André W. Droxler

Neritic and pelagic carbonate sedimentation in the marine environment: ignorance is not bliss

Synthesis of available data allows us to define general patterns of late Quaternary carbonate production and sedimentation in the global ocean. During high stands of sea level, the neritic and pelagic environments appear to sequester approximately similar amounts of carbonate, whereas during low stands of sea level the decreased neritic zone produces and accumulates approximately an order of magnitude less carbonate. Assuming that global accumulation of deep-sea carbonates remains more or less constant during glacially induced changes in sea level, the ocean becomes depleted with respect to calcium carbonate during high stands and recharges during low stands. Before we can achieve a better understanding of the global carbonate system, however, we need a better understanding of key environments and processes: (a) production and accumulation on continental shelves both as potential sinks (accumulation) and as sources (export to the deep sea); (b) a better measure of pelagic carbonate production; and (c) late Quaternary (late Pleistocene and Holocene) mass accumulation rates in the deep sea.

Bundled turbidite deposition in the central Pandora Trough (Gulf of Papua) since Last Glacial Maximum : Linking sediment nature and accumulation to sea level fluctuations at millennial timescale

Since Last Glacial Maximum (23-19 ka), Earth climate warming and deglaciation occurred in two major steps (Bolling-Allerod and Preboreal), interrupted by a short cooling interval referred to as the Younger Dryas (12.5-11.5 ka B. P.). In this study, three cores (MV-33, MV-66, and MD-40) collected in the central part of Pandora Trough (Gulf of Papua) have been analyzed, and they reveal a detailed sedimentary pattern at millennial timescale. Siliciclastic turbidites disappeared during the Bolling-Allerod and Preboreal intervals to systematically reoccur during the Younger Dryas interval. Subsequent to the final disappearance of the siliciclastic turbidites a calciturbidite occurred during meltwater pulse 1B. The Holocene interval was characterized by a lack of siliciclastic turbidites, relatively high carbonate content, and fine bank-derived aragonitic sediment. The observed millennial timescale sedimentary variability can be explained by sea level fluctuations. During the Last Glacial Maximum, siliciclastic turbidites were numerous when the lowstand coastal system was located along the modern shelf edge. Although they did not occur during the intervals of maximum flooding of the shelf (during meltwater pulses 1A and 1B), siliciclastic turbidites reappear briefly during the Younger Dryas, an interval when sea level rise slowed, stopped, or perhaps even fell. The timing of the calciturbidite coincides with the first reflooding of Eastern Fields Reef, an atoll that remained exposed for most of the glacial stages.

Metastable CaCO3 dissolution at intermediate water depths of the Caribbean and western North Atlantic: Implications for intermediate water circulation during the past 200,000 years

We present late Quaternary records of metastable carbonate dissolution determined for sediment cores recovered from intermediate water depths on the Nicaragua Rise (Caribbean Sea, 1000–1894 m) and near the Bahama Banks (western North Atlantic Ocean, 655 and 1934 m). Upper North Atlantic Deep Water is believed to dominate these two regions at present. Both areas are predicted to be good locations to study past variations of average middepth Atlantic chemistry and circulation. However, statistical analyses of metastable carbonate dissolution indices (% Mg calcite, pteropod abundance, % whole pteropods, and % clear pteropods) yielded a composite dissolution index (CDI) which displays different carbonate dissolution histories for Bahama and Nicaragua Rise sediments. The Bahama records resemble deep Atlantic carbonate records with dissolution during glacial oxygen isotope stages 6 and 4 and with preservation during interglacial stages 5 and 1. We observe two dissolution patterns at intermediate water depths of the Caribbean. The “deep” intermediate water pattern (1200–1894 m) resembles deep Caribbean carbonate records with dissolution during interglacial periods and preservation during glacial intervals. The “shallow” intermediate water record (<1200 m) resembles at times the Bahamas record and at other times the deep Caribbean record. These CDI records suggest that different water masses occupied the intermediate depth western North Atlantic and Caribbean during much of the late Quaternary. Observed differences between these regions may be related to variations in the flow of nutrient-rich, low CO3= Antarctic Intermediate Water through the Caribbean or to changes in the ventilation rate of equatorial thermocline and intermediate waters. These water-mass variations have influenced Caribbean carbon chemistry from the base of the thermocline down to abyssal water depths and may have had a significant effect on North Atlantic circulation and nutrients.

