Henry T. Mullins

Oxygen-minimum zone edge effects: Evidence from the central California coastal upwelling system

Geologic and biological data from box cores, gravity cores, and bottom photographs across the modern coastal upwelling system of central California reveal that biogeochemical activity increases along the edges of the oxygen-minimum zone (OMZ; <0.5 ml/1 O2). Macrofauna and benthic foraminifers, as well as relative concentrations of calcium carbonate, glauconite, and fecal pellets, all display edge effects. The major controls appear to be a combination of dissolved oxygen concentration of near-bottom waters, bottom currents, and possibly bacterially mediated nutrient recycling, as well as food supplies. OMZ edge effects should be recognizable in the rock record and may prove to be powerful paleoenvironmental/paleoecological indicators.

Increasing Great Lake–Effect Snowfall during the Twentieth Century: A Regional Response to Global Warming?

Abstract The influence of the Laurentian Great Lakes on the climate of surrounding regions is significant, especially in leeward settings where lake-effect snowfall occurs. Heavy lake-effect snow represents a potential natural hazard and plays important roles in winter recreational activities, agriculture, and regional hydrology. Changes in lake-effect snowfall may represent a regional-scale manifestation of hemispheric-scale climate change, such as that associated with global warming. This study examines records of snowfall from several lake-effect and non-lake-effect sites throughout most of the twentieth century in order to 1) determine whether differences in snowfall trends exist between these settings and 2) offer possible linkages between lake-effect snow trends and records of air temperature, water temperature, and ice cover. A new, historic record of oxygen isotope [δ18O(CaCO3)] data from the sediments of three eastern Finger Lakes in central New York is presented as a means of independently asses...

Carbonate apron models: Alternatives to the submarine fan model for paleoenvironmental analysis and hydrocarbon exploration

Sediment gravity flow deposition along the deep-water flanks of carbonate platforms typically does not produce submarine fans. Rather, wedge-shaped carbonate aprons develop parallel to the adjacent shelf/slope break. The major difference between submarine fans and carbonate aprons is a point source with channelized sedimentation on fans, versus a line source with sheet-flow sedimentation on aprons. Two types of carbonate aprons may develop. Along relatively gentle (< 4°) platform-margin slopes, aprons form immediately adjacent to the shallow-water platform and are referred to as carbonate slope aprons. Along relatively steep (4–15°) platform margin slopes, redeposited limestones accumulate in a base-of-slope setting, by-passing an upper slope via a multitude of small submarine canyons, and are referred to as carbonate base-of-slope aprons. Both apron types are further subdivided into inner and outer facies belts. Inner apron sediments consist of thick, mud-supported conglomerates and megabreccias (Facies F) as well as thick, coarse-grained turbidites (Facies A) interbedded with subordinate amounts of fine-grained, peri-platform ooze (Facies G). Outer apron sediments consist of thinner, grain-supported conglomerates and turbidites (Facies A) as well as classical turbidites (Facies C) with recognizable Bouma divisions, interbedded with approximately equal proportions of peri-platform ooze (Facies G). Seaward, aprons grade laterally into basinal facies of thin, base-cut-out carbonate turbidites (Facies D) that are subordinate to peri-platform oozes (Facies G). Carbonate base-of-slope aprons grade shelfward into an upper slope facies of fine-grained peri-platform ooze (Facies G) cut by numerous small canyons that are filled with coarse debris, as well as intraformational truncation surfaces which result from submarine sliding. In contrast, slope aprons grade shelfward immediately into shoal-water, platform-margin facies without an intervening by-pass slope. The two carbonate apron models presented here offer alternatives to the submarine-fan model for paleoenvironmental analysis and hydrocarbon exploration for mass-transported carbonate facies.

Halimeda bioherms along an open seaway: Miskito Channel, Nicaraguan Rise, SW Caribbean Sea

A recent research cruise to examine small, detached carbonate platforms situated on the Nicaraguan Rise in the SW Caribbean Sea has revealed the presence of numerous Halimeda bioherms. Based upon interpretations from seismic reflection data some exceed 140 m in relief. This is the first documented occurrence of these green-algal buildups in the Caribbean/Bahama Bank region. The Halimeda bioherms form a nearly continuous band that borders the margins of the Miskito Channel—a shallow, open, 125 km long seaway. This 220 m deep channel bisects the Miskito Bank which is a major carbonate shelf. In seismic profile the bioherms appear acoustically “soft” and reveal a local relief of 20–30 m. Tops of these features lie in about 40–50 m of water. Samples from dredge hauls are coarse, poorly cemented packstones/grainstones which are dominated by largely unbroken, disarticulated Halimeda segments set in a poorly sorted sandy matrix. Exposed surfaces were stained brown. Very little living material was brought up in the dredges. The significance of these bioherms and their full extent in the Caribbean are not understood. Undoubtedly, further study will provide important answers concerning their role in the geologic development of Caribbean carbonate platforms.

