James C. Ingle

Stable isotope and salinity systematics in estuarine waters and carbonates: San Francisco Bay

Salinities, δD and δ18O values of water samples collected bimonthly from two stations in San Francisco Bay estuary during 1991–1993, and along a salinity transect in March of 1992, indicate a linear mixing relation between the isotopic compositions of the waters and their salinities. The salinities and stable isotope compositions of samples from two locations in San Francisco Bay vary in response to changes in freshwater inflow. The data from these locations indicate simple mixtures of Pacific Ocean water (salinity ≈33, δ18O ≈ 0 to −1‰, δD ≈ 0 to −10‰) and Sacramento-San Joaquin River water (salinity ≈ 0, δ18O = −10 to −12‰, δD = −75 to −85‰). Preliminary water balance estimates, using isotopic differences between local and upland runoff, suggest that local runoff (including waste water) comprises less than 20% of total freshwater entering the bay. The average δ18O values of mussel shells (Mytilus edulis) collected live from eight locations in San Francisco Bay primarily reflect the δ18O of the water in which they grew. Shells subsampled along growth bands show that seasonal shifts in salinity and δ18O are recorded in the shells. Therefore, the use of stable isotope measurements should be useful in reconstructing pre-instrumental bay salinity and associated freshwater inflow (both annual average values and seasonal variations) to the San Francisco Bay, as well as potentially other estuarine systems.

Stable isotope record of late Holocene salinity and river discharge in San Francisco Bay, California

Abstract Oxygen and carbon isotopic measurements of fossil mollusks from San Francisco Bay are used to derive a record of paleosalinity and paleostreamflow for the past 5,900 years. The δ 18 O and δ 13 C values of river water (−12‰ and −9‰) are markedly different than seawater (0‰ and 1‰), and vary systematically as a function of salinity in the estuary. The data show that annually averaged salinity in the south-central part of the Bay was very close to the modern ‘diversion-corrected’ value of 26.8‰ over the past 2,700 years, and 4‰ lower than modern between 3,800 and 5,100 yr B.P. Based on those salinities, the average annual river inflow to San Francisco Bay is calculated to have been 1290 m 3 /s over the past 2,400 years, and 1990 m 3 /s between 3,800 and 5,100 yr B.P., 1.8 times greater than the modern ‘diversion-corrected’ value of 1100 m 3 /s, assuming a constant bay volume. The inferred river discharge record generally corroborates independent paleohydrologic records in California, including tree-ring, treeline and lake level records.

Neogene stratigraphy, foraminifera, diatoms, and depositional history of Maria Madre Island, Mexico: Evidence of early Neogene marine conditions in the southern Gulf of California

