Peter H. Roth

Middle Cretaceous calcareous nannofossil biogeography and preservation in the Atlantic and Indian oceans: Implications for paleoceanography

Abstract The quantitative study of calcareous nannofossil preservation and biogeographic distribution puts constraints on paleoceanographic models for the Atlantic and Indian oceans during the middle Cretaceous. We establish a dissolution ranking of middle Cretaceous calcareous nannofossils based on susceptibility to dissolution. A positive correlation of organic carbon content of the sediments and the relative abundance of Watznaueria indicates that much of the carbonate dissolution was caused by the release of carbon dioxide during oxidation of organic matter. The depth distribution of the most dissolution resistant species, Watznaueria barnesae , shows relative-abundance maxima at the deepest sites and a maximum where intermediate waters impinge on slopes; this reflects stagnant, oxygen-poor bottom waters and possibly a mid-water oxygen minimum zone with increased deposition and catabolic breakdown of organic matter. Factor analyses are performed on various data sets for three different time slices ranging in age from late Aptian to early Albian, middle Albian and late Albian to Cenomanian. The resulting factors can be related to paleoceanographic conditions such as colder water at high latitudes, high fertility (upwelling conditions) and neritic conditions for all three time intervals. A wide tropical to subtropical zone was flanked by boreal and austral zones. Upwelling conditions probably existed along the North African margin and over the Falkland Plateau. Zonal upwelling is tentatively postulated for the northwestern and northeastern margin of the North Atlantic in the middle Albian and for the eastern margin of the North Atlantic off Western Europe for the late Albian to Cenomanian. We speculate on surface water current patterns, location of upwelling regions and haline-driven deep circulation for the middle Cretaceous.

Floral and solution patterns of coccoliths in surface sediments of the North Pacific

Coccolith assemblages from core-top samples of the North Pacific seafloor follow the distribution of surface water masses. Gephyrocapsa caribbeanica, Umbellosphaera tenuis, and Coccolithus pelagicus are the most important identifiers of surface water mass. Different multivariate statistical techniques group shallow samples in somewhat different spatial patterns. Factor analysis condenses the information contained in sixteen -ies percentages to four new variables that can be interpreted as species assemblages. By canonical correlation analysis these assemblages can in turn be related to the environment. The first canonical correlation indicates that Factor 2, the G. caribbeanica assemblage, is represented in samples underlying cooler surface water. A second possible correlation describes a direct relationship between Factor 3, the Umbilicosphaera sibogae assemblage, and the remaining environmental variable: surface water salinity. Factor 1, the G. oceanica assemblage, and Factor 4, the Emiliania huxleyi assemblage, are not as closely related to the selected environmental measures of temperature, salinity, dissolved oxygen, and inorganic phosphate-phosphorus as are Factor 2 and Factor 3. Because samples were coded according to their overlying water mass, group separations produced by discriminant analysis are easily interpreted in terms of the environment. Overlap of group assignments occurs for samples from the South China Sea and extreme western Pacific. Even partially dissolved samples retain the signature of surface water mass. Group assignment for those samples is generally reliable with the exception of samples from the Central Water Mass, where almost half of the shallow and half of the deep samples show strongest affinities with one of the other three water masses. Selective solution, therefore, only partially obscures the record of floral provinciality. Partial dissolution of coccolith assemblages does not proceed according to one systematic rank order: the sequence of species removal is variable from region to region. The more abundant species tend to consistently appear at an extreme of the ranking, for example: Umbilicosphaera sibogae is among the least resistant and C. leptoporus and G. oceanica are among the most resistant species. Maximum depths of coccolith lysocline and compensation depth occur generally in the central part of the basin and shoal towards its margins, as previously observed for planktonic foraminifera.

Ocean circulation and calcareous nannoplankton evolution during the Jurassic and Cretaceous

