Stable carbon isotope ratios of alkane of IODP Hole 302-M0004A

The Palaeocene/Eocene thermal maximum represents a period of rapid, extreme global warming approx ~55 million years ago, superimposed on an already warm world (Zachos et al., 2003, doi:10.1126/science.1090110; Bowen et al., 2004, doi:10.1038/nature03115; Thomas et al., 2002, doi:10.1130/0091-7613(2002)030<1067:WTFFTF>2.0.CO;2). This warming is associated with a severe shoaling of the ocean calcite compensation depth **4 and a >2.5 per mil negative carbon isotope excursion in marine and soil carbonates (Zachos et al., 2003, doi:10.1126/science.1090110; Bowen et al., 2004, doi:10.1038/nature03115; Thomas et al., 2002, doi:10.1130/0091-7613(2002)030<1067:WTFFTF>2.0.CO;2; Zachos et al., doi:10.1126/science.1109004). Together these observations indicate a massive release of 13C-depleted carbon (Zachos et al., doi:10.1126/science.1109004) and greenhouse-gas-induced warming. Recently, sediments were recovered from the central Arctic Ocean (Backman et al., 2006, doi:10.2204/iodp.proc.302.2006), providing the first opportunity to evaluate the environmental response at the North Pole at this time. Here we present stable hydrogen and carbon isotope measurements of terrestrial-plant- and aquatic-derived n-alkanes that record changes in hydrology, including surface water salinity and precipitation, and the global carbon cycle. Hydrogen isotope records are interpreted as documenting decreased rainout during moisture transport from lower latitudes and increased moisture delivery to the Arctic at the onset of the Palaeocene/Eocene thermal maximum, consistent with predictions of poleward storm track migrations during global warming (Backman et al., 2006, doi:10.2204/iodp.proc.302.2006). The terrestrial-plant carbon isotope excursion (about ~4.5 to ~6 per mil) is substantially larger than those of marine carbonates. Previously, this offset was explained by the physiological response of plants to increases in surface humidity (Bowen et al., 2004, doi:10.1038/nature03115). But this mechanism is not an effective explanation in this wet Arctic setting, leading us to hypothesize that the true magnitude of the excursion - and associated carbon input - was greater than originally surmised. Greater carbon release and strong hydrological cycle feedbacks may help explain the maintenance of this unprecedented warmth.of this unprecedented warmth.

Planktic foraminiferal assemblages in sediments of four ODP Leg 198 holes

A pulse of intense carbonate dissolution occurred during the early late Paleocene at 58.4 Ma. A prominent 5 to 25 cm-thick dark brown clay-rich calcareous nannofossil ooze was found on Shatsky Rise at Sites 1209, 1210, 1211, and 1212 during Ocean Drilling Program Leg 198. The layer corresponds to the lower part of planktonic foraminiferal Zone P4 and coincides with the evolutionary first occurrence of the nannolith Heliolithus kleinpellii, an important component of late Paleocene assemblages and a marker for the base of Zone CP5. The clay-rich layer contains common crystals of phillipsite, fish teeth, and phosphatic micronodules and corresponds to a prominent peak in magnetic susceptibility that probably reflects these high amounts of detrital and authigenic materials. Detailed quantitative analysis of planktonic foraminiferal assemblages across the clay-rich nannofossil ooze layer shows that fundamental changes in faunal composition occurred before, during, and after deposition of the clay-rich ooze. Planktonic foraminifers in the clay-rich layer are characterized by a low-diversity, largely dissolved assemblage dominated by representatives of the genus Igorina (mainly Igorina tadjikistanensis and Igorina pusilla). Conversely, Igorina albeari, morozovellids, acarininids, globanomalinids, subbotinids, and chiloguembelinids are common below the clay-rich layer, almost disappear within it, and reappear in low abundances above the clay-rich layer. These changes in faunal compositions are likely a response to a change in carbonate saturation that caused increased dissolution on the seafloor owing to the shoaling of the lysocline and the carbonate compensation depth.

