Pacific Neogene carbonate accumulation rates

I have compiled CaCO3 mass accumulation rates (MARs) for the period 0-25 Ma for 144 Deep Sea Drilling Project and Ocean Drilling Program drill sites in the Pacific in order to investigate the history of CaCO3 burial in the world's largest ocean basin. This is the first synthesis of data since the beginning of the Ocean Drilling Program. Sedimentation rates, CaCO3 contents, and bulk density were estimated for 0.5 Myr time intervals from 0 to 14 Ma and for 1 Myr time intervals from 14 to 25 Ma using mostly data from Initial Reports volumes. There is surprisingly little coherence between CaCO3 MAR time series from different Pacific regions, although regional patterns exist. A transition from high to low CaCO3 MAR from 23-20 Ma is the only event common to the entire Pacific Ocean. This event is found worldwide. The most likely cause of lowered pelagic carbonate burial is a rising sea-level trend in the early Miocene. The central and eastern equatorial Pacific is the only region with adequate drill site coverage to study carbonate compensation depth (CCD) changes in detail for the entire Neogene. The latitude-dependent decrease in CaCO3 production away from the equator is an important defining factor of the regional CCD, which shallows away from the equatorial region. Examination of latitudinal transects across the equatorial region is a useful way to separate the effects of changes in carbonate production ('productivity') from changes in bottom water chemistry ('dissolution') upon carbonate burial.