Unique and exceptionally long interglacial marine isotope stage 11: window into Earth warm future climate

1 Dept. of Earth Science, Rice University, P.O. Box 1892, Houston, TX 77251, USA andre@rice.edu 2 EMS Environment Inst. and Dept. of Geosciences, Pennsylvania State Univ., University Park, PA,16802, USA 3 Antarctic CRC, University of Tasmania, GPO Box 25280, Hobart, Tasmania 7001, AUSTRALIA 4 U.S. Geological Survey, 12201 Sunrise Valley Drive Reston, VA 20192, USA 5 Lamont-Doherty Earth Observatory, Columbia University, P.O. Box 1000, Palisades, NY 10964-1000 USA

Controls on Carbonate Mineral Accumulation in Bahamian Basins and Adjacent Atlantic Ocean Sediments

ABSTRACT Carbonate mineralogical studies of surface Bahamian periplatform ooze, containing pelagic calcite as well as bank-derived aragonite and magnesian calcite, were coupled with studies of the carbonate chemistry of the waters overlying these sediments. The presence of metastable aragonite and magnesian calcite in the periplatform ooze presents a unique opportunity to assess the relation between the extent of carbonate saturation in the water column and the disappearance of metastable carbonate minerals in an area of the North Atlantic Ocean where intermediate and deep waters are saturated with respect to calcite. Aragonite is present in the fine fraction of the surface sediment to a water depth of 4,800 m, but decreases sharply below 4,000 m. This is also the depth limit of the occurrence of pteropod tests and fragments in the coarse fraction of the sediment. The aragonite compensation depth should be (by extrapolation) between 5,000 and 5,200 m. The saturation depth with respect to aragonite was estimated to be located between 3,800 and 4,500 m. This corresponds fairly well with the depth of a sharp decrease of the fine aragonite as well as the depth of pteropod disappearance. Magnesian calcite in the fine fraction of the sediment decreases irregularly with depth and disappears between 3,700 and 4,000 m. The calculated saturation depth with respect to magnesian calcite (13 mole % MgCO3) is between 900 and 1,500 m. At this depth a significant drop in the concentration of magnesian calcite occurs. This decrease of magnesian calcite from 25 to 15 percent could be caused by both dissolution and the increasing distance of deeper waters from the banks. However, the higher values of specific alkalinity of the waters ranging between 750 and 2,000 m, when compared with the alkalinity values of the Sargasso Sea at the same latitude, are interpreted to be indicative that some dissolution of magnesian calcite occurs in the Bahamian basins at these water dept s. Our results show depressed levels, by 500 m or more, of the aragonite saturation depth in the water column as well as of the depth of aragonite disappearance in the surface sediment, when compared with open ocean studies in the Northwest Atlantic. In addition, our results indicate that fine magnesian calcite (12 to 13 mole % MgCO3) disappears 1,200 to 1,500 m above the extrapolated disappearance depth of aragonite. This observation is consistent with results for experimental studies, showing that aragonite is more stable in seawater than a magnesian calcite of 12 to 13 mole % MgCO3.

Monsoon-induced partial carbonate platform drowning (Maldives, Indian Ocean)

Multibeam maps and high-resolution seismic images from the Maldives reveal that a late Miocene to early Pliocene partial drowning of the platform was linked to strong sea-bottom currents. In the upper Miocene to Holocene, currents shaped the drowned banks, the current moats along the bank edges, and the submarine dune fields. Bottom currents in the Maldives are driven by the monsoon. It is proposed that the onset and the intensification of the monsoon during the Neogene provoked platform drowning through injection of nutrients into surface waters. Since the late Miocene, topographically triggered nutrient upwelling and vigorous currents switched the Maldives atolls into an aggradational to backstepping mode, which is a growth pattern usually attributed to episodes of rising sea level.

