USGS Science⌕ Search

Geology topics

M.A. Arthur

Publications and source records attributed to M.A. Arthur.

At least 19 recordsLinked to original sources
USGS2013

Variations in the global carbon cycle during the Cretaceous related to climate, volcanism, and changes in atmospheric CO2

The stratigraphic record from both deep-sea and shallow-water depositional environments indicates that during late Aptian through Cenomanian time (1) global climates were considerably warmer than at present; (2) latitudinal gradients of atmospheric and oceanic temperatures were considerably less than at present; (3) rates of accumulation of organic matter of both marine and terrestrial origin were as high as or higher than during any other interval in the Mesozoic or Cenozoic; (4) the rate and volume of accumulation of CaCO 3 in the deep sea were reduced in response to a marked shoaling of the carbonate compensation depth; (5) seafloor spreading rates were somewhat more rapid than at any other time in the Cretaceous or Cenozoic; (6) off-ridge volcanism was intense and widespread, particularly in the ancestral Pacific Ocean basin; and (7) sea level was relatively high, forming widespread areas of shallow shelf seas. A marked increase in the rate of CO 2 outgassing due to volcanic activity between about 110 and 70 m.y. ago may have resulted in a buildup of atmospheric CO 2. A significant fraction of this atmospheric CO 2 may have been reduced by an increase in the production and burial of terrestrial organic carbon. Some excess CO 2may have been consumed by marine algal photosynthesis, but marine productivity apparently was low during the Aptian-Albian relative to terrestrial productivity. Terrestrial productivity also may have been stimulated by increased rainfall that resulted 504 from a warm global climate and increased marine transgression as well as by the higher CO 2.

Book chapter↗
USGS2010

Organic-matter production and preservation and evolution of anoxia in the Holocene Black Sea

Dating of sediments collected in gravity cores during Leg 1 of the 1988 R/V Knorr expedition to the Black Sea suggests that the onset of water-column anoxia at ∼7.5 ka was virtually synchronous across the basin over a depth range of ∼200–2250 m. A finely laminated, organic carbon (OC) rich sapropel (unit II) was produced as a result of this anoxia. The trigger for increased OC production and development of anoxia was the spillover of saline waters through the Bosporus that probably began at ∼9.0 ka and peaked between ∼7.0 and 5.5 ka. This spillover enhanced vertical mixing and nutrient cycling and caused a short-term (2–3 kyr) burst in surface-water productivity during the early part of unit II deposition. Continued incursion of saline waters enhanced vertical stability and inhibited mixing of nutrients into surface waters, thus limiting primary production and decreasing the OC flux to sediments beginning ∼5.5 ka. Concentration, accumulation rate, and degree of preservation of organic matter all decreased in the upper part of unit II as a result of decreasing productivity, but anoxia persisted throughout most of the water column. The end of unit II sapropel deposition was synchronous across the Black Sea as the result of the first blooms of the coccolith Emiliania huxleyi , which presumably marked an increase in surface-water salinity above 11 and the beginning of unit I deposition. The high coccolith-carbonate fluxes that occurred during deposition of unit I diluted the OC concentration in the sediments, but OC accumulation rates are about the same as those in upper part of unit II.

Paleoceanography and Paleoclimatology↗
USGS2010

Rhythmic bedding produced in Cretaceous pelagic carbonate environments: Sensitive recorders of climatic cycles

Various types of rhythmic bedding are prominent features of Cretaceous pelagic carbonates. These bedding rhythms are the primary depositional result of variations in carbonate productivity, terrigenous dilution, redox conditions and/or the energy of bottom currents. Each bedding cycle type is different in its expression in the stratigraphic record but ultimately was caused by rhythmic climatic variations related to variations in receipt of solar insolation as the result of the earth's orbital characteristics, the so‐called Milankovitch cycles. Stratigraphic sequences may contain a single bedding cycle type or a composite of several types. Although sediment type and sedimentary structures were influenced by variations in local climatic, oceanographic, tectonic, and diagenetic parameters, pelagic carbonate environments during the Cretaceous were particularly sensitive recorders of orbitally induced changes in worldwide climate.