Abstract Spatial and temporal distribution of calcareous nannoplankton aid in the reconstruction of ocean circulation during the Cretaceous. Biogeographic patterns of calcareous nannoplankton during the late Cretaceous are mostly zonal, reflecting mostly latitudinal gradients of surface water temperature. During the middle Cretaceous calcareous nannofossil assemblages are more strongly affected by surface water fertility (concentration of limiting nutrients) especially in a wide tropical belt between 50° and 45°N latitudes. Major upwelling regions off northwest Africa, in the south Atlantic, and the equatorial Pacific divergence are apparent. Temporal changes in the composition of middle Cretaceous nannofloras are indicative of changes in surface water fertility and vertical mixing at frequencies of tens to hundreds of kiloyears (ka). Changes in the intensity of atmospheric circulation with frequencies close to the Milankovitch cycles appear to be the ultimate cause of these minor changes in the circulation of the upper ocean. Calcareous nannofossil preservation patterns show that during the Middle Cretaceous deep waters were quite homogeneous with respect to carbonate saturation in the Atlantic Ocean but not in the Indo-Pacific Ocean where a more distinctive calcite compensation depth (CCD) is apparent. Water-mass structure of the major oceans during the Mesozoic was thus quite different. Taxonomic evolution of calcareous nannofossils was also strongly affected by ocean circulation and fertility. Major peaks in the rates of diversification coincide with anoxic events, that is periods of increased preservation of organic carbon in the oceans which coincide with high sea level stands. During anoxic events ocean circulation was sluggish, vertical mixing was slow, deep ocean ventilation poor, and the concentration of limiting nutrients in surface waters was low. Increased competition for limiting nutrients and possibly preferential cropping of the dominant forms by zooplankton resulted in increased rates oof speciation during anoxic events. Periods of major production of nannofossil carbonate coincided with low sea-level stands when oceans were more vigorously stirred and surface waters were more fertile. The first such period was the latest Jurassic when calcareous nannoplankton invaded the open oceans and became the major carbonate producers in the pelagic realm during the Mesozoic. The frequency of peaks and troughs in evolutionary rates is on the order of 10–30 Ma and appears to be irregular. The coincidence of periods of anoxia with high sea-level stands, which in turn appear to be caused by more rapid sea-floor spreading indicates that the ultimate diving forces of nannoplankton evolution are processes in the earths mantle. These major tectonically driven fluctuations in ocean-atmosphere circulation with frequencies of a few million to a few tens of millions of years are hereby named Fischer cycles . Superimposed astronomically driven Milankovitch cycles affected relative abundance patterns but not taxonomic evolution of calcareous nannoplankton. Rates of nannoplankton extinction are about five times higher than background rates at the end of the Jurassic and orders of magnitude higher in the latest Cretaceous possibly indicating a terrestrial or extraterrestrial catastrophe.

Stable isotopic and carbonate cyclicity in Lower Cretaceous deep-sea sediments: Dominance of diagenetic effects

Abstract Oxygen and carbon isotopic variability of the dominant ( μ m) carbonate fraction within bedded, organic-carbon rich Lower Cretaceous sediment intervals from various DSDP sites are closely correlated with preservational changes in the carbonates. Isotopic fluctuations are absent where carbonate contents vary little and where the carbonate fraction is dominated by biogenic phytoplankton remains. Within each of the studied intervals oxygen and carbon isotopic ratios become increasingly more negative in samples with carbonate contents higher than about 60% in which the proportion of diagenetic microcarbonate increases rapidly. Carbon isotopic ratios show a trend towards positive values in samples with carbonate contents of less than 40% and strong signs of dissolution. The taxonomic composition of nannofossil assemblages varies little within single intervals, despite significant differential diagenesis among individual beds; this points towards ecological stability of oceanic surface waters during the deposition of alternating beds. Bedding is, however, closely related to changing bioturbation intensity, indicating repeated fluctuations of the deep-water renewal rates and oxygen supply. Various microbial decomposition processes of organic matter leading to bed-specific differential carbonate diagenesis resulted in an amplification of primary bedding features and are considered responsible for most of the observed fluctuations in the stable isotopic ratios and carbonate contents.

Early history of the Atlantic Ocean and gas hydrates on the Blake Outer Ridge: Results of the Deep Sea Drilling Project Leg 76

Leg 76 of the Deep Sea Drilling Project achieved two major scientific objectives. The first objective was met at Site 533, where on the Blake Outer Ridge, gas hydrates were identified by geophysical, geochemical, and geological studies. Gas-hydrate decomposition produced a volumetric expansion of 20:1 of gas volume to pore-fluid volume; this expansion exceeded by about a factor of four the volume of gas that could be released from solution in pore water under similar conditions. The gas hydrate includes methane, ethane, propane, and isobutane but apparently excluded normal butane and higher molecular weight hydrocarbons as predicted from gas hydrate crystallography. For the first time, marine gas hydrates were tested with a pressure core barrel. The second objective was achieved when coring at Site 534 in the Blake-Bahama Basin sampled the oldest oceanic sediments yet recovered. The sequence of oceanic basement and overlying sediments documents the geologic history of the early stages of the opening of the North Atlantic Ocean in detail. The oldest oceanic sediments are red claystones and laminated green and brown claystones of middle Callovian age. This finding supports the interpretation that the beginning of the modern North Atlantic occurred in the early Callovian (∼ 155 m.y. B.P.), as much as 20 m.y. later in time than often previously thought.