Abstract Foraminifera and diatoms have been analyzed from an upper Miocene through Pleistocene(?) sequence of marine sediments exposed on Maria Madre Island, largest of the Tres Marias Islands off the Pacific coast of Mexico. The Neogene stratigraphic sequence exposed on Maria Madre Island includes a mid-Miocene(?) non-marine and/or shallow marine sandstone unconformably overlain by a lower upper Miocene to uppermost Miocene upper to middle bathyal laminated and massive diatomite, mudstone, and siltstone unit. This unit is unconformably overlain by lower Pliocene middle to lower bathyal sandstones and siltstones which, in turn, are unconformably overlain by upper Pliocene through Pleistocene(?) upper bathyal to upper middle bathyal foraminiferal limestones and siltstones. These beds are unconformably capped by Pleistocene terrace deposits. Basement rocks on the island include Cretaceous granite and granodiorite, and Tertiary(?) andesites and rhyolites. The upper Miocene diatomaceous unit contains a low diversity foraminiferal fauna dominated by species of Bolivina indicating low oxygen conditions in the proto-Gulf Maria Madre basin. The diatomaceous unit grades into a mudstone that contains a latest Miocene upper to middle bathyal biofacies characterized by Baggina californica and Uvigerina hootsi along with displaced neritic taxa. An angular unconformity separates the upper Miocene middle bathyal sediments from overlying lower Pliocene siltstones and mudstones that contain a middle to lower bathyal biofacies and abundant planktonic species including Neogloboquadrina acostaensis and Pulleniatina primalis indicating an early Pliocene age. Significantly, this Pliocene unit contains common occurrences of benthic species restricted to Miocene sediments in California including Bulimina uvigerinaformis . Pliocene to Pleistocene(?) foraminiferal limestones and siltstones characterize submarine bank accumulations formed during uplift of the Tres Marias Island area, and include abundant planktonic foraminifera such as Pulleniatina obliquiloculata and Neogloboquadrina duterteri . Common benthic foraminifera in this unit are indicative of upper bathyal water depths. The Neogene depositional history recorded on Maria Madre Island involves an early late Miocene subsidence event marking formation of the Tres Marias Basin with relatively undiluted diatomaceous sediment deposited in a low oxygen setting. Subsidence and deepening of the basin continued into the early Pliocene along with rapid deposition of terrigenous clastics. Uplift of the basinal sequence began in late Pliocene time accompanied by deposition of upper Pliocene-Pleistocene foraminiferal limestones on a rising submarine bank. Continued episodic uplift of the Neogene deposits brought the island above sea level by late Pleistocene time.

Microfacies Analysis of Recent Carbonate Environments in the Southern Gulf of California, Mexico — A Model for Warm-Temperate to Subtropical Carbonate Formation

Abstract The La Paz area in the southwestern Gulf of California, Mexico, provides an ideal site for studying Recent warm-temperate to subtropical carbonate environments. Carbonate factories include small pocket bays, a rhodolith dominated carbonate shelf, and a mixed-carbonate siliciclastic high-energy beach. Underwater mapping and constituent analysis have revealed free-living coralline red algae in the form of rhodoliths to be the main carbonate producers, contributing 33% of the biogenic constituents to the sediment. Other significant contributions come from corals (20%), molluscs (18%), echinoderms (5%), and benthic foraminifera (4%). The benthic foraminiferal community includes mixtures of tropical and temperate species. This compositional pattern stands in marked contrast to (1) better-studied coral and green algae dominated tropical carbonate systems and (2) foraminifer, bryozoan, and mollusc dominated cool-water carbonates. Cluster analyses of biogenic constituents and benthic foraminifera revealed distinctive coral, coralline red algal, and molluscan microfacies. However, subdivisions characterized by benthic foraminifera more closely reflect the bottom facies observed during underwater mapping as opposed to patterns defined by cluster analysis of biogenic components. Diagnostic features for interpreting similar environments found in the fossil record include the (1) co-occurrence of coralline red algae and corals together with the absence of calcareous green algae, (2) presence of 5–10 genera of larger benthic foraminifera, (3) laterally and vertically not extensive character of environments, and (4) changes of microfacies over short distances.