Paleoceanography↗
USGS1999

Diagenesis of lower Cretaceous pelagic carbonates, North Atlantic: Paleoceanographic signals obscured

The stable isotope and minor element geochemistry of Neocomian (Lower Cretaceous) pelagic carbonates of the North Atlantic Basin (Deep Sea Drilling Project Sites 105, 367, 387, 391, and 603) was examined to develop a diagenetic model for pelagic limestones. In particular, we hoped to test the fidelity of whole-rock geochemical records as paleoceanographic indicators for pelagic deposits of pre-Aptian age, in which individual microfossils are not available for analysis. Data indicate that in addition to depth of burial, rhythmic variations in primary carbonate content have strongly controlled diagenetic patterns and associated geochemical signatures in these Neocomian sequences. Samples become increasingly depleted in Sr and 18O with increasing CaCO3 content. Within individual sedimentary sections, substantial decreases in Sr/Ca ratios and ??18O values are evident over a range of 4 to 98% CaCO3. However, even over a relatively narrow range of 50 to 98% CaCO3 a 2.5%c variation in ??18O values and a change of a factor of 1.7 in Sr/Ca ratios are observed. Carbon isotope compositions do not vary as extensively with CaCO3 content, but carbonate-rich intervals tend to be relatively depleted in 13C. Petrographic analysis reveals that these geochemical patterns are related to the transfer of CaCO3 from carbonate-poor intervals (calcareous shales and marlstones) to adjacent carbonate-rich intervals (limestones) during burial compaction and pressure solution. This process results in the addition of diagenetic cement to carbonate-rich intervals to produce a bulk composition that is relatively depleted in Sr and 18O and, at the same time, enables the retention of more-or-less primary carbonate that is relatively enriched in Sr and 18O in adjacent carbonate-poor intervals. Thus, although cyclic variations in CaCO3 content are primary in the Neocomian sequences examined, measured variations in Sr/Ca ratios and ??18O values are not and, as such, do not provide reliable proxies for past variations in climate, oceanographic conditions, or global ice volume.

Journal of Foraminiferal Research↗
USGS1999

Sensitivity of the North Atlantic Basin to cyclic climatic forcing during the early Cretaceous

Striking cyclic interbeds of laminated dark-olive to black marlstone and bioturbated white to light-gray limestone of Neocomian (Early Cretaceous) age have been recovered at Deep Sea Drilling Project (DSDP) and Ocean Drilling Project (ODP) sites in the North Atlantic. These Neocomian sequences are equivalent to the Maiolica Formation that outcrops in the Tethyan regions of the Mediterranean and to thick limestone sequences of the Vocontian Trough of France. This lithologic unit marks the widespread deposition of biogenic carbonate over much of the North Atlantic and Tethyan seafloor during a time of overall low sealevel and a deep carbonate compensation depth. The dark clay-rich interbeds typically are rich in organic carbon (OC) with up to 5.5% OC in sequences in the eastern North Atlantic. These eastern North Atlantic sequences off northwest Africa, contain more abundant and better preserved hydrogen-rich, algal organic matter (type II kerogen) relative to the western North Atlantic, probably in response to coastal upwelling induced by an eastern boundary current in the young North Atlantic Ocean. The more abundant algal organic matter in sequences in the eastern North Atlantic is also expressed in the isotopic composition of the carbon in that organic matter. In contrast, organic matter in Neocomian sequences in the western North Atlantic along the continental margin of North America has geochemical and optical characteristics of herbaceous, woody, hydrogen-poor, humic, type III kerogen. The inorganic geochemical characteristics of the dark clay-rich (<60% CaCO3) interbeds in Neocomian sequences in the North American Basin and the Cape Verde Basin off northwest Africa suggest that most of the detrital clastic material was derived from deep-sea fans off North America and Morocco during relatively wet intervals to dilute pelagic biogenic carbonate. Inorganic geochemical characteristics of the clastic material in the bioturbated, white, carbonate-rich (>80% CaCO3) interbeds in both the eastern and western basins of the North Atlantic suggest that they contain minor amounts of relatively unweathered eolian dust derived from northwest Africa during dry intervals.