Modern Deep-Water Coral Mounds North of Little Bahama Bank: Criteria for Recognition of Deep-Water Coral Bioherms in the Rock Record

ABSTRACT Deep-water, ahermatypic coral mounds are present at water depths of 1,000-1,300 m on the lower slope north of Little Bahama Bank. The mounds are patchily distributed over a minimum area of 2,500 km2 and typically display 5-40 m of relief above the surrounding sea bottom. A diverse benthic community exists on these apparently unlithified mounds, including l 1 genera and 16 species of ahermatypic coral (Bathypsammia, Caryophyllia, Deltocyathus, Desmophyllum, Enallopsammia, Javania, Madrepora, Polymyces, Solenosmilia, Stephanocyathus, and a previously undescribed genus), alcyonaceans, gorgonians, antipatharians, hydroids, ophiuroids, crinoids, barnacles, galatheid crabs, polychaetes, gastropods, bivalves, and sponges. Conspicuously absent from the coral fauna are Lop elia and Dendrophyllia, common deep-water corals in other parts of the Atlantic. Radiocarbon dates on fresh coral and gorgonian fragments of 940 ± 40 and 860 ± 50 years indicate the mounds are at least in part Recent and are probably actively forming today. Bored and stained corals date at around 22 10314C years B. P., which establishes a minimum age for these mounds. We speculate that the mounds develop on sea-floor perturbations in areas where strong bottom currents provide needed oxygen and nutrients to the fauna. The mounds may have undergone multi-stage evolution from colony to thicket to coppice to bank. This evolution may be accomplished through the in situ contribution of skeletal material along with the baffling and trapping of fine-grained carbonate sediment winnowed from adjacent areas by bottom currents. If found in the rock record, the coral mounds north of Little Bahama Bank would probably be considered bioherms. Geologists should thus be aware that scleractinian bioherms are not uniquely shallow-water in origin. The distinction between ancient deep- and shallow-water coral buildups involves using multiple recognition criteria, including: 1) presence or absence of algae; 2) diversity of corals; 3) coral morphology and microstructure; 4) abundance of planktonic/benthonic components; 5) microborings; 6) surrounding litho- and biofacies; 7) stable isotopes of carbon and oxygen; and 8) trace element geochemistry, particularly Sr and U concentrations. The distinction between deep- and shallow-water bioherms is crucial to regional paleoenvironmental and stratigraphic interpretations.

Erosion and infill of New York Finger Lakes: Implications for Laurentide ice sheet deglaciation

A comprehensive seismic reflection profile investigation of the New York Finger Lakes has revealed the extent of bedrock erosion and nature of sediment fill beneath the lakes.Bedrock deepens to the south, where gorgelike profiles have been eroded to as much as 304 m below sea level and infilled with up to 275 m of late Quaternary age (< 14 ka) sediment. We propose a working hypothesis that invokes ice streaming and pressurized subglacial meltwater and sediment to account for regional geomorphology. This working hypothesis is consistent with recent concepts of the collapse of continental ice sheets via ice streaming and the marine isotopic record for a meltwater spike at 13-14 ka. The Finger Lakes may provide a critical testing ground for deglaciation models of the southern Laurentide ice sheet because they contain a thick, well-preserved, and now seismically defined record of the deglaciation process.

Catastrophic collapse of the west Florida carbonate platform margin

A large truncation surface has been discovered on seismic reflection profiles along the distally steepened ramp of the west Florida carbonate platform margin. This structure, interpreted as a submarine slide scar, is at least 120 km across and up to 30 km wide; the slide has removed as much as 300–350 m of late Paleogene and early Neogene strata. Collapse occurred in the middle Miocene within an episode of seaward progradation during a relative sea-level highstand. Postcollapse depositional sequences thicken abruptly across the structure; this suggests catastrophic slope failure. Gravitational instability, created by high rates of sediment accumulation, is believed to have been the triggering mechanism. Such large-scale platform-margin collapse is likely to generate megabreccia debris flows as well as aid in the landward retreat of precipitous, high-relief carbonate escarpments.

