Stephen S. Howe

Tracking the extent of the South Pacific Convergence Zone since the early 1600s

(1) The South Pacific Convergence Zone (SPCZ) is the largest and most persistent spur of the Intertropical Convergence Zone. At the southeastern edge of the SPCZ near 170� W and 15� -20� S a surface ocean salinity frontal zone exists that separates fresher Western Pacific Warm Pool water from saltier and cooler waters in the east. This salinity front is known to shift east and west with the phase of the El Nino Southern Oscillation. We have generated subannually resolved and replicated coral oxygen isotopic time series from Fiji (17� S, 179� E) and Rarotonga (21.5� S, 160� W) that have recorded interannual displacements of the salinity front over the last 380 years and also indicate that at lower frequencies the decadal mean position of the salinity front, and eastern extent of the SPCZ, has shifted east-west through 10� to 20� of longitude three times during this interval. The most recent and largest shift began in the mid 1800s as the salinity front progressively moved eastward and salinity decreased at both sites. Our results suggest that sea surface salinity at these sites is now at the lowest levels recorded and is evidence for an unprecedented expansion of the SPCZ since the mid 1800s. The expansion of the SPCZ implies a gradual change in the South Pacific to more La Nina-like long-term mean conditions. This observation is consistent with the ocean thermostat mechanism for the Pacific coupled ocean-atmosphere system, whereby exogenous heating of the atmosphere would result in greater warming in the western Pacific and a greater east-west surface temperature gradient. Components: 8034 words, 7 figures, 2 tables.

El Niño Southern Oscillation (ENSO) and decadal-scale climate variability at 10°N in the eastern Pacific from 1893 to 1994: A coral-based reconstruction from Clipperton Atoll

We have developed a 101 year (1893–1994) subseasonal oxygen (δ18O) and carbon (δ13C) isotopic time series from Clipperton Atoll in the eastern Pacific using the coral Porites lobata. In agreement with Linsley et al. [1999] we find that seasonal and interannual coral δ18O variability at Clipperton results from variability in both water temperature and salinity. Three new coral time series demonstrate the reproducibility of a secular coral δ18O trend of −0.35‰ since 1910 but show varying δ13C trends. Strong decadal-scale variability in the δ18O record appears related to the Pacific Decadal Oscillation (PDO) through postulated changes in the strengths of the Equatorial Counter Current and North Equatorial Current. Interannual variability in this coral δ18O record is directly related to the El Nino Southern Oscillation (ENSO) and isolation of this frequency band indicates reduced ENSO variability in the eastern equatorial Pacific in the period 1925 to ∼1940, in agreement with instrumental and other Pacific coral records.

Sulphur isotope compositions of sedimentary phosphorites from the basal Cambrian of China: implications for Neoproterozoic-Cambrian biogeochemical cycling

The Meishucun Section (Yunnan Province, South China) is considered to be an important Precambrian–Cambrian boundary section, primarily because of its rich small shelly fossil record. In this article, we report the results of a sulphur isotope study of phosphate-bound sulphate from the Meishucun Section and several correlative sections in South China. Forty clastic, granular phosphorites from Meishucun yield tightly grouped δ34S values averaging 33‰ (CDT), which agree well with published evaporite data for the lower Cambrian of Siberia and elsewhere. We argue that these strongly positive values reflect the sulphur isotopic composition of ambient seawater, confirming further the existence of uniquely high δ34S values in the earliest Cambrian oceans. This novel use of trace-sulphate in phosphate to constrain seawater δ34S represents the first time that sulphate δ34S data for this period have been given precise biostratigraphic assignments. Superimposed on the overall trend are short-term, stratigraphic variations, which might reflect local variations in the sedimentary and early diagenetic environment. Our data, together with other published data, indicate that seawater sulphate δ34S rose from low values (15–20‰) during the pre-750 Ma Proterozoic to possibly all-time high values (>32‰) by the earliest Cambrian. We argue that this rise may, in part, relate to increases in the amount of sulphur isotopic discrimination during microbially mediated sulphate reduction as a result of increased sulphide reoxi-dation. On the other hand, the Neoproterozoic trend to high δ34S values appears to mirror a trend to decreasing seawater δ13C towards the Proterozoic–Phanerozoic transition, implying progressive increases in the efficiency of organic carbon recycling, which would normally be coupled with real increases in sulphate reduction on the global scale. We consider that both these changes in biogeochemical cycling derive ultimately from the introduction of macrofauna around this time and, in particular, from the influence of bioturbation on early diagenesis. Precise constraints on S-isotopic evolution during the Neoproterozoic require additional trace sulphate studies.

