Twentieth Century Delta δC Variability in Surface Water Dissolved Inorganic Carbon Recorded by Coralline Algae in the Northern North Pacific Ocean and the Bering Sea

The oxygen isotopic composition and Mg/Ca ratios in the skeletons of long-lived coralline algae record ambient seawater temperature over time. Similarly, the carbon isotopic composition in the skeletons record delta(13)C values of ambient seawater dissolved inorganic carbon. Here, we measured delta(13)C in the coralline alga Clathromorphum nereostratum to test the feasibility of reconstructing the intrusion of anthropogenic CO(2) into the northern North Pacific Ocean and Bering Sea. The delta(13)C was measured in the high Mgcalcite skeleton of three C. nereostratum specimens from two islands 500 km apart in the Aleutian archipelago. In the records spanning 1887 to 2003, the average decadal rate of decline in delta(13)C values increased from 0.03% yr(-1) in the 1960s to 0.095% yr(-1) in the 1990s, which was higher than expected due to solely the delta(13)C-Suess effect. Deeper water in this region exhibits higher concentrations of CO(2) and low delta(13)C values. Transport of deeper water into surface water (i.e., upwelling) increases when the Aleutian Low is intensified. We hypothesized that the acceleration of the delta(13)C decline may result from increased upwelling from the 1960s to 1990s, which in turn was driven by increased intensity of the Aleutian Low. Detrended delta(13)C records also varied on 4-7 year and bidecadal timescales supporting an atmospheric teleconnection of tropical climate patterns to the northern North Pacific Ocean and Bering Sea manifested as changes in upwelling.

Oxygen Production and Carbon Sequestration in an Upwelling Coastal Margin

We examined high-resolution cross-shelf distributions of particulate organic carbon (POC) and dissolved O(2) during the upwelling season off the Oregon coast. Oxygen concentrations were supersaturated in surface waters, and hypoxic in near-bottom waters, with greatly expanded hypoxic conditions late in the season. Simplified time-dependent mass balances on cross-shelf integrated concentrations of these two parameters, found the following: ( 1) The average net rate of photosynthesis generated 2.1 mmol O(2) m(-3) d(-1) and ( 2) essentially none of the corresponding net carbon fixation of 1.4 mmol m(-3) d(-1) could be accounted for in the observed standing stocks of POC. After examining other possible sinks for carbon, we conclude that most of the net production is being exported to the adjacent deep ocean. A simplified POC budget suggests that about a quarter of the export is via alongshore advection, and the remainder is due to some other process. We propose a simplistic conceptual model of across-shelf transport in which POC sinks to the bottom boundary layer where it comes into contact with mineral ballast material but is kept in suspension by high turbulence. When upwelling conditions ease, the BBL waters move seaward, carrying the suspended, ballasted POC with it where it sinks rapidly into the deep ocean at the shelf break. This suggests a mechanism whereby the duration and frequency of upwelling events and relaxations can determine the extent to which new carbon produced by photosynthesis in the coastal ocean is exported to depth rather than being respired on the shelf.

Salinity of the Eocene Arctic Ocean from oxygen isotope analysis of fish bone carbonate

Stable isotope analysis was performed on the structural carbonate of fish bone apatite from early and early middle Eocene samples (~55 to ~45 Ma) recently recovered from the Lomonosov Ridge by Integrated Ocean Drilling Program Expedition 302 (the Arctic Coring Expedition). The d18O values of the Eocene samples ranged from -6.84 per mil to -2.96 per mil Vienna Peedee belemnite, with a mean value of -4.89 per mil, compared to 2.77 per mil for a Miocene sample in the overlying section. An average salinity of 21 to 25 per mil was calculated for the Eocene Arctic, compared to 35 per mil for the Miocene, with lower salinities during the Paleocene Eocene thermal maximum, the Azolla event at ~48.7 Ma, and a third previously unidentified event at ~47.6 Ma. At the Azolla event, where the organic carbon content of the sediment reaches a maximum, a positive d13C excursion was observed, indicating unusually high productivity in the surface waters.

Dinoflagellate cyst assemblages, oxygen isotope records of foraminifera, and alkenone and Mg/Ca-based SST estimates for the time interval 3.40-3.18 Ma (Late Pliocene) from five North Atlantic and Caribbean sediment cores

The early Late Pliocene (3.6 to ~3.0 million years ago) is the last extended interval in Earth's history when atmospheric CO2 concentrations were comparable to today's and global climate was warmer. Yet a severe global glaciation during marine isotope stage (MIS) M2 interrupted this phase of global warmth ~3.30 million years ago, and is seen as a premature attempt of the climate system to establish an ice-age world. Our geochemical and palynological records from five marine sediment cores along a Caribbean to eastern North Atlantic transect show that increased Pacific-to-Atlantic flow via the Central American Seaway weakened the North Atlantic Current (NAC) and attendant northward heat transport prior to MIS M2. The consequent cooling of the northern high latitude oceans permitted expansion of the Greenland ice sheet during MIS M2, despite near-modern atmospheric CO2 concentrations. Before and after MIS M2, heat transport via the NAC was crucial in maintaining warm climates comparable to those predicted for the end of this century.