Late Pliocene carbonate mineralogy of ODP Hole 133-818B

Shedding of shallow carbonate material toward the deep slopes and basin floors is clearly tied to the position of the carbonate bank tops relative to the photic zone. The onset of bank shedding in periplatform sediments can record either the flooding of the bank tops within the photic zone during a rise in sea level following a period of exposure, referred to in the literature as the "highstand shedding" scenario, or the reentry of the bank tops into the photic zone during a lowering of sea level following a period of drowning, referred to as the "lowstand shedding" scenario. Results from Leg 133 post-cruise research on the Pliocene sequences, drilled in six sites within different slope settings of the Queensland Plateau, seem to point out that the latter "lowstand shedding" scenario can be applied to this particular carbonate system. At the Queensland Plateau sites, the early Pliocene (5.2-3.5 Ma) and the earliest part of the late Pliocene (3.5-2.9 Ma) age sequences were characterized, especially in the ôdeepö Sites 811 and 817, by pelagic sediments (foraminifers and coccoliths) and by typically pelagic sedimentation rates not exceeding 20 mm/k.y. The earliest part of the late Pliocene age section was characterized by well-developed hardgrounds in the "shallow" Sites 812 and 814 and by normal pelagic sediments mixed with reworked phosphatized planktonic foraminifers in Site 813. Finally, the early part of the late Pliocene (2.9-2.4 Ma) section was characterized by high sedimentation rates, related to the shedding and admixture into the pelagic sediments of bank-derived materials. These bank-derived materials consist of either diagenetically unaltered fine aragonite with traces of dolomite in Site 818 or micritic calcite resulting from seafloor and/or shallow burial alteration in the deepest Sites 817 and 811. The highest sedimentation rates (163 mm/k.y.) were recorded in Site 818, drilled nearest the modern carbonate bank of Tregrosse Reef. The sedimentation rates decrease with increasing distance from Tregrosse Reef - 120 mm/k.y. in Site 817 and 47.5 mm/k.y. in Site 811. The initial appearance of fine aragonite in Site 818, corresponding to the transition from pelagic to periplatform sedimentation rates, has been dated at 2.9 Ma. This Pliocene sediment pattern on the Queensland Plateau is different from the pattern observed in sediments from two earlier ODP legs (i.e., Leg 101 in the Bahamas and in Leg 115 in the Maldives), where aragonite-rich sediments, characterized by high periplatform sedimentation rates, were observed in the lower Pliocene section (5.2-3.5 Ma), whereas the upper Pliocene (3.5-1.6 Ma) sediments are more pelagic in nature and are characterized by low sedimentation rates or major hiatuses. These Pliocene periplatform sequences in the Bahamas and in the Maldives and late Quaternary age periplatform sequences worldwide have pointed out that "highstand shedding" was the typical response of carbonate platforms to fluctuations in sea level, just opposite to a "lowstand shedding" response to sea-level fluctuations, typical of siliciclastic shelves. Assuming that the envelope of Haq et al.'s (1987) sea-level curve, showing a well-defined lowering of sea level between 3.5 and 2.9 Ma, can also be applied to the southwest Pacific Ocean, based on a high-resolution Pliocene d18O record from the Ontong Java Plateau recently published by Jansen et al. (1993, doi:10.2973/odp.proc.sr.130.028.1993), the Pliocene periplatform sequences on the Queensland Plateau would have recorded the reentry of the bank tops into the photic zone during a general lowering of sea level, following an interval characterized by high sea level, during which the shallow carbonate system on the Queensland Plateau was drowned. The early Pliocene age (5.2-3.5 Ma) sediments deposited on the Queensland Plateau, an established interval of eustatic sea-level highstand, are typically pelagic in character. In addition, relatively cold surface temperatures (estimated to have ranged from 18° to 20°C by Isern et al. [this volume]) might have also stressed the reefs during early Pliocene time and contributed to the drowning of the Queensland Plateau carbonate system during the late Miocene and early Pliocene. Differential and relatively high subsidence rates, inferred by variations in paleodepth of water (based upon benthic foraminifer assemblages; Katz and Miller, this volume) may also have influenced the drowning of the carbonate bank tops on the Queensland Plateau during the late Miocene and early Pliocene. The sediments of early late Pliocene age (2.9-2.4 Ma), a well-established interval of lowering of sea level, are clearly periplatform and cyclic in nature. High-frequency (~40 k.y.) aragonite cycles, well-developed between 2.9 and 2.45 Ma, correlate with the planktonic high-resolution Pliocene d18O record from the Ontong Java Plateau, a good sea-level proxy (Jansen et al., in press). Contrary to late Quaternary age aragonite cycles from the Bahamas, the Nicaragua Rise, the Maldives, and the Queensland Plateau, the late Pliocene aragonite cycles in Hole 818B display high levels of aragonite during glacial stages and, therefore, lowstands of sea level. In addition, sediments deposited during the earliest part of the late Pliocene (3.5-2.9 Ma), transition between the early Pliocene highstand and the late Pliocene lowering in sea level, have recorded the first evidence of a fall in sea level, by (1) the occurrence of synchronous submarine hardgrounds in the two shallowest sites (Sites 812 and 814), (2) the deposition of reworked material from the shallower part of the slope into the intermediate Sites 813 and 818, and (3) the deposition of pelagic sediments in the deepest Sites 817 and 817. In summary, contrary to previous findings, the Pliocene periplatform sediments on the Queensland Plateau appear to have recorded a regional shedding of shallow carbonate bank tops during an interval of sea-level lowering, a good illustration of the "carbonate lowstand shedding" scenario, occurring during the reentry of previously drowned carbonate bank tops into the photic zone related to a decrease in sea level.

