Robert B. Dunbar

Eastern Pacific sea surface temperature since 1600 A.D.: The δ18O record of climate variability in Galápagos Corals

We measured stable oxygen isotope ratios and skeletal growth rates in the massive corals Pavona clavus and P. gigantea from the west coast of Isabela Island, Galapagos, to assess interannual to decadal climate variability in the eastern Pacific. Comparisons of instrumental data sets show that sea surface temperatures (SST) in the Galapagos region are representative of a broad portion of the eastern equatorial Pacific. The site is especially well-suited for long-term studies of the El Nino/Southern Oscillation (ENSO) phenomenon, as it lies within the eastern Pacific “center of action” for thermal anomalies associated with ENSO. The P. gigantea isotope record is nearly monthly in resolution, spans the period 1961–1982, and shows strong correlation with a Galapagos instrumental SST record (r = −0.90 for annual averages). Cross-spectral analysis shows that SST can explain greater than 80% of the variance in δ18O at both the annual cycle and within the high-frequency portion of the ENSO band (3-5 years). The P. clavus record is annual in resolution, extends from 1587 to 1953 A.D., and was obtained from a 10-m diameter colony preserved within the Urvina Bay uplift. Because seawater δ18O variations in the region are very small, we interpret the Urvina Bay coral δ18O record in terms of annual average SST. The isotopic record appears to be a very good, but not perfect, indicator of ENSO events and shows good correspondence with the historical ENSO reconstruction of Quinn et al. (1987). A number of low δ18O excursions that we observe during the 17th and 18th centuries very likely represent ENSO events that are missing from the historical tabulations. Most interannual δ18O variations between 1607 and 1953 A.D. represent annual average temperature excursions of 1° to 2.5°C. During the Little Ice Age, the annual δ18O series correlates well with many North American tree ring records and shows low temperatures during the early 1600s and early 1800s, and relatively warmer conditions during the 1700s. Unlike most northern hemisphere tree ring and instrumental records, we see no evidence at this site for warming between 1880 and 1940 but rather observe a slight cooling ( 28% of the total variance. The main ENSO mode is centered at 4.6 years and accounts for 12% of the total variance. Additional significant oscillations occur at periods of 3.3, 6, 8, 11, 17, 22, and 34 years. Both annual growth rate and δ18O show variance at periods equivalent to the solar and solar magnetic periods (e.g., 11 and 22 years, respectively). In addition, the amplitude of the 11-year δ18O cycle generally varies with the amplitude of the solar cycle, supporting previous suggestions that the solar cycle may modulate interannual to decadal climate variability in the tropics. The dominant oscillatory modes, both within the ENSO and interdecadal frequency bands, shift to shorter periods from the early to middle 1700s and again from the middle to late 1800s. This may reflect major reorganizations within the tropical ocean-atmosphere system and suggests that tropical Pacific climate variability is linked across timescales ranging from years to decades.

Experimental Dendroclimatic Reconstruction of the Southern Oscillation

Abstract Exactly dated tree-ring chronologies from ENSO-sensitive regions in subtropical North America and Indonesia together register the strongest ENSO signal yet detected in tree-ring data worldwide and have been used to reconstruct the winter Southern Oscillation index (SOI) from 1706 to 1977. This reconstruction explains 53% of the variance in the instrumental winter SOI during the boreal cool season (December-February) and was verified in the time, space, and frequency domains by comparisons with independent instrumental SOI and sea surface temperature (SST) data. The large-scale SST anomaly patterns associated with ENSO in the equatorial and North Pacific during the 1879-1977 calibration period are reproduced in detail by this reconstruction. Cross-spectral analyses indicate that the reconstruction reproduces over 70% of the instrumental winter SOI variance at periods between 3.5 and 5.6 yr, and over 88% in the 4-yr frequency band. Oscillatory modes of variance identified with singular spectrum ana...

Rapid and early export of Phaeocystis antarctica blooms in the Ross Sea, Antarctica.

The Southern Ocean is very important for the potential sequestration of carbon dioxide in the oceans and is expected to be vulnerable to changes in carbon export forced by anthropogenic climate warming. Annual phytoplankton blooms in seasonal ice zones are highly productive and are thought to contribute significantly to pCO2 drawdown in the Southern Ocean. Diatoms are assumed to be the most important phytoplankton class with respect to export production in the Southern Ocean; however, the colonial prymnesiophyte Phaeocystis antarctica regularly forms huge blooms in seasonal ice zones and coastal Antarctic waters. There is little evidence regarding the fate of carbon produced by P. antarctica in the Southern Ocean, although remineralization in the upper water column has been proposed to be the main pathway in polar waters. Here we present evidence for early and rapid carbon export from P. antarctica blooms to deep water and sediments in the Ross Sea. Carbon sequestration from P. antarctica blooms may influence the carbon cycle in the Southern Ocean, especially if projected climatic changes lead to an alteration in the structure of the phytoplankton community.