MODERN WARM-TEMPERATE AND SUBTROPICAL SHALLOW-WATER BENTHIC FORAMINIFERA OF THE SOUTHERN GULF OF CALIFORNIA, MEXICO

The La Paz area of the southwestern Gulf of California, Mexico, provides an ideal setting for studying the distribution of Recent warm-temperate to subtropical littoral to inner neritic foraminiferal assemblages in a carbonate-dominated setting. Four representative subenvironments including two pocket bays, a rhodolith-dominated carbonate bioherm, and a mixed carbonate-siliciclastic high-energy beach were selected for detailed foraminiferal analysis. Assemblages of benthic foraminifera were compared to bottom observations, time-series temperature recordings, and occurrences of other carbonate-producing biota. Foraminifera are of intermediate importance among carbonate producers in the La Paz environments, making up 4% of the total calcareous biota. However, they reach abundances of up to 27% in middle shelf localities where species numbers are also highest. Miliolid foraminifera, dominated by the genus Quinqueloculina, are the most abundant group and frequently comprise more than 60% of foraminiferal assemblages. Larger foraminifera include Peneroplis pertusus pertusus, P. pertusus arietina, Amphisorus hemprichii, Gypsina vesicularis, and Sphaerogypsina globulus. The distribution of shallow-water assemblages in the La Paz environments is largely controlled by substrate type. Q- and R-mode cluster analyses revealed eight distinct foraminiferal biofacies that appear to be related to microlithofacies defined on the basis of biogenic components. Assemblages include mixtures of tropical and temperate species as demonstrated by the abundance of Peneroplis pertusus, Amphisorus hemprichii , and Quinqueloculina spp., which are representative of tropical communities. In contrast, the most abundant species with temperate affinities include Buccella parkerae, Gavelinopsis sp., Haplophragmoides sp., and Oolina melo. Application of the Murray model for distinguishing cool-temperate, warm-temperate, and subtropical faunas clearly points to the warm-temperate to subtropical character of the foraminiferal assemblages associated with the carbonate factories and environments of the southern Gulf of California.

Subsidence and uplift of the Late Cretaceous-Cenozoic margin of California: New evidence from the Gualala and Point Arena basins

Cretaceous-Paleocene deposits of the Gualala basin and overlying deposits of the Neogene Point Arena basin record the complex tectonic and depositional history of the California continental margin as it changed from a convergent tectonic regime to a translational regime during the evolution of the San Andreas transform fault system. The Gualala and Point Arena deposits are among the northernmost onshore exposures of upper Cretaceous and Cenozoic marine strata west of the San Andreas fault and, as such, constitute a critically important but not intensely studied succession of rocks. In this paper, lithologic, stratigraphic, and paleobathymetric data from the Gualala and Point Arena stratigraphic successions are used to produce quantitative geohistory analyses that illustrate the subsidence and uplift of a portion of the California continental margin over the past 80 million yr. The history of subsidence and uplift revealed through geohistory analysis of the Gualala and Point Arena basin-fill deposits clearly reflects two episodes of basin development directly associated with the convergent and transform phases in the evolution of the California margin. Beginning in Late Cretaceous time (∼80 Ma), the Gualala basin underwent slow rates of tectonic subsidence (∼30 m/m.y.). In late Paleocene time (∼57 Ma), tectonic subsidence increased to ∼140 m/m.y. and was accompanied by rapid rates of sediment accumulation (∼440 m/m.y.) that continued through the Eocene until terminated by basin filling, cessation of subduction tectonics, and flexing and uplift of the continental margin. Initial development of the Neogene Point Arena basin occurred in latest Oligocene time (∼24 Ma), some 2 to 4 m.y. after the birth of the San Andreas transform system. The earliest history of the Point Arena basin was marked by volcanism and extremely high rates of tectonic subsidence (∼1,060 m/m.y.) characteristic of a transtensional origin. Based on our analysis, the tectonic and depositional histories of the Gualala and Point Arena basins can be summarized as follows: (1) during Late Cretaceous to middle Eocene, submarine fans of the Gualala and German Rancho Formations were deposited in a >2,000-m-deep subsiding basin that probably formed as a fore-arc basin in a setting characterized by oblique convergence and subduction along the California margin; (2) flexing and erosion of Cretaceous-Paleocene bathyal deposits of the Gualala basin occurred in the Oligocene epoch as the Pacific-Farallon spreading ridge approached the California margin, and as the accretionary prism outboard of the Gualala fore-arc basin migrated landward; (3) initial development of the Point Arena basin in earliest Miocene time (∼24 Ma) was accompanied by extrusion of the Iversen Basalt during the transitional period between convergent and transform tectonic regimes; (4) rapid subsidence of the Neogene Point Arena basin during Miocene time was related to transtension associated with the San Andreas fault, a pattern common to many of the small pull-apart basins formed along the California margin at this time; (5) early deposition in the Point Arena basin was dominated by submarine-fan deposits of the lower Miocene Skooner Gulch and Gallaway Formations, whereas siliceous biogenic sediments of the overlying Point Arena Formation signalled reduced deposition of coarse-grained terrigenous sediment during middle to late Miocene time; and (6) margin flexure and uplift of the Point Arena basin occurred in middle Pliocene (∼3.5 Ma) and early Pleistocene (∼1.6 Ma) time as a function of transpressional deformation of the crust in response to Pacific-North American plate interaction.