Mineral and foraminifer composition of sediments across the Paleocene/Eocene Thermal Maximum from ODP Leg 198 sites

The Paleocene/Eocene Thermal Maximum (PETM) was a transient interval of global warming ~55 m.y. ago associated with transformation of ecosystems and changes in carbon cycling. The event was caused by the input of massive amounts of CO2 or CH4 to the ocean-atmosphere system. Rapid shoaling of the lysocline and calcite compensation depth (CCD) is a predicted response of CO2 or CH4 input; however, the extent of this shoaling is poorly constrained. Investigation of Ocean Drilling Program (ODP) Sites 1209-1212 at Shatsky Rise, which lies along a depth transect, suggests a minimum lysocline shoaling of ~500 m in the tropical Pacific Ocean during the PETM. The sites also show evidence of CaCO3 dissolution within the sediment column, carbonate "burn-down" below the level of the carbon isotope excursion, and a predicted response to a rapid change in deepwater carbonate saturation. Close examination of several foraminiferal preservation proxies (i.e., fragmentation, benthic/planktonic foraminiferal ratios, coarse fraction, and CaCO3 content) and observations of foraminifers reveal that increased fragmentation levels most reliably predict intervals with visually impoverished foraminiferal preservation as a result of dissolution. Low CaCO3 content and high benthic/planktonic ratios also mirror intervals of poorest preservation.

Seawater Nd and Pb isotopic record in central Pacific and Caribbean sediments

The shoaling and final closure of the Central American Seaway (CAS) resulted in a major change of the global ocean circulation and has been suggested as an essential driver for strengthening of Atlantic Meridional Overturning Circulation (AMOC). The exact timing of CAS closure is key to interpreting its importance. Here we present a reconstruction of deep and intermediate water Nd and Pb isotope compositions obtained from fossil fish teeth and the authigenic coatings of planktonic foraminifera in the eastern equatorial Pacific (Ocean Drilling Program (ODP) Site 1241) and the Caribbean (ODP Sites 998, 999, and 1000) covering the final stages of CAS closure between 5.6 and 2.2 Ma. The data for the Pacific site indicate no significant Atlantic/Caribbean influence over this entire period. The Caribbean sites show a continuous trend to less radiogenic Nd isotope compositions during the Pliocene, consistent with an enhancement of Upper North Atlantic Deep Water (UNADW) inflow and a strengthening of the AMOC. Superimposed onto this long-term trend, shorter-term changes of intermediate Caribbean Nd isotope signatures approached more UNADW-like values during intervals when published reconstructions of seawater salinity suggested complete closure of the CAS. The data imply that significant deep water exchange with the Pacific essentially stopped by 7 Ma and that shallow exchange, which still occurred at least periodically until approximately 2.5 Ma, may have been linked to the strength of the AMOC but did not have any direct effect on the intermediate and deep Caribbean Nd isotope signatures through mixing with Pacific waters.

Calcareous plankton response to orbital and millennial-scale climate changes from ODP Leg 161-975