Chronology of the Palmer Deep site, Antarctic Peninsula: a Holocene palaeoenvironmental reference for the circum-Antarctic

Palmer Deep sediment cores are used to produce the first high-resolution, continuous late Pleistocene to Holocene time-series from the Antarctic marine system. The sedimentary record is dated using accelerator mass spectrometer radiocarbon methods on acid insoluble organic matter and foraminiferal calcite. Fifty-four radiocarbon analyses are utilized in the dating which provides a calibrated timescale back to 13 ka BP. Reliability of resultant ages on organic matter is assured because duplicates produce a standard deviation from the surface age of less than laboratory error (i.e., ±50 years). In addition, surface organic matter ages at the site are in excellent agreement with living calcite ages at the accepted reservoir age of 1260 years for the Antarctic Peninsula. Spectral analyses of the magnetic susceptibility record against the age model reveal unusually strong periodicity in the 400,–200 and 50-70 year frequency bands, similar to other high-resolution records from the Holocene but, so far, unique for the circum-Antarctic. Here we show that comparison to icecore records of specific climatic events (e.g., the ’Little Ice Age‘, Neoglacial, Hypsithermal, and the Bølling/Allerød to Younger Dryas transition) provides improved focus upon the relative timing of atmosphere/ocean changes between the northern anid southern high latitudes.

A coral‐based reconstruction of Intertropical Convergence Zone variability over Central America since 1707

Seasonal movements of the Intertropical Convergence Zone (ITCZ) control precipitation patterns and cloud cover throughout the tropics. In this study we have reconstructed seasonal and interannual variability of the eastern Pacific ITCZ from 1984 to 1707 using subseasonal δ18O analyses on a massive coral from Secas Island (7°59′N, 82°3′W) in the Gulf of Chiriqui, Panama. The land area that drains into the Gulf of Chiriqui has served to amplify the rainfall effect on nearshore surface waters and coral δ18O composition. During the protracted wet season in Panama, the δ18O of precipitation (δ18Oppt) is reduced on average by 10‰ and sea surface salinity (SSS) along the western coast is reduced up to 11‰. Calibration of the coral δ18O from Secas Island against instrumental sea surface temperature (SST), SSS, precipitation and δ18Oppt data indicate that seasonal rainfall induced variations in seawater δ18O are responsible for ∼80% of the annual δ18O variance. Past El Nino-Southern Oscillation (ENSO) events are recorded as minor 0.2 to 0.4‰ δ18O changes superimposed on the dominant annual δ18Oseawater and salinity variations. The annual cycle in coral δ18O (average 0.9‰) accounts for the largest component of variance at 51% and is the direct result of the annual northward expansion of the eastern Pacific ITCZ. The regularity of the reconstructed seasonal ITCZ cycle indicates that over the length of the record the zone of maximum rainfall in the eastern Pacific has always expanded north to at least Panama in every northern hemisphere summer. Significant interannual and interdecadal δ18O oscillations occur at average periods near 9, 3–7 (ENSO band), 17 and 33 years (listed in order of decreasing variance). Over the past 20 years similar decadal shifts are apparent in coral δ18O from nearshore in the Gulf of Panama. SST data spanning the last 40 years show no decadal changes. This indicates that decadal oscillations in the Gulf of Chiriqui δ18O record are regional features not related to SST changes, but are caused by ITCZ precipitation effects on the δ18O of seawater. A 9-year period in Panama precipitation supports this conclusion and provides a potential link between interannual coral δ18O variations and ITCZ precipitation. It is also shown that the period of the average 9-year interannual period in coral δ18O varies from ∼7.5 years to ∼11.8 years. Variance near 11 years is strongest throughout the 1800s, however, a poor direct correlation with sunspot number and solar irradiance leaves the origin of this interannual oscillation in question. The δ18O time series also contains a long-term trend of −0.40‰ suggesting an increase in precipitation and/or SST since the early 1800s. As the Gulf of Chiriqui coral δ18O time series is the first paleoclimatic record of past variations in the ITCZ, other seasonal-resolution reconstructions of the past behavior of the ITCZ are required to test whether the interannual and long-term variability observed in the eastern Pacific ITCZ is more than regional in scale.