Sea level influence on the nature and timing of a minibasin sedimentary fill (northwestern slope of the Gulf of Mexico)

This study focuses on the sedimentary fill of basin 4, the termination of the Brazos-Trinity minibasin slope system in the northwestern Gulf of Mexico. Results from multistratigraphic analyses of 15 giant piston cores provided (1) important information regarding the nature (hemipelagic versus gravity-induced mud and sand deposits) and the timing of the sedimentary fill; (2) some key chronostratigraphic constraints for the evolution of this system; and (3) strong links between well-known cycles of sea level change to clearly imaged deposits in the fill of basin 4. Gravity-flow–induced sedimentation in basin 4 occurred and increased in importance during the stepwise sea level regression that developed between 115 and 15 ka and clearly ceased just prior to the meltwater spike in the Gulf of Mexico dated at about 14 ka. The onset of gravity-induced deposition in basin 4 is dated at marine isotope stage (MIS) 5d (115 ka). This finding implies that sandy turbidity currents reached this distal setting as a consequence of a higher frequency sea level fall within a time of general high sea level (MIS 5). An interval of hemipelagic sedimentation lasting from 90 to 45 ka illustrates cessation of gravity-induced deposits in basin 4. Turbidite sandy deposits resumed in mid-MIS 3 and increased toward MIS 2 (approximately from 30 to 15 ka). The largest proportion of reservoir-grade sandy sediment was deposited during the maximum sea level lowstand of the last glacial maximum, consistent with the prevailing view of sequence-stratigraphic models for deep-water deposition.

Neogene evolution of the mixed carbonate-siliciclastic system in the Gulf of Papua, Papua New Guinea

This paper outlines the evolution of the late Cenozoic mixed carbonate-siliciclastic depositional system in the Gulf of Papua (GoP), using seismic, gravity, multibeam bathymetry, well data sets, and Landsat imagery. The deposition of the mixed sedimentary sequences was influenced by dynamic interplay of tectonics, eustasy, in situ carbonate production, and siliciclastic sediment supply. The roles of these major factors are estimated during different periods of the GoP margin evolution. The Cenozoic mixed system in the GoP formed in distinct phases. The first phase ( Late Cretaceous-Paleocene) was mostly driven by tectonics. Rifting created grabens and uplifted structural blocks which served later as pedestals for carbonate edifices. Active neritic carbonate accumulation characterized the second phase (Eocene-middle Miocene). During this phase, mostly eustatic fluctuations controlled the large-scale sedimentary geometries of the carbonate system. The third phase ( late Miocene-early Pliocene) was characterized by extensive demise of the carbonate platforms in the central part of the study area, which can be triggered by one or combination of several factors, such as eustatic sea level fluctuations, increased tectonic subsidence, uplift, sudden influx of siliciclastics, or dramatic changes in environmental conditions and climate. The fourth phase ( late Pliocene-Holocene) was dominated by siliciclastics, which resulted in the burial of drowned and/or active carbonate platforms, although some platforms still remain alive until present-day.

Deep water geomorphology of the mixed siliciclastic‐carbonate system, Gulf of Papua