The paleoenvironmental conditions through MIS 15-9 at the Mediterranean Ocean Drilling Program (ODP) Site 975 were interpreted by high resolution study of calcareous plankton assemblages compared with available d18O and d13C records and high resolution paleoclimate proxies from the Atlantic Ocean. Sea Surface Temperatures (SSTs) have been estimated from planktonic foraminiferal assemblages using the artificial neural networks method. Calcareous plankton varied dominantly on a glacial-interglacial scale as testified by the SST record, foraminiferal diversity, total coccolith abundance and changes in warm-water calcareous nannofossil taxa. A general increase in foraminiferal diversity and of total coccolith abundance is observed during interglacials. Warmest SSTs are reached during MIS 11, while MIS 12 and MIS 10 represent the coldest intervals of the studied record. During MIS 12, one of the most extreme glacials of the last million years, occurrence of Globorotalia inflata and of neogloboquadrinids indicates a shoaling of the interface between Atlantic inflowing and Mediterranean outflowing waters. Among calcareous nannofossils the distribution of Gephyrocapsa margereli-G. muellerae > 4 µm also supports a reduced Atlantic-Mediterranean exchange during MIS 12. Superimposed on glacial-interglacial variability, six short-terms coolings are recognized during MIS 12 and 10, which appear comparable in their distribution and amplitude to the Heinrich - type events documented in the Atlantic Ocean in the same interval. During these H-type events, N. pachyderma (s) and G. margereli-G. muellerae > 4 µm increase as a response to the enhanced inflow of cold Atlantic water into the Mediterranean via the Strait of Gibraltar. Mediterranean surface water hydrography appears to have been most severely affected at Termination V during the H-type event Ht4, possibly as a response to a large volume of Atlantic meltwater inflow via the Strait of Gibraltar and/or to freshwater/terrigenous input deriving from local mountain glaciers. Three additional SST coolings are recorded through MIS 14-16, but these are not well correlated with Heinrich - type events documented in the Atlantic Ocean in the same interval; during these cooling episodes only the subpolar Turborotalita quinqueloba increases. These results highlight the sensitive response of the Mediterranean basin to millennial-scale climate variations related to Northern Hemisphere ice-sheet instability and support the hypothesis that the tight connection between high latitude climate dynamics and Mediterranean sea surface water features can be traced through the Middle Pleistocene.

Sea surface temperature estimates and alkenone C37:4 abundances in ODP Site 145-882 and 177-1090

The cold upwelling 'tongue' of the eastern equatorial Pacific is a central energetic feature of the ocean, dominating both the mean state and temporal variability of climate in the tropics and beyond. Recent evidence for the development of the modern cold tongue during the Pliocene-Pleistocene transition has been explained as the result of extratropical cooling that drove a shoaling of the thermocline. We have found that the sub-Antarctic and sub-Arctic regions underwent substantial cooling nearly synchronous to the cold tongue development, thereby providing support for this hypothesis. In addition, we show that sub-Antarctic climate changed in its response to Earth's orbital variations, from a subtropical to a subpolar pattern, as expected if cooling shrank the warm-water sphere of the ocean and thus contracted the subtropical gyres.

Sedimentary and carbonate preservation of ODP Sites 1262, 1263 and 1266

Rapid carbon input into the ocean-atmosphere system caused a dramatic shoaling of the lysocline during the Paleocene-Eocene thermal maximum (PETM), a transient (~170 kyr) global warming event that occurred roughly 55 Ma. Carbon cycle models invoking an accelerated carbonate-silicate feedback mechanism to neutralize ocean acidification predict that the lysocline would subsequently deepen to depths below its original position as the marine carbonate system recovered from such a perturbation. To test this hypothesis, records of carbonate sedimentation and preservation for PETM sections in the Weddell Sea (ODP Site 690) and along the Walvis Ridge depth transect (ODP Sites 1262, 1263, and 1266) were assembled within the context of a unified chronostratigraphy. The meridional gradient of undersaturation delimited by these records shows that dissolution was more severe in the subtropical South Atlantic than in the Weddell Sea during the PETM, a spatiotemporal pattern inconsistent with the view that Atlantic overturning circulation underwent a transient reversal. Deepening of the lysocline following its initial ascent is signaled by increases in %CaCO3 and coarse-fraction content at all sites. Carbonate preservation during the recovery period is appreciably better than that seen prior to carbon input with carbonate sedimentation becoming remarkably uniform over a broad spectrum of geographic and bathymetric settings. These congruent patterns of carbonate sedimentation confirm that the lysocline was suppressed below the depth it occupied prior to carbon input, and are consistent with the view that an accelerated carbonate-silicate geochemical cycle played an important role in arresting PETM conditions.