Distribution of magnesium in coral skeleton

Ion micro-probe imaging of the aragonite skeleton of Pavona clavus, a massive reef-building coral, shows that magnesium and strontium are distributed very differently. In contrast to strontium, the distribution of magnesium is strongly correlated with the fine-scale structure of the skeleton and corresponds to the layered organization of aragonite fibers surrounding the centers of calcification, which have up to ten times higher magnesium concentration. This indicates a strong biological control over the magnesium composition of all structural components within the skeleton. Magnesium may be used by the coral to actively control the growth of the different skeletal crystal components.

Phytoplankton taxonomic variability in nutrient utilization and primary production in the Ross Sea

Patterns of nutrient utilization and primary productivity (PP) in late austral spring and early summer in the southwestern Ross Sea were characterized with respect to phytoplankton taxonomic composition, polynya dynamics, and upper ocean hydrography during the 1996–1997 oceanographic program Research on Ocean-Atmosphere Variability and Ecosystem Response in the Ross Sea. Phytoplankton biomass in the upper 150 m of the water column ranged from 40 to 540 mg chlorophyll a (Chl a) m−2, exceeding 200 mg Chl a m−2 everywhere except the extreme northern and eastern boundaries of the Ross Sea polynya. Diatom biomass was greatest in the shallow mixed layers of Terra Nova Bay, while the more deeply mixed waters of the Ross Sea polynya were dominated by Phaeocystis antarctica. Daily production computed from the disappearance of NO3 (1.14 g C m−2 d−1) and total dissolved inorganic carbon (TDIC, 1.29 g C m−2 d−1) is consistent with estimates made from an algorithm forced with satellite measurements of Chl a (1.25 g C m−2 d−1) and from measurements of 14C uptake (1.33 g C m−2 d−1). Phytoplankton PP in the Ross Sea averaged 100 g C m−2 yr−1 during 1996–1997. Despite the early formation of the Terra Nova Bay polynya the diatom bloom there did not reach its peak PP until middle to late January 1997 (most likely because of more intense wind mixing in November), ∼6 weeks after the P. antarctica bloom in the Ross Sea polynya had reached the same stage of development. From 70 to 100% of the C and N deficits in the upper 150 m could be accounted for by particulate organic matter, indicating that there had been little dissolved organic matter production or export of particulate material prior to our cruise. This suggests that early in the season, PP and zooplankton grazing are decoupled in the southwestern Ross Sea. The NO3∶PO4 disappearance ratio in waters dominated by P. antarctica (19.0±0.61) was significantly greater than in waters where diatoms were most common (9.52±0.33), and both were significantly different from the Redfield N∶P ratio of 16. Vertical profiles of TDIC suggest that P. antarctica took up 110% more CO2 per mole of PO4 removed than did diatoms, an important consideration for climate models that estimate C uptake from the removal of PO4.

Cycling of organic carbon and biogenic silica in the Southern Ocean: Estimates of water-column and sedimentary fluxes on the Ross Sea continental shelf

We examined the cycling of organic carbon and biogenic silica in the water column and upper sediments of the Ross Sea, seeking to understand the processes leading to the formation of opal-rich, organic-poor sediments over much of the Southern Ocean. Between January, 1990 and December, 1994 we conducted three cruises, performing tracer incubation studies (14C, 15N, 30Si, 32Si) to measure rates of primary production, nitrate-based “new” production, biogenic silica production and biogenic silica dissolution in the upper 50 m over most of the Ross Sea shelf in spring, mid summer and late summer. We deployed sediment traps from January, 1990 to early March, 1992 to measure the mid-water (250 m) and near-bottom gravitational fluxes of particulate organic carbon, nitrogen and biogenic silica year-round at three sites, and obtained sediment cores at 15 sites to assess the accumulation rates of organic carbon and biogenic silica in all known sediment regimes on the shelf. At 9 of those sites we also measured nutrient efflux from the sediments, enabling us to calculate benthic recycling fluxes of organic matter and opal. These data permit estimates of the annual production, near-surface recycling, vertical sinking flux, delivery to the seabed, benthic regeneration and long-term burial of both organic and siliceous material, integrated over a 3.3 × 105 km2 area that covers 75–80% of the Ross Sea shelf. The resulting annual budgets for carbon and silica indicate highly selective preservation of biogenic silica over organic carbon between 50 and 250 m in the water column, as well as in the upper seabed. Selective preservation of silica within the upper 50 m is not indicated, and both organic matter and silica are transported from 250 m to the sea floor with virtually 100% efficiency. The SiO2/C mass ratios for surface-layer production, 250-m sinking flux, delivery to the seabed and long-term burial are approximately 0.85, 6.1, 6.2 and 27, respectively. This progressive enrichment in silica results in long-term burial of 5.8% of the biogenic silica and 0.17% of the organic carbon produced by phytoplankton in the surface layer, a factor of 30 greater preservation efficiency for silica than for carbon. Nevertheless, the ratio of opal burial to opal production in the Ross Sea is only about twice the apparent global average of 3% and <1/3 of the estimated burial/production ratio for the Southern Ocean as a whole. It thus appears that both silica preservation and the decoupling between the cycles of silica and carbon must be even more effective in the waters overlying abyssal Southern Ocean sediments than they are over the Ross Sea shelf.