A Permian–Triassic boundary section at Quinn River Crossing, northwestern Nevada, and implications for the cause of the Early Triassic chert gap on the western Pangean margin

The Upper Permian–Lower Triassic Quinn River Formation in northwestern Nevada was previously thought to represent an incomplete Permian–Triassic boundary sequence, owing to an inferred disconformable relationship between Permian radiolarian- and spicule-rich chert and overlying Triassic siltstone. Petrographic and geochemical studies demonstrate that the “siltstone” is in fact a radiolarian-bearing early authi-genic dolomicrite, with both the chert and dolomicrite deposited conformably in deep water. Chert production declined or ceased in the Late Permian and reappeared in the Spathian, forming a widespread “chert gap” in Permian–Triassic sequences. Given the conformable lithofacies relationships, deep-water depositional setting, new radiolarian data extending ranges of key taxa, and the presence of the global chert gap, sedimentation in the Quinn River Formation was apparently continuous across the Permian–Triassic boundary. This represents the first Permian–Triassic boundary section in the United States portion of the North American Cordillera, and one of the few deep-water sections worldwide. Organic carbon isotope stratigraphy of the Quinn River Formation displays multiple excursions through sediments of Wuchiapingian–Anisian age, with a negative excursion 1.54 m above the chert-shale transition likely representing the Permian–Triassic boundary. The multiple excursions in the organic carbon record verify studies of the carbonate carbon record in China that suggest instability in the isotopic record throughout the Early Triassic, and demonstrate that the Permian–Triassic boundary isotope excursion was not an isolated event. Stratigraphic variation in redox-sensitive trace metals indicates that seawater became less oxic slightly before the chert-shale transition, in turn impacting siliceous sponge communities and creating the widespread chert gap. The distinctive dolomicrites in the Quinn River Formation represent a widespread lithofacies deposited in many localities during the Late Permian–Early Triassic and express early authigenic formation of dolomite via microbial sulfate reduction in organic-rich, low-oxygen environments.

Pacific neogene stratigraphy

The Fourth International Congress of Pacific Neogene Stratigraphy, was held July 29–31, 1987, at the University of California, Berkeley. This very successful congress was organized by the Regional Committee on Pacific Neogene Stratigraphy (RCPNS) of the International Union of Geological Sciences (IUGS) and the International Geological Correlation Program (IGCP) Project 246 “Pacific Neogene Events in Time and Space.” The meeting was attended by 180 scientists from 16 different countries, and more than 90 presentations were made, on topics ranging from paleoclimatology, biostratigraphy, regional stratigraphy and geologic history, new techniques in stratigraphy, evolutionary studies, and modern biofacies and sediment relationships. A. R. Edwards of the New Zealand Geological Survey (Lower Hutt, New Zealand) spoke on climatic events that he recognizes in the late Neogene strata of New Zealand. The carbon isotope shift during chron 6 (6.3–6.5 Ma) is identified in the sequence at Blind River (Marlborough, New Zealand). The extinction of ∼25% of New Zealand molluscan genera during the latest Micoene (Kapitean Stage) accompanied the greatly accelerated diversification of planktonic foraminifera lineages at this time. The New Zealand events are also coeval with the Messinian “salinity crisis” in the Mediterranean. A series of events (extinctions of Mollusca, appearance of glacial rock types, foraminifera speciation, and nannofossil appearances) in New Zealand late Pliocene rocks reflect the climatic deterioration. One type of sub-Antarctic molluscan fauna abruptly appeared in central North Island at 2.4 m.y., coeval with the onset of major Northern Hemisphere glaciation.

