Paleoceanography

Eolian grain size and flux were measured on samples from 11 Arabian Sea sediment traps deployed 200‐1250 km offshore. The timing of increased grain size is coincident with the onset of strong summer monsoon winds and dust storm activity over the Arabian Peninsula and Middle East. Data spanning a full annual cycle show that eolian grain size is highly correlated with barometric pressure (r=−0.91) and wind speed (r=0.84), enabling calibration of the downcore record in terms of these primary meteorological variables. Eolian flux is highly correlated with organic carbon flux (r=0.80); both increase 6–8 weeks after the grain size increase and summer monsoon onset. This lag, and the low correlation between eolian grain size and eolian flux (r=0.36), likely result from the differential sinking rates of large and small dust particles in the surface waters as well as biological scavenging associated with monsoon‐induced productivity.

Three Cenomanian/Turonian (C/T, ∼93.5 Ma) black shale sections along a northeast‐southwest transect in the southern part of the proto‐North Atlantic Ocean were correlated by stable carbon isotope stratigraphy using the characteristic excursion in δ¹³C values of both bulk organic matter (OM) and molecular fossils of algal chlorophyll and steroids. All three sites show an increase in marine organic carbon (OC) accumulation rates during the C/T Oceanic Anoxic Event (OAE). The occurrence of molecular fossils of anoxygenic photosynthetic green sulfur bacteria, lack of bioturbation, and high abundance of redox sensitive trace metals indicate sulfidic conditions, periodically reaching up into the photic zone before as well as during the C/T OAE. During the C/T OAE, there was a significant rise of the chemocline as indicated by the increase in concentrations of molecular fossils of green sulfur bacteria and Mo/Al ratios. The presence of molecular fossils of the green strain of green sulfur bacteria indicates that euxinic conditions periodically even occurred at very shallow water depths of 15 m or less during the C/T OAE. However, bottom water conditions did not dramatically change as indicated by more or less constant V/Al and Zn/Al ratios at site 367. This suggests that the increase in OC burial rates resulted from enhanced primary productivity rather than increased anoxia, which is supported by stable carbon isotopic evidence and a large increase in Ba/Al ratios during the C/T OAE. The occurrence of the productivity event during a period of globally enhanced organic carbon burial rates (i.e., the C/T OAE) points to a common cause possibly related to the formation of a deep water connection between North and South Atlantic basins.

Oceanic Anoxic Event 2 (OAE2), spanning the Cenomanian‐Turonian boundary (CTB), represents one of the largest perturbations in the global carbon cycle in the last 100 Myr. The δ¹³C_carb, δ¹³C_org, and δ¹⁸O chemostratigraphy of a black shale–bearing CTB succession in the Vocontian Basin of France is described and correlated at high resolution to the European CTB reference section at Eastbourne, England, and to successions in Germany, the equatorial and midlatitude proto‐North Atlantic, and the U.S. Western Interior Seaway (WIS). Δ¹³C (offset between δ¹³C_carb and δ¹³C_org) is shown to be a good pCO₂ proxy that is consistent with pCO₂ records obtained using biomarker δ¹³C data from Atlantic black shales and leaf stomata data from WIS sections. Boreal chalk δ¹⁸O records show sea surface temperature (SST) changes that closely follow the Δ¹³C pCO₂ proxy and confirm TEX₈₆ results from deep ocean sites. Rising pCO₂ and SST during the Late Cenomanian is attributed to volcanic degassing; pCO₂ and SST maxima occurred at the onset of black shale deposition, followed by falling pCO₂ and cooling due to carbon sequestration by marine organic productivity and preservation, and increased silicate weathering. A marked pCO₂ minimum (∼25% fall) occurred with a SST minimum (Plenus Cold Event) showing >4°C of cooling in ∼40 kyr. Renewed increases in pCO₂, SST, and δ¹³C during latest Cenomanian black shale deposition suggest that a continuing volcanogenic CO₂ flux overrode further drawdown effects. Maximum pCO₂ and SST followed the end of OAE2, associated with a falling nutrient supply during the Early Turonian eustatic highstand.

