Gerald R. Dickens

Dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the Paleocene

Isotopic records across the “Latest Paleocene Thermal Maximum“ (LPTM) indicate that bottom water temperature increased by more than 4°C during a brief time interval (<104 years) of the latest Paleocene (∼55.6 Ma). There also was a coeval −2 to −3‰ excursion in the δ13C of the ocean/atmosphere inorganic carbon reservoir. Given the large mass of this reservoir, a rapid δ13C shift of this magnitude is difficult to explain within the context of conventional hypotheses for changing the mean carbon isotope composition of the ocean and atmosphere. However, a direct consequence of warming bottom water temperature from 11 to 15°C over 104 years would be a significant change in sediment thermal gradients and dissociation of oceanic CH4 hydrate at locations with intermediate water depths. In terms of the present-day oceanic CH4 hydrate reservoir, thermal dissociation of oceanic CH4 hydrate during the LPTM could have released greater than 1.1 to 2.1 × 1018 g of carbon with a δ13C of approximately −60‰. The release and subsequent oxidation of this amount of carbon is sufficient to explain a −2 to −3‰ excursion in δ13C across the LPTM. Fate of CH4 in oceanic hydrates must be considered in developing models of the climatic and paleoceanographic regimes that operated during the LPTM.

A blast of gas in the latest Paleocene: Simulating first-order effects of massive dissociation of oceanic methane hydrate

Carbonate and organic matter deposited during the latest Paleocene thermal maximum is characterized by a remarkable -2.5% excursion in delta 13C that occurred over approximately 10(4) yr and returned to near initial values in an exponential pattern over approximately 2 x 10(5) yr. It has been hypothesized that this excursion signifies transfer of 1.4 to 2.8 x 10(18) g of CH4 from oceanic hydrates to the combined ocean-atmosphere inorganic carbon reservoir. A scenario with 1.12 x 10(18) g of CH4 is numerically simulated here within the framework of the present-day global carbon cycle to test the plausibility of the hypothesis. We find that (1) the delta 13C of the deep ocean, shallow ocean, and atmosphere decreases by -2.3% over 10(4) yr and returns to initial values in an exponential pattern over approximately 2 x 10(5) yr; (2) the depth of the lysocline shoals by up to 400 m over 10(4) yr, and this rise is most pronounced in one ocean region; and (3) global surface temperature increases by approximately 2 degrees C over 10(4) yr and returns to initial values over approximately 2 x 10(6) yr. The first effect is quantitatively consistent with the geologic record; the latter two effects are qualitatively consistent with observations. Thus, significant CH4 release from oceanic hydrates is a plausible explanation for observed carbon cycle perturbations during the thermal maximum. This conclusion is of broad interest because the flux of CH4 invoked during the maximum is of similar magnitude to that released to the atmosphere from present-day anthropogenic CH4 sources.

Methane hydrate stability in seawater

Experimental data are presented for methane hydrate stability conditions in seawater (S ≈ 33.5‰). For the pressure range of 2.75–10.0 MPa, at any given pressure, the dissociation temperature of mcthane hydrate is depressed by approximately −1.1°C relative to the pure methane-pure water system. These experimental results are consistent with previously reported thermodynamic predictions and experimental results obtained with artificial seawater. Collectively these results provide a minimum constraint concerning depth ranges over which methane hydrate is stable in the oceanic environment.

Quantitative resolution of eolian continental crustal material and volcanic detritus in North Pacific surface sediment

Proxy records of continental climate and atmospheric circulation provided by analyses of eolian continental material extracted from marine sediment have resulted in significant new information concerning the behavior of these climate systems on various timescales. These studies, however, currently are limited to certain geographic areas because no chemical or physical extraction procedure provides an unambiguous separation of eolian continental crustal material from other contaminants like volcanic detritus. We employ a combined analytical and statistical procedure in an effort to extract a more refined eolian “signal” from areas that may be affected by volcanic detritus. Bulk surface sediment samples from 33 locations in the North Pacific were treated using a conventional sequential extraction procedure to remove the carbonate, silica and oxyhydroxide components, and the residue was analyzed by instrumental neutron activation analysis (INAA) for La, Ce, Sm, Eu, Yb, Lu, Hf, Sc, and Th. Q-mode factor analysis of these data shows that > 99% of the variance is explained by two end-members, which we interpret to be continental crustal material and volcanic detritus. Five least squares normative analysis models were evaluated to estimate the relative amount of these end-members in each sample. The continental crustal component was approximated using a fine-grained fraction of China loess and bulk loess. The volcanic component was approximated using the compositions of average Kurile-Kamchatka volcanic material, a median Ocean Drilling Program (ODP) Leg 145 Ash, and a median Kurile basalt. The model based on average Kurile-Kamchatka volcanic material and the fine loess fraction gives the most accurate results. Central and east Pacific samples typically contain up to 100% of the eolian continental crustal component, while samples near Japan, Kamchatka, and the Aleutians contain a majority of volcanic detritus. However, the highly variable composition of volcanic material can result in systematic errors up to 25% in samples dominated by volcanic detritus. The geographic distribution of the compositional end-members is consistent with a continental dust source originating in Asia and being diluted by ash from the volcanic arcs of the Pacific rim. This improved identification and resolution of the eolian continental component realized in this approach should permit paleoclimatic reconstructions to be developed from sediments in significant portions of the world that were previously precluded from analysis because of limitations with chemical or physical extraction procedures.

