A chemical indicator of trade wind reversal in corals from the western tropical Pacific

Abstract

Historical levels of manganese have been measured in the annual rings of a reef coral from Tarawa atoll (1ø30'N,173øE) in order to assess the temporid variability of this trace element in surface waters of the western tropical Pacific. Seasonal variabitity associated with vertical mixing is not apparent as is the case in the eastern Pacific; however, Mn/Ca ratios are highly perturbed during three E1 Nifio-Southern Oscillation (ENSO) events (1965, 1972, 1976) that occurred within the growth period 1960-1977. For approximately 6 months during each of these events, Mn/Ca ratios increase by 50-80% from a background of 27 nmol Mn/mol Ca. These features co-occur with negative •180 pulses recorded in the same coral which result from anomalously abundant rainfall. Wet deposition of Mn, however, does not appear to be an adequate source to account for the inferred surface ocean enrichments. The most plausible explanation has to do with the transient appearance of westerly winds along the equator during ENSO periods. Particularly vigorous westerly wind "bursts" (sometimes averaging 10 m/sec or more over 16 hours) during the aforementioned years may have generated wind waves of sufficient strength to create an aura of particulate and diagenetically remobitized Mn from within the shallow westward facing lagoon of Tarawa. This interpretation is supported by relatively high Mn concentrations measured in pore waters of Tarawa lagoon sediments. The results suggest a useful new chemical tracer of historical ENSO conditions in the western Pacific basin. 12,689 12,690 SHEN ET AL.' CORALS AND TRADE WIND REVERSAL Weber, J.N., and P.M.J. Woodhead, Temperature dependence of oxygen-18 concentration in reef coral carbonates, J. Geophys. Res., 77, 463-473, 1972. Wynki, K., El Nifio, The dynamic response of the equatorial Pacific Ocean to atmospheric forcing, J. Phys. Oceanogr., 9, 1223-1231, 1975.

New views of tropical paleoclimates from corals

“…Corals from sites in the equatorial Paci"c record past ENSO variability through its impact on SST, surface water salinity, or a combination of the two. Work on corals in the ENSOsensitive central Paci"c has focused on the islands of Kiribati (near 13N, 1723E; Cole and Fairbanks, 1990;Cole et al, 1993, Shen et al, 1992 and the Line Islands (&1603W; Dru!el, 1987;Evans et al, 1998). In this region, warm and wet anomalies during ENSO warm phases work together to generate lower coral skeletal O values.…”

“…In oceanic settings where the oxygen isotopic composition of seawater is constant, coral skeletal O records SST variability, usually according to the standard paleotemperature relationship for carbonates (Epstein et al, 1953). The isotopic composition is o!set by a biological non-equilibrium component that appears to be stable through time, as long as a consistent, maximum growth axis is sampled within a coral colony (Weber and Woodhead, 1972;Dunbar and Wellington, 1981; Winter et al, 1991;Shen et al, 1992;Gagan et al, 1994;Leder et al, 1996;Swart et al, 1996;Wellington et al, 1996;Cohen and Hart, 1997). When seawater O varies in response to changes in the balance between precipitation, evaporation, and water advection, the coral O changes accordingly (Swart and Coleman, 1980;Dunbar and Wellington, 1981;Cole et al, 1993;Gagan et al, 1994;Linsley et al, 1994).…”

“…In the eastern Paci"c, a comprehensive network of coral climate reconstructions is developing (Dru!el, 1981;Dru!el et al, 1990;Linn et al, 1990;Shen et al, 1991Shen et al, , 1992Dunbar et al, 1994Dunbar et al, , 1996Carriquiry et al, 1994Carriquiry et al, , 1998Linsley et al, 1994Linsley et al, , 1999Wellington and Dunbar, 1995;Shen, 1996;Wellington et al, 1996). The 280-yr O record from a rainfall-sensitive site near Panama indicates decadal periods in the strength/position of Intertropical Convergence Zone (ITCZ) precipitation (Fig.…”

Climate records from corals

“…Analytical capabilities for measuring geochemical tracers in coral skeletons have advanced considerably since early work first suggested their usefulness for providing information on coral seawater environments 60, 61. Now the most commonly used analytical techniques include stable isotope ratio mass spectrometry (for δ 18 O and δ 13 C), inductively coupled plasma mass spectrometry (ICP‐MS), inductively coupled plasma atomic emission spectrometry (ICP‐AES), thermal ionisation mass spectrometry (TIMS) and multi collector‐inductively coupled plasma mass spectrometry (Mc‐ICP‐MS) for trace and minor elemental ratios such as Sr/Ca, Mg/Ca, U/Ca, and Ba/Ca 8, 62–69. Uranium/thorium ratios measured with TIMS and ICP‐MS are also used to date fossil coral samples 70, 71.…”

Coral Cd/Ca and Mn/Ca records of ENSO variability in the Gulf of California

“…Also, but in a different manner, the ENSO variability has been documented in the Mn/Ca ratios of corals from Tarawa atoll, in the central Pacific Ocean (Shen et al, 1992b). In this region, as a consequence of Trade wind reversal (development of intense Westerlies) that takes place during ENSO years, bottom lagoonal sediments become resuspended, enriching the water column with diagenetic-Mn; corals record these Mn-pulse events in their growth bands (Shen et al, 1992b).…”