The cold climate anomaly about 8200 years ago is investigated with CLIMBER‐2, a coupled atmosphere‐ocean‐biosphere model of intermediate complexity. This climate model simulates a cooling of about 3.6 K over the North Atlantic induced by a meltwater pulse from Lake Agassiz routed through the Hudson strait. The meltwater pulse is assumed to have a volume of 1.6 × 10¹⁴ m³ and a period of discharge of 2 years on the basis of glaciological modeling of the decay of the Laurentide Ice Sheet (LIS). We present a possible mechanism which can explain the centennial duration of the 8.2 ka cold event. The mechanism is related to the existence of an additional equilibrium climate state with reduced North Atlantic Deep Water (NADW) formation and a southward shift of the NADW formation area. Hints at the additional climate state were obtained from the largely varying duration of the pulse‐induced cold episode in response to overlaid random freshwater fluctuations in Monte Carlo simulations. The model equilibrium state was attained by releasing a weak multicentury freshwater flux through the St. Lawrence pathway completed by the meltwater pulse. The existence of such a climate mode appears essential for reproducing climate anomalies in close agreement with paleoclimatic reconstructions of the 8.2 ka event. The results furthermore suggest that the temporal evolution of the cold event was partly a matter of chance.

Here we present a high‐resolution marine sediment record from the El Niño region off the coast of Peru spanning the last 20,000 years. Sea surface temperature, photosynthetic pigments, and a lithic proxy for El Niño flood events on the continent are used as paleo–El Niño–Southern Oscillation proxy data. The onset of stronger El Niño activity in Peru started around 17,000 calibrated years before the present, which is later than modeling experiments show but contemporaneous with the Heinrich event 1. Maximum El Niño activity occurred during the early and late Holocene, especially during the second and third millennium B.P. The recurrence period of very strong El Niño events is 60–80 years. El Niño events were weak before and during the beginning of the Younger Dryas, during the middle of the Holocene, and during medieval times. The strength of El Niño flood events during the last millennium has positive and negative relationships to global and Northern Hemisphere temperature reconstructions.

We present oxygen and carbon isotope records of benthic foraminifera from the glacial stage 6 through interglacial substage 5e (Eemian) sections of several cores from the subpolar North Atlantic. The cores range in water depth from 1451 to 2658 m. In one core, we generated estimates of sea surface temperature (SST) and of ice‐rafted detritus (IRD) delivery for all of stage 5. We reconstruct bathymeric profiles of δ¹³C for stage 6, Termination II, and three time slices of substage 5e. The δ¹³C profiles indicate that local deep water geometry during stage 6 was similar to that of the last glaciation, with glacial North Atlantic Intermediate Water (GNAIW) overlying deeper waters partially of southern origin. An anomalously large peak in IRD, coupled with planktonic δ¹⁸O evidence for iceberg melting immediately precedes Termination II and is otherwise similar to stage 2 and 3 Heinrich events. During the termination, low δ¹³C values are observed in cores near and above 2 km, providing evidence of reduced GNAIW production in association with deglacial melting. In the shallowest cores, low δ¹³C values persist into early substage 5e, indicating that the southward retreat of southern‐source waters was not completed until well after the beginning of the substage. In contrast to Termination I, SSTs remained cold until the end of the deglaciation; this may be why there is little evidence from marine and terrestrial sequences for a pronounced climate reversal on Termination II despite what is now clear evidence of a significant reduction in ocean ventilation. The faunal data suggest that SSTs during early 5e were about 7°C warmer than during the glaciation. SST rose several degrees from early to middle substage 5e, peaked in middle substage 5e at about 10°C above glacial values, and then gradually declined by about 5°C. These changes were linked to variations in water mass configuration, as interpreted from benthic δ¹³C evidence. Most of the evidence suggests that oscillations superimposed on the gradual SST trend were 1°–2°C, in contrast to the larger SST changes (3°–4°C) documented for substage 5d.