Correlation of Continental-Margin and Deep-Sea Sequences--Neogene Examples from Pacific: ABSTRACT

The Deep Sea Drilling Project (DSDP) has produced fundamental advances in Cenozoic planktonic biostratigraphy and our understanding of the Cenozoic paleo-oceanographic and paleoclimatic history of the world ocean. These new insights and data serve to increase the precision and resolution of provincial biostratigraphies and correlations commonly applied to continental-margin sequences, and thus result in more accurate reconstructions of margin histories. DSDP-IPOD Legs 18, 19, 31, 57, and 63 included drill sites close to tectonically active continental margins surrounding the North Pacific. They provide clear examples of correlation of deep-sea and epicontinental marine deposits in Mexico, California, Japan, and Korea. In particular, DSDP Site 173 off northern California h s yielded an important lower Miocene through Pleistocene (N4-N22) reference section, demonstrating the usefulness of DSDP data for interpretation of margin biostratigraphic, sedimentologic, and tectonic events on a regional scale. Multiple siliceous and calcareous plankton zones within this sequence provide an average biostratigraphic resolution of 0.5 m.y. with paleo-oceanographically induced biofacies trends marking zones of special value for interbasin correlation across latitude. Many of the planktonic datums and biofacies trends clearly defined in the thin (320 m), but nearly complete, Neogene column at DSDP Site 173 can be readily recognized in the thick paleoenvironmentally diverse and structurally disordered, continental-margin deposits now exposed along the Pacific Coast of Nort America. These correlations provide a framework for calibrating provincial biostratigraphic units; estimating rates of sediment accumulation, subsidence, and uplift in margin sequences; and hindcasting periods of increased primary productivity, variations in the oxygen-minimum layer, and deposition of sediments favorable for hydrocarbon generation. End_of_Article - Last_Page 473------------

Effect of seawater temperature, pH, and nutrients on the distribution and character of low abundance shallow water benthic foraminifera in the Galápagos

In order to help predict the effects of anthropogenic stressors on shallow water carbonate environments, it is important to focus research on regions containing natural oceanographic gradients, particularly with respect to interactions between oceanography and ecologically sensitive carbonate producers. The Galápagos Archipelago, an island chain in the eastern equatorial Pacific, spans a natural nutrient, pH, and temperature gradient due to the interaction of several major ocean currents. Further, the region is heavily impacted by the El Niño—Southern Oscillation (ENSO) and the Galápagos exhibited widespread coral bleaching and degradation following the strong ENSO events of 1982–1983 and 1997–1998. These findings are coupled with reports of unusually low abundances of time-averaged benthic foraminiferal assemblages throughout the region. Foraminifera, shelled single-celled protists, are sensitive to environmental change and rapidly respond to alterations to their surrounding environment, making them ideal indicator species for the study of reef water quality and health. Here, statistical models and analyses were used to compare modern shallow water benthic foraminiferal assemblages from 19 samples spanning the Galápagos Archipelago to predominant oceanographic parameters at each collection site. Fisher α diversity indices, Ternary diagrams, Canonical Correspondence Analysis, regression tree analysis and FORAM-Index (FI; a single metric index for evaluating water quality associated with reef development) implied a combined impact from ENSO and upwelling from Equatorial Undercurrent (EUC) waters to primarily impact foraminiferal abundances and drive assemblage patterns throughout the archipelago. For instance, repeated ENSO temperature anomalies might be responsible for low foraminiferal density, while chronically high nutrients and low aragonite saturation and low pH—induced by EUC upwelling and La Niña anomalies—likely inhibited post-ENSO recovery, and caused foraminiferal assemblages to exhibit a heterotrophic dominance in the southern archipelago. What resulted are low FI values in the southern collection sites, indicating environments not conducive to endosymbiont development and/or recovery.