Calibration of stable oxygen isotope signatures in Galápagos corals

A 2-year (1993-1994) study was conducted in the Galapagos Islands (Ecuador) to determine the relationship between δ18O in skeletal carbonate and sea surface temperature (SST) in three species of reef-building corals: Pavona clavus, Pavona gigantea, and Porites lobata. Coral samples were grown at 3, 10, and 3 m depth at Bartolome Island, Champion iIsland, and Urvina Bay (Isabela Island), respectively. Hourly measurements of SST and sea surface salinity (SSS) were taken at each site immediately adjacent to colonies which were stained biannually to establish the chronology of growth. In addition, surface waters were sampled periodically (bimonthly to monthly) at each site to determine variation in δ18O seawater. Results indicate the mean annual SSTs were similar between sites, varying from 22.9°C at Champion to 23.8°C at Urvina Bay. Comparisons of monthly SST averages between instrumental and remote sensing (satellite, 1° × 1° grid) data show a high correspondence (r2 ranging from 0.84 to 0.94), indicating that remote sensing data are useful for interpreting the δ18O record in corals when instrumental data are lacking. Here δ18Ocoral analyses of eight specimens show that coralline aragonite is a reliable indicator of SST in Galapagos. In general, higher-resolution coral sampling/year resolved more of the monthly variation in SST, up to 97% at a sampling resolution of 1.4 samples per millimeter of linear skeletal growth. Comparisions of the δ18Ocoral signal among and between species at the same site showed consistent seasonal patterns of variation closely tracking SST. In addition, comparisons between sites were highly concordant, with some differences reflecting local variation in SST. Seasonal patterns, however, were essentially the same over the entire region. Thus we conclude that the δ18Ocoral signal from coral skeletons in Galapagos can be used to interpret regional changes in SST variation.

Environmental controls on uranium in reef corals

A survey of corals from a variety of tropical settings reveals previously unseen seasonal variations in skeletal UCa. Based upon two corals from the Galapagos Islands, a comparison of UCa with δ180 suggests a possible temperature dependence of +3–4% per degree centigrade cooling. An overall range in UCa of 1.03–1.37 μmol U/mol Ca (2.45–3.25 ppm) between corals from warm and cool water settings supports this interpretation. An alternative control, however, cannot be ruled out. Changes in the carbonate ion content of surface waters are sufficient to drive comparable variations, provided uranium is incorporated as UO22+ or a carbonate complex thereof. In addition to these possible controls, we identify a probable salinity influence on coral UCa which suggests that uranium uptake depends upon [U]seawater and not [UCa]seawater. Within individual corals, artifacts associated with “vital” influences appear minimal. Extension/calcification rate effects as assessed via comparisons of contemporaneous growth trajectories in individual colonies appear small relative to typical seasonal UCa variations of 10–20%. Excluding corals from the Galapagos cool regime, five species from warmer tropical settings exhibit remarkably little variability in mean UCa (1.03–1.09 μmol U/mol Ca). Our findings suggest that the range of UCa in modern corals defined by published data reflects a combination of interspecific variability and environmental control. The possibility that coral UCa is associated with temperature, alkalinity, and salinity suggests many uses for this new paleotracer. Rapid and precise measurement of uranium by isotope dilution ICP-MS will expedite future development and application. Key among the next steps must be studies to isolate the influences of the above mentioned environmental parameters. Additionally, potential microsampling artifacts (e.g., caused by drilling) and an apparent 6% “leachable” uranium fraction in a single test coral should be investigated through further pretreatment studies.