Stable oxygen isotope record and Sr/Ca ratios of corals from IODP Holes M0009D and M0023A

We present monthly resolved records of strontium/calcium (Sr/Ca) and oxygen isotope (d18O) ratios from well-preserved fossil corals drilled during the Integrated Ocean Drilling Program (IODP) Expedition 310 'Tahiti Sea Level' and reconstruct sea surface conditions in the central tropical South Pacific Ocean during two time windows of the last deglaciation. The two Tahiti corals examined here are uranium/thorium (U/Th)-dated at 12.4 and 14.2 ka, which correspond to the Younger Dryas (YD) cold reversal and the Bølling-Allerød (B-A) warming of the Northern Hemisphere, respectively. The coral Sr/Ca records indicate that annual average sea surface temperature (SST) was 2.6-3.1 °C lower at 12.4 ka and 1.0-1.6 °C lower at 14.2 ka relative to the present, with no significant changes in the amplitude of the seasonal SST cycle. These cooler conditions were accompanied by seawater d18O (d18Osw) values higher by ~0.8 per mill and ~0.6 per mill relative to the present at 12.4 and 14.2 ka, respectively, implying more saline conditions in the surface waters. Along with previously published coral Sr/Ca records from the island [Cohen and Hart (2004), Deglacial sea surface temperatures of the western tropical Pacific: A new look at old coral. Paleoceanography 19, PA4031, doi:10.1029/2004PA001084], our new Tahiti coral records suggest that a shift toward lower SST by ~1.5 °C occurred from 13.1 to 12.4 ka, which was probably associated with a shift toward higher d18Osw by ~0.2 per mill. Along with a previously published coral Sr/Ca record from Vanuatu [Corrège et al. (2004), Interdecadal variation in the extent of South Pacific tropical waters during the Younger Dyras event. Nature 428, 927-929], the Tahiti coral records provide new evidence for a pronounced cooling of the western to central tropical South Pacific during the Northern Hemisphere YD event.

Stable oxygen isotope record from a high-latitude reef off Western Australia

A core from a coral colony of Porites lutea was analysed for stable oxygen isotopic composition*. A 200-year proxy record of sea surface temperatures from the Houtman Abrolhos Islands off west Australia was obtained from coral delta18O. At 29°S, the Houtman Abrolhos are the southernmost major reef complex of the Indian Ocean. They are located on the path of the Leeuwin Current, a southward flow of warm, tropical water, which is coupled to Indonesian throughflow. Coral delta18O primarily reflects local oceanographic and climatic variability, which is largely determined by spatial variability of the Leeuwin Current. However, coherence between coral delta18O and the current strength itself is relatively weak. Evolutionary spectral and singular spectrum analyses of coral delta18O demonstrate a high variability in spectral composition through time. Oscillations in the 5-7-y, 14-15-y, and quasi-biennial bands reflect teleconnections of local sea surface temperature (SST) to tropical Pacific climate variability. Deviations between local (coral-based) and regional (instrument) SST contain a cyclic component with a period of 15 y. Coral delta18O suggests a rise in SST by 0.6°C since AD 1944, consistent with available instrumental SST records. A long-term warming by 1.4°C since AD 1795 is inferred from the coral record.

Growth rates and stable carbon and oxygen isotope record of corals from the Red Sea

Monthly delta18O records of 2 coral colonies (Porites cf. lutea and P. cf. nodifera) from different localities (Aqaba and Eilat) from the northern Gulf of Aqaba, Red Sea, were calibrated with recorded sea surface temperatures (SST) between 1988 and 2000. The results show high correlation coefficients between SST and delta18O. Seasonal variations of coral delta18O in both locations could explain 91% of the recorded SST. Different delta18O/SST relations from both colonies and from the same colonies were obtained, indicating that delta18O from coral skeletons were subject to an extension rate effect. Significant delta18O depletions are associated with high extension rates and higher values with low extension rates. The relation between coral skeletal delta18O and extension rate is not linear and can be described by a simple exponential model. An inverse relationship extends over extension rates from 1 to 5 mm/yr, while for more rapidly growing corals and portions of colonies the relation is constant and the extension rate does not appear to have a significant effect. We recommend that delta18O values be obtained from fast-growing corals or from portions in which the isotopic disequilibrium is fairly constant (extension rate >5 mm/yr). The results show that interspecific differences in corals may produce a significant delta18O profile offset between 2 colonies that is independent of environmental and extension-rate effects. We conclude that the rate of skeletal extension and the species of coral involved have an important influence on coral delta18O and must be considered when using delta18O records for paleoclimatic reconstructions.

Monthly Tahiti coral Sr/Ca and oxygen isotope data from IODP Hole 310-M0024A

The early last glacial termination was characterized by intense North Atlantic cooling and weak overturning circulation. This interval between ~18,000 and 14,600 years ago, known as Heinrich Stadial 1, was accompanied by a disruption of global climate and has been suggested as a key factor for the termination. However, the response of interannual climate variability in the tropical Pacific (El Niño-Southern Oscillation) to Heinrich Stadial 1 is poorly understood. Here we use Sr/Ca in a fossil Tahiti coral to reconstruct tropical South Pacific sea surface temperature around 15,000 years ago at monthly resolution. Unlike today, interannual South Pacific sea surface temperature variability at typical El Niño-Southern Oscillation periods was pronounced at Tahiti. Our results indicate that the El Niño-Southern Oscillation was active during Heinrich Stadial 1, consistent with climate model simulations of enhanced El Niño-Southern Oscillation variability at that time. Furthermore, a greater El Niño-Southern Oscillation influence in the South Pacific during Heinrich Stadial 1 is suggested, resulting from a southward expansion or shift of El Niño-Southern Oscillation sea surface temperature anomalies.