Journal of Foraminiferal Research↗
USGS1998

Organic carbon accumulation and preservation in surface sediments on the Peru margin

Concentrations and characteristics of organic matter in surface sediments deposited under an intense oxygen-minimum zone on the Peru margin were studied in samples from deck-deployed box cores and push cores acquired by submersible on two transects spanning depths of 75 to 1000 m at 12??and 13.5??S. The source of organic matter to the seafloor in these areas is almost entirely marine material as confirmed by the narrow range of ??13C of organic carbon obtained in the present study (-20.3 to -21.6???; PDB) and the lack of any relationship between pyrolysis hydrogen index and carbon isotope composition. Organic carbon contents are highest (up to 16%) on the slope at depths between 75 and 350 m in sediments deposited under intermediate water masses with low dissolved oxygen concentrations (< 5 ??mol/kg). Even at these low concentrations of dissolved oxygen, however, the surface sediments that were recovered from these depths are dominantly unlaminated. Strong currents (up to 30 cm/s) associated with the poleward-flowing Peru Undercurrent were measured at depths between 160 and 300 m on both transects. The seafloor in this range of water depths is characterized by bedforms stabilized by bacterial mats, extensive authigenic mineral crusts, and (or) thick organic flocs. Constant advection of dissolved oxygen, although in low concentrations, active resuspension of surficial organic matter, activity of organisms, and transport of fine-grained sediment to and from more oxygenated zones all contribute to greater degradation and poorer initial preservation of organic matter than might be expected under oxygen-deficient conditions. Dissolved-oxygen concentrations ultimately may be the dominant affect on organic matter characteristics, but reworking of fine-grained sediment and organic matter by strong bottom currents and redeposition on the seafloor in areas of lower energy also exert important controls on organic carbon concentration and degree of oxidation in this region.

Chemical Geology↗
USGS1997

History of water-column anoxia in the Black Sea indicated by pyrite framboid size distributions

A detailed study of size distributions of framboidal pyrite in Holocene Black Sea sediments establishes the timing of a change from deposition under an oxic water column to deposition under an anoxic and sulfidic water column. In the most recent carbonate-rich sediments (Unit I) and in the organic carbon-rich sapropel (Unit II), framboid size distributions are remarkably uniform (mean diameter = 5 μm); over 95% of the framboids in Unit I and Unit II are < 7 μm in diameter. These properties of framboidal pyrite are consistent with framboid nucleation and growth within an anoxic and sulfidic water column, followed by transport to the sediment-water interface, cessation of pyrite growth due to the exhaustion of reactive iron, and subsequent burial. In contrast, the organic carbon-poor sediments of lacustrine Unit III contain pyrite framboids that are generally much larger in size (mean diameter = 10 μm). In Unit III, over 95% of the framboids are < 25 μm in diameter, 40% of framboids are between 7 μm and 25 μm, and framboids up to 50 μm in diameter are present. This distribution of sizes suggests framboid nucleation and growth within anoxic sediment porewaters. These new data on size distributions of framboidal pyrite confirm that the development of water-column anoxia in the Black Sea coincided with the initiation of deposition of laminated Unit II sapropels.

Earth and Planetary Science Letters↗
USGS1997

Evidence for Milankovitch periodicities in Cenomanian-Turonian lithologic and geochemical cycles, western interior U.S.A.