(Table 1) Biostratigraphic datums of ODP Leg 133 sites

We use benthic foraminifers to reconstruct the Neogene paleobathymetric history of the Marion Plateau, Queensland Plateau, Townsville Trough, and Queensland Trough on the northeastern Australian margin (Ocean Drilling Program Leg 133). Western Queensland Plateau Site 811/825 (present depth, ~938 m) deepened from the neritic zone (0-200 m) to the upper bathyal zone (200-600 m) during the middle Miocene (~13-14 Ma), with further deepening into the middle bathyal zone (600-1000 m) occurring during the late Miocene (~7 Ma). A depth transect across the southern Queensland Plateau shows that deepening from the outer neritic zone (100-200 m) to the upper bathyal zone began during the latest Miocene (~6 Ma) at the deepest location (Site 813, present depth, 539.1 m), whereas the shallower Sites 812 and 814 (present depths, 461.6 and 520.4 m, respectively) deepened during the late Pliocene (~2.7 and ~2.9 Ma). At Marion Plateau Site 815 (present depth, 465.5 m), water depth increased during the late Miocene (~6.7 Ma) from the outer neritic to the upper bathyal zone. Nearby Site 816 (present water depth, 437.3 m) contains Pliocene upper bathyal assemblages that directly overlie middle Miocene shallow neritic deposits; the timing of the deepening is uncertain because of a late Miocene hiatus. On the northern slope of the Townsville Trough (Site 817, present depth, 1015.8 m), benthic foraminifers and sponge spicules indicate deepening from the lower upper bathyal (400-600 m) to the middle bathyal zone in the late Miocene (by ~6.8 Ma). Benthic foraminiferal faunas at nearby Site 818 (present water depth, 752.1 m) do not show evidence of paleobathymetric change; however, a late Pliocene (~2-3 Ma) increase in downslope transport may have been related to the drowning of the Queensland Plateau. Site 822 (present depth, 955.2 m), at the base of the Great Barrier Reef slope, deepened from the upper bathyal to the middle bathyal zone during the late Pliocene (by ~2.3 Ma). Queensland Trough Site 823 (present depth, 1638.4 m) deepened from the middle bathyal to the lower bathyal (1000-2000 m) zone during the late Miocene (~6.5 Ma). Benthic foraminiferal faunal changes at these Leg 133 sites indicate that rapid deepening occurred during the middle Miocene (~13-14 Ma), late Miocene (6-7 Ma), and late Pliocene (2-3 Ma) along the northeastern Australian margin.

Granulometry of two sediment cores from Maxwell Bay, South Shetland Islands, West Antarctica

The climate evolution of the South Shetland Islands during the last c. 2000 years is inferred from the multiproxy analyses of a long (928 cm) sediment core retrieved from Maxwell Bay off King George Island. The vertical sediment flux at the core location is controlled by summer melting processes that cause sediment-laden meltwater plumes to form. These leave a characteristic signature in the sediments of NE Maxwell Bay. We use this signature to distinguish summer and winter-dominated periods. During the Medieval Warm Period, sediments are generally finer which indicates summer-type conditions. In contrast, during the Little Ice Age (LIA) sediments are generally coarser and are indicative of winter-dominated conditions. Comparison with Northern and Southern Hemisphere, Antarctic, and global temperature reconstructions reveals that the mean grain-size curve from Maxwell Bay closely resembles the curve of the global temperature reconstruction. We show that the medieval warming occurred earlier in the Southern than in the Northern Hemisphere, which might indicate that the warming was driven by processes occurring in the south. The beginning of the LIA appears to be almost synchronous in both hemispheres. The warming after the LIA closely resembles the Northern Hemisphere record which might indicate this phase of cooling was driven by processes occurring in the north. Although the recent rapid regional warming is clearly visible, the Maxwell Bay record does not show the dominance of summer-type sediments until the 1970s. Continued warming in this area will likely affect the marine ecosystem through meltwater induced turbidity of the surface waters as well as an extension of the vegetation period due to the predicted decrease of sea ice in this area.

Particulate and phytoplankton absorption during POLARSTERN cruises ANT-XXVI/3 and ANT-XXVIII/3