Middle Miocene oceanographic event in the eastern Gulf of Mexico: Implications for seismic stratigraphic succession and Loop Current/Gulf Stream circulation

Analysis of ∼1,500 km of high-resolution, single-channel air-gun seismic-reflection profiles, correlated with two drill cores, demonstrates that the central west Florida carbonate-ramp slope experienced an oceanographic event in the middle Miocene that resulted in a major stratigraphic break or succession of depositional sequences. During the long-term eustatic sea-level rise in the early to middle Miocene, carbonate shelf-margin deposition was characterized by seaward-prograding, sigmoidal clinoforms that built both laterally and vertically. Shelf-margin sediments were relatively fine grained and rich in dolomite, total organics, and clay minerals. Approximately 12–15 m.y. ago, tectonically controlled oceanographic closure of the Isthmus of Panama, and/or increased climatic gradients, resulted in an intensification of oceanic circulation. Increased flow velocity of the Loop Current truncated antecedant clinoforms along the mid-Miocene shelf margin as the Loop Current, for the first time, became a dynamic oceanographic barrier to off-shelf sediment transport. Carbonate deposition along the west Florida slope changed dramatically from a series of prograding clinoforms to a pelagic slope-front-fill system bounded on its landward margin by contour-current flow of an invigorated Loop Current. Shelf-margin sediments, presently represented as a condensed sequence, became more carbonate rich, coarser grained, lower in organic content, and depleted in dolomite. Our results suggest that oceanographic events can generate major seismic stratigraphic breaks that result in the succession of depositional sequences and systems along continental margins and that they may be as important as eustatic oscillations. Our data also suggest that “modern” Loop Current/Gulf Stream circulation was initiated in the middle Miocene, controlled by tectono-oceanographic and/or climatic processes. The challenge now is to identify seismic stratigraphic-scale oceanographic events in the rock record, differentiate them from eustatic responses, and evaluate their paleoenvironmental significance.

Late glacial–Holocene atmospheric circulation and precipitation in the northeast United States inferred from modern calibrated stable oxygen and carbon isotopes

As global climate changes because of anthropogenic influences, it has become critical to better understand past climate and its various forcing mechanisms as a baseline for future comparison. To this end, we present a continental isotopic record from an 11.2-m-long wetland piston core sampled at 10‐50 yr resolution; the core was taken in the heavily populated, economically vibrant northeastern United States (adjacent to Fayetteville Green Lake) and spans 14,600‐3200 cal. yr B.P. We use a historically based correlation between d 18 Ocalcite obtained from individual varves in a box core from Fayetteville Green Lake and winter atmospheric circulation over the northeast United States to examine the way in which changes in winter circulation have influenced d 18 O in precipitation from 14,600 to 3200 cal. yr B.P. Our correlation analysis suggests that in periods during which the circumpolar westerlies are expanded, storms track more frequently from the Gulf of Mexico region, delivering precipitation with relatively high d 18 O values to the study site. By contrast, contracted westerlies result in more frequent low-d 18 Oprecipitation cross-continental storms. By using this relationship we model winter-vortex latitudes over the northeast United States for the prehistoric oxygen isotope record, focusing on millennial-scale

Megabreccia shedding from modern, low-relief carbonate platforms, Nicaraguan Rise

Single-channel seismic reflection data from the margins of lowrelief (150-250 m, measured from edge of bank to basin) carbonate platforms on the northern Nicaraguan Rise reveal complex seismic intervals consisting of mounded, chaotic seismic facies interspersed with discontinuous, parallel/laminated seismic facies. We interpret that these intervals contain megabreccias (chaotic facies) and sandy turbidites (parallel/laminated facies). One megabreccia is exposed on the sea floor displaying an overall fan shape having individual blocks measuring nearly 300 m across and >110 m high. The source area consists of a scalloped embayment with a headwall scarp 180 m high. Reflections within the platform are sharply truncated by this escarpment. This single megabreccia is ∼120 m thick and extends ∼27 km along slope and ∼16 km out into the basin. Other megabreccias within the basin have individual blocks measuring >400 m across. Rocks from dredge hauls are a mixture of shallow- and deep-water facies. Shallow-water facies consist of mixed, skeletal grain-stones and Halimeda packstones. Deep-water facies are massive chalks, chalks with shallow-water skeletal grains, and chalk-block breccias. This indicates that the megabreccias formed as a result of bank-margin collapse, during which the ensuing debris flow eroded into slope and basin facies, mixing rock types together. We speculate that bank-margin-collapse events, resulting in megabreccia formation, may have been seismically triggered, and we emphasize that these large-scale, mass-wasting events occurred along margins of low-relief carbonate platforms.