The limestone/marlstone bedding couplets of the Bridge Creek Limestone Member, Cenomanian-Turonian Greenhorn Formation, were analyzed by applying spectral techniques to high-resolution lithologic and geochemical data from a core. The results suggest that the Bridge Creek contains a complex record of orbital cyclicity. The dominant signal appears to be obliquity, but signals corresponding to precession and eccentricity were also observed. The development of the bedding couplets is interpreted to have resulted from a combination of factors, including insolation-controlled changes in higher-latitude precipitation leading to dilution/redox cycles, and in lower-latitude evaporation, leading to changes in surface water conditions and productivity cycles in the calcareous plankton. The data interpreted to reflect redox cycles appear to be more strongly influenced by obliquity, and show a weak precessional signal. In contrast, trends in the carbonate record show the opposite response. The complex bedding pattern observed in the Bridge Creek Limestone is interpreted to result from the competing influences of different orbital cycles expressed through different pathways of the depositional system, and was also affected by changes in sedimentation rates related to relative sea level fluctuations, aperiodic dilution by volcanic ash, and changes in organic-matter production and redox conditions related to a global "oceanic anoxic event". These factors complicate cycle analysis in the lower part of the member but leave a relatively undisturbed record in the upper Bridge Creek Limestone.

Journal of Sedimentary Research↗
USGS1989

Geochemical evidence for suppression of pelagic marine productivity at the Cretaceous/Tertiary boundary

The normal, biologically productive ocean is characterized by a gradient of the 13 C/ 12 C ratio from surface to deep waters. Here we present stable isotope data from planktonic and benthic micro-fossils across the Cretaceous/Tertiary boundary in the North pacific, which reveal a rapid and complete breakdown in this biologically mediated gradient. The fluxes of barium (a proxy for organic carbon) and CaCO 3 also decrease significantly at the time of the major marine plankton extinctions. The implied substantial reduction in oceanic primary productivity persisted for ∼ 0.5 Myr before the carbon isotope gradient was gradually re-established. In addition, the stable isotope and preservational data indicate that environmental change, including cooling, began at least 200 kyr before the Cretaceous/Tertiary boundary, and a peak warming of ∼ 3 °C occurred 600 kyr after the boundary event.

Nature↗
USGS1989

Geochemical expression of early diagenesis in middle Eocene-lower Oligocene pelagic sediments in the southern Labrador Sea, Site 647, ODP Leg 105

Geochemical analyses of the middle Eocene through lower Oligocene lithologic Unit IIIC (260-518 meters below sea floor [mbsf]) indicate a relatively constant geochemical composition of the detrital fraction throughout this depositional interval at Ocean Drilling Program (ODP) Site 647 in the southern Labrador Sea. The main variability occurs in redox-sensitive elements (e.g., iron, manganese, and phosphorus), which may be related to early diagenetic inability in anaerobic pore waters during bacterial decomposition of organic matter. Initial preservation of organic matter was mediated by high sedimentation rates (36 m/m.Y.). High iron (Fe) and manganese (Mn) contents are associated with carbonate concretions of siderite, manganosiderite, and rhodochrosite. These concretions probably formed in response to elevated pore-water alkalinity and total dissolved carbon dioxide (CO2) concentrations resulting from bacterial sulfate reduction, as indicated by nodule stable-isotope compositions and pore-water geochemistry. These nodules differ from those found in upper Cenozoic hemipelagic sequences in that they are not associated with methanogenesis. Phosphate minerals (carbonate-fluorapatite) precipitated in some intervals, probably as the result of desorption of phosphorus from iron and manganese during reduction. The bulk chemical composition of the sediments differs little from that of North Atlantic Quaternary abyssal red clays, but may contain a minor hydrothermal component. The silicon/ aluminum (Si/Al) ratio, however, is high and variable and probably reflects original variations in biagenic opal, much of which is now altered to smectite and/or opal CT. An increase in the sodium/potassium (Na/K) ratio in the upper Eocene corresponds to the beginning of coarser-grained feldspar flux to the site, possibly marking the onset of more vigorous deep currents. Although the Site 647 cores provide a nearly complete high-resolution, high-latitude Eocene-Oligocene record, the high sedimentation rate and somewhat unusual diagenetic conditions have led to variable alteration of benthic foraminifers and fine-fraction carbonate and have overprinted the original stable-isotope records. Planktonic foraminifers are less altered, but on the whole, there is little chance of sorting out the nature and timing of environmental change on the basis of our stable-isotope analyses.

