index showed improvement compared to the previous round-robin study. The reproducibility, indicating interlaboratory variation, of TEX86 values ranged from 1.3 to 3.0 degrees C when translated to temperature. These results are similar to those of other temperature proxies used in paleoceanography. Comparison of the results obtained from one of the three sediments showed that TEX86 and BIT indices are not significantly affected by interlaboratory differences in sediment extraction techniques. BIT values of the sediments and extracts were at the extremes of the index with values close to 0 or 1, and showed good reproducibility (ranging from 0.013 to 0.042). However, the measured BIT values for the two GDGT mixtures, with known molar ratios of crenarchaeol and branched GDGTs, had intermediate BIT values and showed poor reproducibility and a large overestimation of the true (i.e., molar-based) BIT index. The latter is likely due to, among other factors, the higher mass spectrometric response of branched GDGTs compared to crenarchaeol, which also varies among mass spectrometers. Correction for this different mass spectrometric response showed a considerable improvement in the reproducibility of BIT index measurements among laboratories, as well as a substantially improved estimation of molar-based BIT values. This suggests that standard mixtures should be used in order to obtain consistent, and molar-based, BIT values.

  • 9. Screaton, E.
    et al.
    Kimura, G.
    Curewitz, D.
    Moore, G.
    Chester, F.
    Fabbri, O.
    Fergusson, C.
    Girault, F.
    Goldsby, D.
    Harris, R.
    Inagaki, F.
    Jiang, T.
    Kitamura, Y.
    Knuth, M.
    Li, C. -F
    Liljedahl, L. Claesson
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Louis, L.
    Milliken, K.
    Nicholson, U.
    Riedinger, N.
    Sakaguchi, A.
    Solomon, E.
    Strasser, M.
    Su, X.
    Tsutsumi, A.
    Yamaguchi, A.
    Ujiee, K.
    Zhao, X.
    Interactions between deformation and fluids in the frontal thrust region of the NanTroSEIZE transect offshore the Kii Peninsula, Japan: Results from IODP Expedition 316 Sites C0006 and C00072009In: Geochemistry Geophysics Geosystems, ISSN 1525-2027, E-ISSN 1525-2027, Vol. 10, p. Q0AD01-Article in journal (Refereed)
    Abstract [en]

    Integrated Ocean Drilling Program (IODP) Expedition 316 Sites C0006 and C0007 examined the deformation front of the Nankai accretionary prism offshore the Kii Peninsula, Japan. In the drilling area, the frontal thrust shows unusual behavior as compared to other regions of the Nankai Trough. Drilling results, integrated with observations from seismic reflection profiles, suggest that the frontal thrust has been active since similar to 0.78-0.436 Ma and accommodated similar to 13 to 34% of the estimated plate convergence during that time. The remainder has likely been distributed among out-of-sequence thrusts further landward and/or accommodated through diffuse shortening. Unlike results of previous drilling on the Nankai margin, porosity data provide no indication of undercompaction beneath thrust faults. Furthermore, pore water geochemistry data lack clear indicators of fluid flow from depth. These differences may be related to coarser material with higher permeability or more complex patterns of faulting that could potentially provide more avenues for fluid escape. In turn, fluid pressures may affect deformation. Well-drained, sand-rich material under the frontal thrust could have increased fault strength and helped to maintain a large taper angle near the toe. Recent resumption of normal frontal imbrication is inferred from seismic reflection data. Associated decollement propagation into weaker sediments at depth may help explain evidence for recent slope failures within the frontal thrust region. This evidence consists of seafloor bathymetry, normal faults documented in cores, and low porosities in near surface sediments that suggest removal of overlying material. Overall, results provide insight into the complex interactions between incoming materials, deformation, and fluids in the frontal thrust region.