The isotopic composition of the dissolved inorganic carbon (DIC) collected at sites of active methane discharge on Hydrate Ridge, Oregon, reveals anaerobic methane oxidation mediated by bacteria, with δ¹³C_DIC reaching values as low as −48‰ in the upper 4 cm of the sediment. In spite of the high sulfide levels in the discharging fluids, living specimens of the benthic foraminifera Uvigerina peregrina are abundant in the vents, probably owing to the rich bacterial food source. Although pore water δ¹³C_DIC is extremely low (−6 to −48‰), the δ¹³C values of living (Rose Bengal stained) foraminifera shells collected from active methane seeps are not significantly lower than those observed in nonventing pelagic sediments, and are within the range expected from local organic matter decomposition (0 to −4‰). The apparent δ¹³C disequilibrium between biogenic calcite and DIC suggests that at seep localities, foraminifera calcify mostly during periods when there is little methane discharge or during intermittent episodes of seawater flow into the sediments. The isotopic composition and Mg/Ca ratios of fossil (unstained) foraminifera recovered at carbonate‐rich sites on the northern Hydrate Ridge reveals overprinting of the biogenic record by inorganic calcite with high Mg/Ca and anomalously low δ¹³C values. Thus overprinting of the original isotopic composition of foraminifera by overgrowths or recrystallization at or below the sediment surface, rather than primary calcification in contact with ¹³C depleted DIC, can explain extreme ¹³C depletion observed in fossil foraminifera recovered from sites of active methane discharge.

To study the effects of temperature, salinity, and life processes (growth rates, size, metabolic effects, and physiological/genetic effects) on newly precipitated bivalve carbonate, we quantified shell isotopic chemistry of adult and juvenile animals of the intertidal bivalveMytilus edulis(Blue mussel) collected alive from western Greenland and the central Gulf of Maine and cultured them under controlled conditions. Data for juvenile and adultM. edulisbivalves cultured in this study, and previously by Wanamaker et al. (2006), yielded statistically identical paleotemperature relationships. On the basis of these experiments we have developed a species‐specific paleotemperature equation for the bivalveM. edulis[T °C = 16.28 (±0.10) − 4.57 (±0.15) {δ¹⁸O_cVPBD −δ¹⁸O_wVSMOW} + 0.06 (±0.06) {δ¹⁸O_cVPBD −δ¹⁸O_wVSMOW}²; r²= 0.99; N = 323; p < 0.0001]. Compared to the Kim and O'Neil (1997) inorganic calcite equation,M. edulisdeposits its shell in isotope equilibrium (δ¹⁸O_calcite) with ambient water. Carbon isotopes (δ¹³C_calcite) from sampled shells were substantially more negative than predicted values, indicating an uptake of metabolic carbon into shell carbonate, andδ¹³C_calcitedisequilibrium increased with increasing salinity. Sampled shells ofM. edulisshowed no significant trends inδ¹⁸O_calcitebased on size, cultured growth rates, or geographic collection location, suggesting that vital effects do not affectδ¹⁸O_calciteinM. edulis. The broad modern and paleogeographic distribution of this bivalve, its abundance during the Holocene, and the lack of an intraspecies physiologic isotope effect demonstrated here make it an ideal nearshore paleoceanographic proxy throughout much of the North Atlantic Ocean.

We present a method for determining the δ¹⁸O of seawater in the deep ocean during the last glacial maximum from the measured δ¹⁸O values of deep sea pore fluids. Using data from Deep Sea Drilling Project (DSDP) site 576 in the Western Pacific, this method yields a glacial to interglacial change in δ¹⁸O_sw of 1.0±0.25‰. This value for Δδ¹⁸O_sw is the first direct measurement of deep ocean δ¹⁸O for the last glacial maximum and avoids the problems of spatial and temporal variability of the δ¹⁸O of surface water implicit in previous determinations. More precise, higher‐resolution pore fluid measurements are required to improve this determination.

Geochemical analyses as well as X ray diffraction measurements were carried out on five sediment cores from the eastern Angola Basin and the equatorial divergence of the South Atlantic. Barite concentrations were calculated from total barium concentrations by subtracting the estimated barium background supplied by “nonbarite” barium carriers. Barite concentrations assessed by this geochemical method show a good correspondence to barite concentrations determined by quantitative X ray diffraction measurements. Barite proved to be an important carrier of barium in the pelagic cores, contributing up to 90% of the total barium concentrations in the sediment, while clastic material provides an important source of barium at sites closer to the African continent. Barite accumulation rates were calculated in order to eliminate the diluting effects of varying inputs of terrigenous and biogenic material. Barite accumulation rates show cyclic variations with maxima corresponding to glacial and minima to interglacial stages. Absolute paleoproduction rates were computed from barite accumulation rates. At the Congo fan and the equatorial divergence they are consistent with calculations based on total organic carbon (TOC) accumulation. At the Walvis Ridge, glacial‐interglacial cycles contrast to a constant paleoproductivity computed from TOC accumulation.