3500 yr record of centennial-scale climate variability from the Western Pacific Warm Pool

We use geochemical data from a sediment core in the shallowsilled and intermittently dysoxic Kau Bay in Halmahera (Indo nesia, lat 1°N, long 127.5°E) to reconstruct century-scale climate variability within the Western Pacifi c Warm Pool over the past ~3500 yr. Downcore variations in bulk sedimentary δ 15 N appear to refl ect century-scale variability in basin ventilation, attributed to changes in oceanographic conditions related to century-scale fl uctuations in El Nino Southern Oscillation (ENSO). We infer an increase in century-scale El Nino activity beginning ca. 1700 yr B.P. with peaks in El Nino activity ca. 1500 yr B.P., 1150 yr B.P., and ca. 700 yr B.P. The Kau Bay results suggest that there was diminished ENSO amplitude or frequency, or a departure from El Nino–like conditions during the Medieval Warm Period, and distinctive, but steadily decreasing, El Nino activity during and after the Little Ice Age.

Geochemical and fluid zonation in the skarn environment at the tomboy—minnie gold deposits, Lander County, Nevada

The Tomboy—Minnie gold deposits are related to the middle Tertiary porphyry copper system centered at Copper Canyon. Gold-silver ores in the deposits occur mostly in a pyrrhotite- and pyrite-rich basal 30-m-thick sequence of altered calcareous conglomerate belonging to the Middle Pennsylvanian Battle Formation. The entire mineralized system contained at least 3.3 million troy oz gold before large-scale mining operations began. Alteration in the Tomboy—Minnie deposits includes actinolite- and chlorite-dominant assemblages, in marked contrast to the skarn, potassic, and phyllic assemblages characterizing the copper-gold-silver deposits of the system. Introduction of gold occurred penecontemporaneously with replacement of early diopside-alteration assemblages by actinolite and chlorite. Metals are zoned strongly in the Copper Canyon system: the West and East ore bodies occur in a copper-gold-silver zone that is followed outward by a gold-silver zone which includes the Tomboy deposit and in turn, is succeeded by a lead-zinc-silver zone. Locations of drill holes that have Au/Ag assay ratios of ⩾ 1 clearly outline the Tomboy—Minnie deposits within an area of rocks with Au/Ag ratios of ≈ 0.5. Fluid-inclusion studies suggest wide variations in temperature and chemistry prevailed in the fluids associated with mineralization at the Tomboy. Early fluids associated with diopsidequartz assemblages probably were dominantly CaCl2-rich brines and were boiling at temperatures higher than 500°C. These fluids were progressively enriched in sodium and potassium over time, and during the hydrosilicate stages, temperatures probably ranged from 320 to 500°C at the time actinolite formed, and from 220 to 320°C at the time chlorite was dominant. Sulfur isotopic data suggest that sulfur, mostly from a magmatic or deep-seated crustal source, was transported by hydrothermal fluids as aqueous H2S with a δ 34S of about 4 ± 1‰ to the West, East, and Tomboy deposits.

Late Shortening and Extensional Structures and Veins in the Western Margin of the Taconic Orogen (New York to Vermont)

Along the western margin of the Taconic orogen in New York and Vermont, undeformed quartz‐calcite veins commonly occur in the belt of melange that formed beneath the westward‐advancing Taconic Allochthon during the Middle‐Late Ordovician Taconic orogeny. The veins have mineral slickenfibers recording either reverse or normal slip. In New York, the reverse‐motion veins recording the latest phase of shortening are crosscut by the normal‐motion veins and faults. The shortening indicated by the reverse‐motion veins is correlated with the convergence along the Champlain thrust in Vermont, which is also crosscut by a significant, strike‐parallel normal fault. Fluid inclusion data from the veins, complemented by stable isotope data, lead to a reconstruction of the sequence of events in the context of a cooling of the fluids, which is consistent with crosscutting relationships among the veins. Following cessation of the convergence, there was regional extension of the western margin of the Taconic orogen, analogous to modern arc‐continent collisional orogens. Extension progressed to normal faulting without vein precipitation, and the normal faulting significantly modified the Allochthon‐melange contact. The timing of extension is constrained to follow the late Taconic thrusting and predate the latest Silurian, based on the similar fluid temperature/salinity of the reverse‐ and the normal‐motion veins, and contrast with veins nearby in Devonian rocks. Extensional, partial collapse of the orogen was accompanied or followed by rebound of the foreland basin, perhaps due to reduction of the thrust load and/or subducted slab breakoff. The systematic gradient of homogenization temperatures exhibited by the reverse‐motion veins along the orogen margin is interpreted to be caused by real differences in temperature of the vein‐forming fluids.