  • 10.
    Stranne, Christian
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Center for Coastal & Ocean Mapping/Joint Hydrographic Center, USA.
    O'Regan, Matthew
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Dickens, Gerald R.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Rice University, USA.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Miller, C.
    Preto, Pedro
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Dynamic simulations of potential methane release from East Siberian continental slope sediments2016In: Geochemistry Geophysics Geosystems, ISSN 1525-2027, E-ISSN 1525-2027, Vol. 17, no 3, p. 872-886Article in journal (Refereed)
    Abstract [en]

    Sediments deposited along continental margins of the Arctic Ocean presumably host large amounts of methane (CH4) in gas hydrates. Here we apply numerical simulations to assess the potential of gas hydrate dissociation and methane release from the East Siberian slope over the next 100 years. Simulations are based on a hypothesized bottom water warming of 3 degrees C, and an assumed starting distribution of gas hydrate. The simulation results show that gas hydrate dissociation in these sediments is relatively slow, and that CH4 fluxes toward the seafloor are limited by low sediment permeability. The latter is true even when sediment fractures are permitted to form in response to overpressure in pore space. With an initial gas hydrate distribution dictated by present-day pressure and temperature conditions, nominally 0.35 Gt of CH4 are released from the East Siberian slope during the first 100 years of the simulation. However, this CH4 discharge becomes significantly smaller (approximate to 0.05 Gt) if glacial sea level changes in the Arctic Ocean are considered. This is because a lower sea level during the last glacial maximum (LGM) must result in depleted gas hydrate abundance within the most sensitive region of the modern gas hydrate stability zone. Even if all released CH4 reached the atmosphere, the amount coming from East Siberian slopes would be trivial compared to present-day atmospheric CH4 inputs from other sources.

  • 11. van Peer, Tim E.
    et al.
    Xuan, Chuang
    Lippert, Peter C.
    Liebrand, Diederik
    Agnini, Claudia
    Stockholm University, Faculty of Science, Department of Geological Sciences. University of Padova, Italy.
    Wilson, Paul A.
    Extracting a Detailed Magnetostratigraphy From Weakly Magnetized, Oligocene to Early Miocene Sediment Drifts Recovered at IODP Site U1406 (Newfoundland Margin, Northwest Atlantic Ocean)2017In: Geochemistry Geophysics Geosystems, ISSN 1525-2027, E-ISSN 1525-2027, Vol. 18, no 11, p. 3910-3928Article in journal (Refereed)
    Abstract [en]

    Fine-grained magnetic particles in deep-sea sediments often statistically align with the ambient magnetic field during (and shortly after) deposition and can therefore record geomagnetic reversals. Correlation of these reversals to a geomagnetic polarity time scale is an important geochronological tool that facilitates precise stratigraphic correlation and dating of geological records globally. Sediments often carry a remanence strong enough for confident identification of polarity reversals, but in some cases a low signal-to-noise ratio prevents the construction of a reliable and robust magnetostratigraphy. Here we implement a data-filtering protocol, which can be integrated with the UPmag software package, to automatically reduce the maximum angular deviation and statistically mask noisy data and outliers deemed unsuitable for magnetostratigraphic interpretation. This protocol thus extracts a clearer signal from weakly magnetized sediments recovered at Integrated Ocean Drilling Program (IODP) Expedition 342 Site U1406 (Newfoundland margin, northwest Atlantic Ocean). The resulting magnetostratigraphy, in combination with shipboard and shore-based biostratigraphy, provides an age model for the study interval from IODP Site U1406 between Chrons C6Ar and C9n (approximate to 21-27 Ma). We identify rarely observed geomagnetic directional changes within Chrons C6Br, C7r, and C7Ar, and perhaps within Subchron C8n.1n. Our magnetostratigraphy dates three intervals of unusual stratigraphic behavior within the sediment drifts at IODP Site U1406 on the Newfoundland margin. These lithostratigraphic changes are broadly concurrent with the coldest climatic phases of the middle Oligocene to early Miocene and we hypothesize that they reflect changes in bottom water circulation.

  • 12. Walin, G.
    et al.
    Hieronymus, Jenny
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Source-related variables for the description of the oceanic carbon system2014In: Geochemistry Geophysics Geosystems, ISSN 1525-2027, E-ISSN 1525-2027, Vol. 15, no 9, p. 3675-3687Article in journal (Refereed)
    Abstract [en]