Proceedings of the Ocean Drilling Program: Scienti↗
USGS1988

Sediment deposition in the Late Holocene abyssal Black Sea with climatic and chronological implications

The temporal sedimentary patterns in the Late Holocene central eastern and western Black Sea are very similar. The sedimentary history was most visibly affected by the coccolithophorid species Emiliania huxleyi which briefly invaded the Black Sea for the first time (“First Invasion Period”), nearly disappeared again shortly afterwards (“Transition Sapropel”), but returned permanently several centuries later (“Final Invasion Period”). The temporary near-disappearance of E. huxleyi was probably caused by a temporary drop in salinity. Accumulation of E. huxleyi was on average about 40% higher in the western than in the eastern Black Sea. Highest coccolithophorid production occurred basin-wide during part of the Little Ice Age. The accumulation of terrigenous matter was generally higher in the eastern than in the western Black Sea by about 20%.

Deep Sea Research Part A, Oceanographic Research P↗
USGS1988

Geochemical and climatic effects of increased marine organic carbon burial at the Cenomanian/Turonian boundary

Perhaps the most significant event in the Cretaceous record of the carbon isotope composition of carbonate 1,2 , other than the 1–2.5 ‰ negative shift in the carbon isotope composition of calcareous plankton at the Cretaceous/Tertiary boundary 3 , is the rapid global positive excursion of ∼ 2 ‰ ( 13 C enrichment) which took place between ∼ 91.5 Myr and 90.3 Myr (late Cenomanian to earliest Turonian (C/T boundary event)) 1,4,5 . This excursion has been attributed to a change in the isotope composition of the marine total dissolved carbon (TDC) reservoir resulting from an increase in rate of burial of 13 C-depleted organic carbon, which coincided with a major global rise in sea level 5 during the so-called C/T oceanic anoxic event (OAE) 6 . Here we present new data, from nine localities, which demonstrate that a positive excursion in the carbon isotope composition of organic carbon at or near the C/T boundary 7,8 is nearly synchronous with that for carbonate and is widespread throughout the Tethys and Atlantic basins (Fig. 1), as well as in more high-latitude epicontinental seas. The postulated increase in the rate of burial of organic carbon may have had a significant effect on CO 2 and O 2 concentrations in the oceans and atmosphere, and consequent effects on global climate and sedimentary facies.

Nature↗
USGS1987

15N/14N variations in Cretaceous Atlantic sedimentary sequences: Implication for past changes in marine nitrogen biogeochemistry

At two locations in the Atlantic Ocean (DSDP Sites 367 and 530) early to middle Cretaceous organic-carbon-rich beds (“black shales”) were found to have significantly lower δ 15 N values (lower 15 N/ 14 N ratios) than adjacent organic-carbon-poor beds (white limestones or green claystones). While these lithologies are of marine origin, the black strata in particular have ° 15 N values that are significantly lower than those previously found in the marine sediment record and most contemporary marine nitrogen pools. In contrast, black, organic-carbon-rich beds at a third site (DSDP Site 603) contain predominantly terrestrial organic matter and have C- and N-isotopic compositions similar to organic matter of modern terrestrial origin. The recurring 15 N depletion in the marine-derived Cretaceous sequences prove that the nitrogen they contain is the end result of an episodic and atypical biogeochemistry. Existing isotopic and other data indicate that the low 15 N relative abundance is the consequence of pelagic rather than post-depositional processes. Reduced ocean circulation, increased denitrification, and, hence, reduced euphotic zone nitrate availability may have led to Cretaceous phytoplankton assemblages that were periodically dominated by N 2 -fixing blue-green algae, a possible source of this sediment 15 N-depletion. Lack of parallel isotopic shifts in Cretaceous terrestrially-derived nitrogen (Site 603) argues that the above change in nitrogen cycling during this period did not extend beyond the marine environment.

Earth and Planetary Science Letters↗