Late Miocene‐Early Pliocene manganese redirection in the central Indian Ocean: Expansion of the Intermediate Water oxygen minimum zone

Decomposition of organic matter combined with density stratification generate a pronounced intermediate water oxygen minimum zone (OMZ) in the northwest Indian Ocean. This zone currently lies between water depths of 200 and 2000 m and extends approximately 5000 km southeast from the Arabian coast. Based upon benthic foraminiferal assemblage changes, it has been suggested that this OMZ was even more extensive during the late Miocene-early Pliocene (6.5–3.0 Ma), with a maximum volume and/or intensity at approximately 5.0 Ma. While this inference may contribute to an understanding of the history of northwest Indian Ocean upwelling, corroborating geochemical evidence for this interpretation has heretofore been lacking. Ocean Drilling Program (ODP) sites 752, 754, and 757 on Broken and Ninetyeast ridges are located within central Indian Ocean intermediate water depths (1086–1650 m) but outside the present lateral dimensions of the Indian Ocean OMZ. High-resolution chemical analyses of sediment from these sites indicate significant reductions in the flux of Mn and normalized Mn concentrations between 6.5 and 3.0 Ma that are most pronounced at approximately 5.0 Ma. Because late Miocene-Pliocene paleodepths for these sites were essentially the same as at present and because extremely low sedimentation rates (0.3–1.3 cm/ky) most likely precluded sedimentary metal oxide diagenesis, we suggest that the observed Mn depletions reflect diminished deposition of reducible Mn oxyhydroxide phases within O2 deficient intermediate waters and that this effect was most intense at approximately 5.0 Ma. This interpretation implies that waters with less than 2.0 mL/L O2 extended at least 1500 km beyond their present limits and is consistent with changes in benthic foraminifera assemblages. We further suggest this expanded Indian Ocean OMZ is related to regionally and/or globally increased biological productivity.

CAUSES AND IMPLICATIONS OF THE MIDDLE RARE EARTH ELEMENT DEPLETION IN THE EOLIAN COMPONENT OF NORTH PACIFIC SEDIMENT

Abstract Previous studies clearly demonstrate that the detrital fraction of central North Pacific sediment is derived almost exclusively from wind-born particles from the arid and semi-arid regions in Asia. These conclusions are based, in part, on trajectories observed for westerly wind systems and on a grainsize distribution for central North Pacific sediment which can only be explained by eolian transport. These observations have been verified by comparing the mineralogy, neodymium isotopic composition, and trace element geochemistry of the sediment with its Asian source region. However, recent geochemical investigations consistently highlight compositional differences exemplified by a MREE (middle rare earth element) depletion and a lower Th/Sc ratio in eolian material extracted from North Pacific sediment when compared to bulk China loess, the continental analog of the eolian material. These geochemical differences persist even when the bulk loess is subjected to the same extraction procedure. Here we present experiments demonstrating that these compositional differences are caused by a combination of grainsize fractionation during transport and partial dissolution of REE- and Th-bearing phosphatic phases during the extraction procedure. Two bulk loess samples from China were separated into several different grain size fractions and a split of each fraction was subjected to the extraction procedure commonly used to isolate terrigenous material from marine sediment. All extracted and unextracted sample pairs were analyzed for P, Th, Sc, Fe, La, Ce, Sm, Eu, Yb, and Lu. The amount of P removed by the extraction procedure correlates well with both the observed MREE depletion and the decrease in Th/Sc and is most pronounced in the finest grained samples. These correlations strongly suggest that the extraction procedure dissolves inorganic phosphate phases which are rich in MREEs and Th. A comparison of the compositions of the 2–3 mm loess size fractions with central North Pacific sediment shows that both of these materials are identical.