    The oceanic carbon system is commonly described in terms of the two state variables total carbon, DIC, and alkalinity, Alk. Here we suggest the use of alternative source adapted state variables, Acidic Carbon, AC and Basic Carbon, BC, defined by and related to (DIC, Alk) with a simple linear transformation. (AC, BC) can be interpreted as representing respectively the supply to the system of carbon dioxide and dissolved carbonate, keeping in mind that supply of hydrogen ions acts to transform from basic carbon to acidic carbon. Accordingly these variables tell us how much carbon dioxide or dissolved carbonate we actually have in the water, despite the fact that the major part of the carbon resides in bicarbonate ions. We claim that using these source-related variables as a compliment to the traditional variables, offers a number of advantages in the formulation of continuity equations, as well as in the interpretation of observations and modeling results. The traditional definition of alkalinity is related to a measuring procedure rather than to the supply of material to the system. Here we demonstrate that alkalinity, though defined in the traditional way, may be interpreted in terms of sources and sinks acting on the system. In the case of ocean water this amounts to twice the supply of dissolved carbonate minus the net supply of free hydrogen ions. We argue that this interpretation is a useful complement to the traditional definition. Every process that affects the state of the carbon system may be quantified in terms of supply of carbon dioxide, F-a, carbonate ions, F-b, or hydrogen ions, E.

  • 13. Weis, Franz A.
    et al.
    Skogby, Henrik
    Troll, Valentin R.
    Deegan, Frances M.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Uppsala University, Sweden.
    Dahren, Börje
    Magmatic water contents determined through clinopyroxene: Examples from the Western Canary Islands, Spain2015In: Geochemistry Geophysics Geosystems, ISSN 1525-2027, E-ISSN 1525-2027, Vol. 16, no 7, p. 2127-2146Article in journal (Refereed)
    Abstract [en]

    Water is a key parameter in magma genesis, magma evolution, and resulting eruption styles, because it controls the density, the viscosity, as well as the melting and crystallization behavior of a melt. The parental water content of a magma is usually measured through melt inclusions in minerals such as olivine, a method which may be hampered, however, by the lack of melt inclusions suitable for analysis, or postentrapment changes in their water content. An alternative way to reconstruct the water content of a magma is to use nominally anhydrous minerals (NAMs), such as pyroxene, which take up low concentrations of hydrogen as a function of the magma's water content. During magma degassing and eruption, however, NAMs may dehydrate. We therefore tested a method to reconstruct the water contents of dehydrated clinopyroxene phenocrysts from the Western Canary islands (n=28) through rehydration experiments followed by infrared and Mossbauer spectroscopy. Employing currently available crystal/melt partitioning data, the results of the experiments were used to calculate parental water contents of 0.710.07 to 1.490.15 wt % H2O for Western Canary magmas during clinopyroxene crystallization at upper mantle conditions. This H2O range is in agreement with calculated water contents using plagioclase-liquid-hygrometry, and with previously published data for mafic lavas from the Canary Islands and comparable ocean island systems elsewhere. Utilizing NAMs in combination with hydrogen treatment can therefore serve as a proxy for pre-eruptive H2O contents, which we anticipate becoming a useful method applicable to mafic rocks where pyroxene is the main phenocryst phase.

  • 14. Wiers, Steffen
    et al.
    Snowball, Ian
    O'Regan, Matt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Almqvist, Bjarne
    Late Pleistocene Chronology of Sediments From the Yermak Plateau and Uncertainty in Dating Based on Geomagnetic Excursions2019In: Geochemistry Geophysics Geosystems, ISSN 1525-2027, E-ISSN 1525-2027, Vol. 20, no 7, p. 3289-3310Article in journal (Refereed)
    Abstract [en]

    The Yermak Plateau is one of several regions in the Arctic Ocean where paleomagnetism yields controversial results. Despite low sedimentation rates, late Pleistocene paleomagnetic excursions have been reconstructed from many cores in the region, but they are characterized by considerably longer durations when compared to established ones. Self-reversal during maghemitization of (titano)magnetite has been proposed as one explanation. Rock magnetic, C-14 dating, sedimentological and stable isotope (delta O-18) methods were employed to three new sediment cores to put paleomagnetic results in the context of the regional stratigraphy and chronology. Coherence of lithological parameters and delta O-18 variations validated the ratio of anhysteretic remanent susceptibility to bulk magnetic susceptibility (kappa(ARM)/kappa) as a parameter for cross-core correlation. As established by earlier studies, we use the link between glacial/interglacial cycles and kappa(ARM)/kappa to tune our records to a global delta O-18 stack, which provides age models that are independent of radicarbon ages and paleomagnetic data. Our results show that zones of negative magnetic inclination are asynchronous across the plateau. Alternating field demagnetization data revealed that negative inclinations are contained in a medium-high-coercivity (>25-35mT) magnetic phase that may be the result of postdepositional alteration of (titano)magnetite. We note a positive relationship between water depth and excursion duration, which may be driven by changes in water mass circulation on glacial/interglacial timescales.

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