Scanning electron and light microscopy of the same small object

Method for mounting microfossils (especially nannofossils) on slide which can be used for both microscopes

Sedimentation history and biostratigraphy of ophiolite-related Tertiary sediments, Luzon, Philippines

Pelagic and hemipelagic sediments deposited on the Zambales Ophiolite contain a nearly continuous depositional record of the original setting and emplacement history of this large ophiolite from the late Eocene through the Miocene. Pelagic limestone with thin ash layers (the Aksitero Formation) caps the volcanic complex of the ophiolite along its east flank. Calcareous nannofossil biostratigraphy of this limestone gives sedimentation rates of 3–5 m/m.y. from the late Eocene through the early Oligocene. Rates increase to 10 m/m.y. or more in the upper Aksitero Formation, where sandy turbidites appear in the middle and upper Oligocene sections. Lower Miocene mudstone, sandstone, and conglomerate of the Moriones Formation were deposited at much higher rates; this formation includes channels and debris-flow deposits characteristic of deep-sea fans. Oligocene sandstones are predominantly volcaniclastic, whereas sandstones in the lower Miocene section contain serpentine and other components derived from the ultramafic complex of the ophiolite. Sedimentary facies and sandstone composition show that the Zambales was deeply eroded by the early Miocene and probably first emerged above sea level in the middle or late Oligocene, only 10 to 15 m.y. after it formed as new ocean crust. A comparison of the Aksitero Formation with Deep Sea Drilling Project (DSDP) sites in the western Pacific suggests that the Zambales Ophiolite was originally part of a marginal basin and not an island arc.

Late Neogene calcareous nannofossil biostratigraphy of the Gulf of Aden region

Abstract The hemilpelagic sedimentary sequence in the Gulf of Aden has yielded a continuous succession of relatively well preserved late Neogene nannofossils. Detailed quantitative study of the nannofossils recognized forty-two datum planes which are potentially useful in the late Neogene biostratigraphy. A high rate of sedimentation (52.2 mm/k.y.) allowed us to recognize even relatively closely spaced datum planes and locate them within the sequence. Absolute ages of the nannofossil datum planes younger than 4 Ma have been estimated by interpolation between tephra layers, for which ages were determined by correlation with paleomagnetically and radiometrically dated tephra layers exposed in East African terrestrial sequences. Ages of the older late Neogene datum planes are estimated by interpolation between the FOD of D. hamatus and the Moitu Tuff, and by extrapolation below the FOD of D. hamatus . Previously defined and newly introduced datum planes define twenty nannofossil zones, six each in the Quaternary, Pliocene and late Miocene, and two in the middle Miocene, with durations of 0.13 to 0.78, 0.37 to 0.81 and 0.14 to 2.49 m.y. in the Quaternary, Pliocene and late Miocene time period respectively. Nine new zones and five new subzones are proposed in this paper, three zones in the Pleistocene, two zones in the Pliocene, two zones and five subzones in the late Miocene, and two zones in the upper part of the middle Miocene. Six new datum planes are introduced, FOD of P. lacunosa in the Pliocene, FOD and LOD of D. neohamatus , LODs of D. loeblichii and D. calcaris in the late Miocene and LOD of D. extensus in the latest middle Miocene.

Stable isotopic and carbonate cyclicity in deep sea sediments from different DSDP Holes, supplement to: Thierstein, Hans R; Roth, Peter H (1991): Stable isotopic and carbonate cyclicity in Lower Cretaceous deep-sea sediments: Dominance of diagenetic effects. Marine Geology, 97(1-2), 1-34

Oxygen and carbon isotopic variability of the dominant (<38 µm) carbonate fraction within bedded, organic-carbon rich Lower Cretaceous sediment intervals from various DSDP sites are closely correlated with preservational changes in the carbonates. Isotopic fluctuations are absent where carbonate contents vary little and where the carbonate fraction is dominated by biogenic phytoplankton remains. Within each of the studied intervals oxygen and carbon isotopic ratios become increasingly more negative in samples with carbonate contents higher than about 60% in which the proportion of diagenetic microcarbonate increases rapidly. Carbon isotopic ratios show a trend towards positive values in samples with carbonate contents of less than 40% and strong signs of dissolution. The taxonomic composition of nannofossil assemblages varies little within single intervals, despite significant differential diagenesis among individual beds; this points towards ecological stability of oceanic surface waters during the deposition of alternating beds. Bedding is, however, closely related to changing bioturbation intensity, indicating repeated fluctuations of the deep-water renewal rates and oxygen supply. Various microbial decomposition processes of organic matter leading to bed-specific differential carbonate diagenesis resulted in an amplification of primary bedding features and are considered responsible for most of the observed fluctuations in the stable isotopic ratios and carbonate contents.