The Gulf of Papua (GoP) has become a focal point for understanding the deposition and accumulation of siliciclastic and carbonate material along and across a low-latitude continental margin. Although studies have addressed submarine geomorphological features on the inner and middle shelves, as well as processes that may have led to their formation, the seafloor of adjacent slope regions remains poorly documented. This study presents and interprets results from approximately 13,000 line-km of multibeam bathymetry, 9500 line-km of 3.5 kHz seismic, and 122 sediment cores that were collected from the GoP shelf edge and slope, primarily on two cruises (PANASH and PECTEN). Bathymetric maps, in conjunction with the seismic profiles and cores, were used to make extensive observations, descriptions, and interpretations of seafloor geomorphology and begin to address several key issues regarding the delivery and accumulation of sediment. This study divided the GoP slope region into physiographic regions including intraslope basins: Ashmore Trough, southern Pandora Trough, northern Pandora Trough, Moresby Trough and intraslope plateaus/platforms: carbonate platforms and atolls and Eastern Plateau. Ashmore Trough contains a very linear northern margin capped by a drowned barrier reef system. This shelf edge is also defined by a broad promontory with channels extending from its apex, interpreted as a relict shelf-edge delta. Southern Pandora Trough is characterized by pervasive slope channels and slump scars extending down slope to a thick depocenter and an extensive mass-transport complex. In contrast, northern Pandora Trough has few visible slope channels. Seismic observations reveal a wedge of sediment extending down slope from northern Pandora Trough shelf edge and filling preexisting bathymetry. Large fold-and-thrust-belt ridges are also present on the seafloor in this region and may act to divert and/or catch sediment, depending on sediment transport direction. Moresby Trough contains a large axial submarine channel that extends almost the entire length of the intraslope basin. In addition, an extensive system of canyons lines the NE margin of Moresby Trough. Mass-transport deposits have been fed from the canyons and in one case deposited a large (similar to 2000 km(2)) mass-transport complex. Fold-and-thrust-belt ridges also extend into Moresby Trough. Here they trend perpendicular to slope and catch gravity flow deposits on their updip side. GoP carbonate platforms/atolls all display very pronounced scalloped-margin morphology, which may indicate pervasive mass-wasting processes on carbonate margins. Northwest Eastern Plateau is dominantly carbonate and displays the characteristic scalloped margin morphology; however, most of the plateau is characterized by parallel seismic reflectors. These seismic observations in conjunction with core data indicate that accumulation on Eastern Plateau is primarily mixed pelagic and hemipelagic sediment. Observations and interpretations of the bathymetry have revealed the deep water GoP to contain very diverse geomorphology and suggest it is a dynamic system influenced by a variety of sediment transport processes, particularly mass wasting and other gravity flow processes.

Controls and Development of Late Quaternary Periplatform Carbonate Stratigraphy in Walton Basin (Northeastern Nicaragua Rise, Caribbean Sea)

The late Quaternary sedimentary record in periplatform oozes and muds deposited on the northern Nicaragua Rise results from the interplay of four main controls: (1) input of pelagic carbonates and bank-derived fine aragonite and magnesian calcite; (2) input of siliciclastic sediments; (3) dispersal and removal of sediments by the Caribbean Current; and (4) partial seafloor dissolution of metastable carbonates. High accumulation rates of the calcite coarse sediment fraction throughout the study area demonstrate that planktonic foraminiferal productivity peaked during interglacial stages. Neritic carbonate productivity also peaked during interglacial stages, when bank and shelf tops along the Nicaragua Rise were submerged within the photic zone. Because the bank and shelf tops remained mostly exposed during glacial stages, the surface area available for the neritic carbonate productivity was drastically reduced to a narrow band along the margins of Pedro Bank and the southern shelf of Jamaica. A large volume of siliciclastic sediments, transported from the coastal area of South America and the eastern and western regions of Jamaica, were deposited during glacial stages within the eastern and western deep extensions of Walton Basin. This finding contrasts with the input pattern of siliciclastic sediments in Walton Basin itself, where the largest input of siliciclastic sediments occurred at each of the interglacial climatic optima, corresponding to intervals of maximum sea level transgression. Finally, cores in water depths exceeding 1100 m display during interglacial stages aragonite accumulation rates systematically lower than rates in cores from water depths shallower than 1100 m. This indicates that some bank-derived aragonite and magnesian calcite has been partially removed by seafloor dissolution in areas from water depths exceeding 1100 m. Results from this study on the northeastern Nicaragua Rise indicate that not only do offbank transport and water column saturation state influence the late Quaternary record but also that the variable strength of the Caribbean Current and the proximity of sources for siliciclastic sediments have played a major role in the development of the late Quaternary periplatform high resolution stratigraphy.