The euphotic depth (Zeu) is a key parameter in modelling primary production (PP) using satellite ocean colour. However, evaluations of satellite Zeu products are scarce. The objective of this paper is to investigate existing approaches and sensors to estimate Zeu from satellite and to evaluate how different Zeu products might affect the estimation of PP in the Southern Ocean (SO). Euphotic depth was derived from MODIS and SeaWiFS products of (i) surface chlorophyll-a (Zeu-Chla) and (ii) inherent optical properties (Zeu-IOP). They were compared with in situ measurements of Zeu from different regions of the SO. Both approaches and sensors are robust to retrieve Zeu, although the best results were obtained using the IOP approach and SeaWiFS data, with an average percentage of error (E) of 25.43% and mean absolute error (MAE) of 0.10 m (log scale). Nevertheless, differences in the spatial distribution of Zeu-Chla and Zeu-IOP for both sensors were found as large as 30% over specific regions. These differences were also observed in PP. On average, PP based on Zeu-Chla was 8% higher than PP based on Zeu-IOP, but it was up to 30% higher south of 60°S. Satellite phytoplankton absorption coefficients (aph) derived by the Quasi-Analytical Algorithm at different wavelengths were also validated and the results showed that MODIS aph are generally more robust than SeaWiFS. Thus, MODIS aph should be preferred in PP models based on aph in the SO. Further, we reinforce the importance of investigating the spatial differences between satellite products, which might not be detected by the validation with in situ measurements due to the insufficient amount and uneven distribution of the data.

Nannofossil biohorizons for ODP Leg 133 sites

Paleontological studies conducted subsequent to the completion of Leg 133 led to refinements of the biostratigraphy for the Leg 133 sites. These biostratigraphic refinements bear on the calculations of sedimentation rates and on the age-depth plots prepared for the Initial Reports volume for Leg 133. To make available the revised data to anyone who may wish to make use of it, the revised biostratigraphic information is presented here in tabulated form. Revised age-depth plots also are presented for all of the sites to facilitate comparison of sedimentation rate curves and to identify intervals where significant changes have been made based on post-cruise studies. The revised age-depth plots include calcareous nannofossils only, and the revised data have been taken from thechapters contributed for this volume (Gartner et al., 1993, doi:10.2973/odp.proc.sr.133.213.1993; Wei and Gartner, 1993, doi:10.2973/odp.proc.sr.133.216.1993). Planktonic foraminifer biostratigraphy revisions became available subsequently and could not be readily incorporated. The age-depth plots for Sites 812 through 818 were made with the (ADP) program provided to ODP by Dave Lazarus.

Pleistocene sedimentology of ODP Hole 133-820A from the northeastern Australian margin

A principal objective at Site 820, situated on the outer shelf, upper slope of the northeastern Australian continental margin, was to test the relationships between changes in Pleistocene sea level and sedimentary packages produced on a mixed carbonate-siliciclastic continental margin. To this end, we have examined the downcore distribution of grain size, magnetic susceptibility, and calcium-carbonate content throughout Hole 820A and, in particular, the top 35 meters below the seafloor (mbsf). These data are compared with variations in the oxygen-isotope signal defined for the same hole and are interpreted as indicating sea-level oscillations. The distribution of sand, mud, calcium carbonate of the mud fraction and total sample, and magnetic susceptibility during the last 20,000 yr defines the position of a sea-level regression (41,000-18,000 yr B.P.), a lowstand, early (18,000-9,400 yr B.P.) and late transgressions (9400-900 yr B.P.), and a highstand (4900 yr to the present). The regression is seen first in a high-carbonate content peak. Calcium carbonate constituents mainly comprise skeletal carbonate grains, with abundant planktonic and benthic foraminifers, and lime muds. The lowstand is characterized by a maximum abundance of the sand fraction, which contains dominantly skeletal carbonate grains and a minor abundance of lithoclasts. Sand-sized terrigenous sediments are proposed to have bypassed the continental shelf during a lowstand of sea level. Sedimentation rates throughout the regression and lowstand are low (3.0 cm/k.y.). The early transgression, marked by highest values in magnetic susceptibility, displays a rapid increase in sedimentation rate that coincided with an increase in terrigenous mud. Highest sedimentation rates of 82.3 cm/k.y. occurred during the late transgression, with increasing percentages of lime-mud. A decrease in noncarbonate constituents in the mud fraction during the late transgression and highstand of sea level is thought to be the result of restricted inner-shelf sedimentation of terrigenous sediments. The same relationship is also seen in the major sea-level oscillation, which is interpreted as isotope stage 6.