Sediment geochemical evidence for an early-middle Gilbert (early Pliocene) productivity peak in the North Pacific Red Clay Province

Abstract Current attempts to understand climatic variability during the early to middle Pliocene require paleoceanographic information from the Pacific and Indian Oceans that may serve to test and/or constrain future circulation models. Ocean Drilling Program (ODP) Sites 885/886 are located in the central subarctic North Pacific at water depths exceeding 5700 m. Recent studies of rock magnetic properties suggest that the fine-grained Fe oxide component in sediment at Sites 885/886 experienced reductive dissolution during the early-middle Gilbert. Because such an interval in the North Pacific Red Clay Province suggests a maximum in the sedimentary flux of organic carbon and/or a minimum in bottom water dissolved O 2 concentrations (and Hence, a peak change in North Pacific oceanographic conditions), a geochemical investigation was conducted to test the hypothesis. Quaternary sediment at Hole 886B was subjected to an oxyhydroxide removal procedure, and chemical analyses indicate that bulk sediment concentrations of Fe and the Fe/Sc ratio decrease significantly upon reductive dissolution. Downcore chemical analyses of untreated sediment at Hole 886B demonstrate that similar depletions also occur across the proposed interval of reduced sediment. Downcore chemical analyses also indicate that a pronounced increase in the Ba/Sc ratio occurs across the interval. These results are consistent with an interpretation that abyssal sediment of the North Pacific experienced a decrease in redox conditions during the early-middle Gilbert, and that this change in oxidation state was related to a peak in paleoproductivity. If the zenith of late Miocene to middle Pliocene enhanced productivity observed at other Indo-Pacific divergence regions similarly can be constrained to the early-middle Gilbert, there exists an oceanographic boundary condition in which to test future models concerning Pliocene warmth.

Global change and manganese deposition at the Cenomanian‐turonian boundary

Abstract Several of the worlds large stratiform Mn deposits formed in shallow water environments at the Cenomanian‐Turonian boundary (CTB; ca. 92 mya). Earth history at this time is also characterized by increased tectonism, the highest Mesozoic‐Cenozoic sea level stand, and widespread reducing conditions in the oceans. Here we present evidence for a causal relationship between these global‐scale phenomena. A high resolution geochemical analysis of pelagic sediments from two sites on the Exmouth Plateau (off northwest Australia) indicates significant Mn depletion and reducing conditions coincident with deposit formation. We suggest the coupling between deep water Mn depletions and shallow water Mn enrichments involved the sequestering of hydrothermally derived Mn within expanded oxygen minimum zones via diagenetic remobilization and/or direct entrapment of hydrothermal effluents, and its subsequent deposition on shelf substrates at the top of a redoxcline.

A rapidly deposited pennate diatom ooze in Upper Miocene-Lower Pliocene sediment beneath the North Pacific polar front

Abstract Rapidly deposited Thalassionema-Thalassiothrix pennate diatom oozes previously have been described in Upper Miocene-Lower Pliocene sediment beneath the frontal boundary of the eastern equatorial Pacific. Here we document a new occurrence of Thalassionema-Thalassiothrix ooze in Upper Miocene-Lower Pliocene sediment beneath the frontal boundary of the subarctic North Pacific. The ooze is a 6 m interval of siliceous sediment at Ocean Drilling Program (ODP) sites 885/886 that was rapidly deposited between approximately 5.0 and 5.9 Ma. Bulk sediment in this interval may contain greater than 85% pennate diatom tests. There are also abundant laminae and pockets that are composed entirely of Thalassionema and Thalassiothrix diatoms. The presence of a rapidly deposited ooze dominated by pennate diatoms indicates unusual past conditions in the overlying surface waters. Time coincident deposition of such oozes at two distinct frontal boundary locations of the Pacific suggests that the unusual surface water conditions were causally linked to large-scale oceanographic change. This same oceanographic change most likely involved (1) addition of nutrients to the ocean, or (2) redistribution of nutrients within the ocean. The occurrence and origin of pennate diatom oozes may be a key component to an integrative understanding of late Neogene paleoceanography and biogeochemical cycling.

Rare earth element deposition in pelagic sediment at the Cenomanian‐Turonian Boundary, Exmouth Plateau

ODP Site 762 (eastern Indian Ocean) includes a section of sediment that spans the Cenomanian-Turonian Boundary (CTB) and was deposited along a continental margin during a period of widespread oceanic 09. deficiency. The rare earth element (REE) content of pre- and post-boundary sediment is similar to that of present-day continental slope material deposited in well-oxygenated seawater, whereas the CTB section is characterized by significantly depleted REE abundances, a bulk Ce anomaly that increases to maximum of 1.0, and a REE pattern that resembles that of present-day fluvial material. We suggest the change in REE patterns reflects release of scavenged REEs upon reductive dissolution of authigenic Fe-Mn oxyhydroxides, such that sediment deposited during the CTB is dominated by the lithogenous REE fraction. These results are consistent with recent models concerning pervasive fractionation of Mn and Fe and redirection of Mn in pelagic CTB waters.