Transport of dredged sediment placed in the nearshore zone--Currituck sand-bypass study (phase I) /

New dredge-disposal techniques may serve the dual role of aiding sand by-passing across coastal inlets, and beach nourishment, provided the dredged sediments placed seaward of the surf zone move shoreward into that zone. During the summer of 1976, 26,750 cubic meters of relatively coarse sediment was dredged from New River Inlet, North Carolina, moved down coast by a split-hull barge, and placed in a 215-meter coastal reach between the 2- and 4-meter depth contours. Bathymetric changes on the disposal piles and in the adjacent beach and nearshore area were studied for a 13-week period (August to November 1976) to determine the modification of the surrounding beach and nearshore profile, and the net transport direction of the disposal sediment. The sediment piles initially created a local shoal zone with minimum depths of 0.6 meter. Disposal sediment was coarser (Mn = 0.49 millimeter) than the native sand at the disposal site (Mn = 0.14 millimeter) and coarser than the composite mean grain size of the entire profile (Mn = 0.21 millimeter). Shoaling and breaking waves caused rapid erosion of the pile tops and a gradual coalescing of the piles to form a disposal bar located seaward (= 90 meters) of a naturally occurring surf zone bar. As the disposal bar relief was reduced, the disposal bar-associated breaker zone was restricted to low tide times or periods of high wave conditions.

Relation between immersed weight and volume rates of longshore transport /

As presently used, the immersed weight rate, I sub l, is the volume rate, Q, of longshore transport, multiplied by a constant. For use in engineering problems, I sub l must be converted back to the equivalent Q. The I sub l formulation may be important where the unit weight of sand differs significantly from the unit weight of sand at the open-coast sites contributing data to the design curve. Increase in void ratio may result in a 10- to 20-percent increase in actual (as compared to predicted) shoaling volumes where sand accumulates in protected water. Void ratio should be measured in field studies of longshore transport.

Dynamics of a transient wave group breaking on a beach

New experimental data are presented on the dynamics of a transient wave group breaking on a beach. The transient group is tracked during shoaling and wave breaking, together with the long waves forced during those processes. High spatial sampling enables novel resolution of the evolution of the wave envelope during breaking and the correlation between the envelope and the long waves. The data show a strong dynamic long wave setup in front of the group in shallow water. The amplitude of the dynamic setup is likely to be a function of beach slope, and larger on steeper beaches.

Sedimentology, palaeontology and diagenesis of the Much Wenlock limestone formation

Lithofacies distribution indicates that the Much Wenlock Limestone Formation of England and South Wales was desposited on a shelf which was flat and gently subsiding in the north, but topographically variable in the south. Limestone deposition in the north began with 12m of alga-rich limestone, which formed an upward shoaling sequence. Deepening then led to deposition of calcareous silty mudstones on the northern shelf. The remainder of the formation in this area formed during a shelf-wide regression, culminating in the production of an E to W younging sandbody. Lithofacies distribution on the southern shelf was primarily controlled by local subsidence. Six bedded lithofacies are recognised which contain 14 brachiopod/bryozoan dominated assemblages, of which 11 are in situ and three consist of reworked fossils. Microfacies analysis is necessary to distinguish assemblages which reflect original communities from those which reflect sedimentary processes. Turbulence, substrate-type, ease of feeding and other organisms in the environment controlled faunal distribution. Reefs were built dominantly by corals, stromatoporoids, algae and crinoids. Coral/stromatoporoid (Type A) reefs are common, particularly on the northern shelf, where they formed in response to shallowing, ultimately growing in front of the advancing carbonate sandbody. Algae dominate Type B and Type C reefs, reflecting growth in areas of poor water circulation. Lithification of the formation began in the marine-phreatic environment with precipitation of aragonite and high Mg calcite, which was subsequently altered to turbid low Mg calcite. Younger clear spars post-date secondary void formation. The pre-compactional clear spars have features which resemble the products of meteoric water diagenesis, but freshwater did not enter the formation at this time. The pre-compactional spars were precipitated by waters forced from the surrounding silty mudstones at shallow burial depths. Late diagenetic products are stylolites, compaction fractures and burial cements.