Geology

The expected changes over the past 500 million years in the principal astronomical frequencies influencing the Earth's climate may be strong enough to be detectable in the geological records, and such effects have been inferred in several cases. Calculations suggest that the shortening of the Earth-moon distance and of the length of the day back in time induced a shortening of the fundamental periods for the obliquity and climatic precession, from 54 to 3 5 , 4 1 to 2 9 , 2 3 to 19, and 19 to 16 thousand years over the last half-billion vears. At the same time. the precessional constant increked from 50 to 6 1 arc secbnds per year. The changes in the frequencies of the planetary system due to its chaotic motion are much smaller; their influence on the changes of the periods of climatic precession, obliquity, and eccentricity of the Earth's orbit around the sun can be neglected. Eccentricity periods used for Quaternary climate studies may therefore be considered to have been more or less constant for pre-Quaternary times.

Corals o!er a rich archive of past climate variability in tropical ocean regions where instrumental data are limited and where our knowledge of multi-decadal climate sensitivity is incomplete. In the eastern equatorial Paci"c, coral isotopic records track variations in ENSO-related changes in sea-surface temperature; further west, corals record variability in sea-surface temperature and rainfall that accompanies zonal displacement of the Indonesian Low during ENSO events. These multi-century records reveal previously unrecognised ENSO variability on time scales of decades to centuries. Outside the ENSO-sensitive equatorial Paci"c, long-term trends towards recent warmer/wetter conditions suggest the tropics respond to global forcings. New coral paleothermometers indicate that surface-ocean temperatures in the tropical southwestern Paci"c were depressed by 4}63C during the Younger Dryas climatic event and rose episodically during the next 4000 yr. High temporal-resolution measurements of Sr/Ca and O in corals provide information about the surface-ocean hydrologic balance and can resolve the seasonal balance between precipitation and evaporation. Radiocarbon measurements in corals, coupled with ocean circulation models, may be used to reconstruct near-surface ocean circulation, past mixing rates, and the distribution of fossil fuel CO in the upper ocean. Most recently, seasonal to interannual variations in the radiocarbon of corals from the equatorial Paci"c have been linked to the redistribution of surface waters associated with the ENSO.

The climate of the Eastern Mediterranean region of the last 60 ky was determined by a high resolution study of the oxygen and carbon isotopic composition (1500 measurement pairs) of speleothems from the Soreq cave, Israel, with chronology provided by 53 precise 230Th–234U (TIMS) ages. The high precision of the speleothem TIMS ages permits us to determine the timing of regional climatic events in the Eastern Mediterranean region and to see if they correlate with global events. During the period from 60 to 17 ky, the δ18O and δ13C values were generally 2–2.5‰ higher than during the period from 17 ky to present. This is consistent with the climatic transition from glacial to interglacial. Within the 60 to 17 ky period, the Soreq cave stable isotope profile includes four cold peaks (at 46, 35, 25 and 19 ky) and 2 warm peaks (at 54 and 36 ky). In addition, the period <17 ky has two more cold peaks at 16.5 and from 13.2 to 11.4 ky. The ages of four of the six cold peaks correlate well with the ages of three Heinrich events (H1, H2, H5) and with the age of the Younger Dryas. However, the other two Heinrich events are not reflected in the Soreq cave record. Several other isotope peaks which appear during the last 7 ky are contemporaneous with regional climatic events in the Middle East and North Africa.In addition to the drop in δ18O and δ13C observed between the last glacial and the Holocene, sharp simultaneous drops in (234U/238U)0 ratios, Sr concentrations and in 87Sr/86Sr are also observed, suggesting that the latter are climate related. These variations are interpreted in terms of major changes in the temperature, the mean annual rainfall and its isotopic composition, the isotopic composition of the Mediterranean vapor source, the soil moisture conditions, and in the mixing proportions of sources with different 87Sr/86Sr ratios (sea spray, dust particles and dolomitic host rock).

Book reviews acquired both ocean and atmosphere, nor how these evolved over eons to our present, turbulent, but astonishingly stable, balance over the last several hundred millions of years. Set in that context the possible future trends are merely planetary variations, but ones with daunting consequences for humankind!

New Sr and C isotopic data, both obtained on the same samples of marine carbonates, provide a relatively detailed record of isotopic variation in seawater through the latest Proterozoic and allow, for the first time, direct correlation of these isotopic changes in the Vendian (∼ 540–610 Ma). The strong isotope variations determined in this study record significant environmental and tectonic changes. Together with a fairly poorly constrained Nd isotopic record, the Sr and C isotopic records can be used to constrain rates of erosion, hydrothermal alteration and organic C burial. Further, comparison of these records with those of the Cenozoic permit investigation of the general relationship between global tectonics and continental glaciation. In particular, results of this study show a very large change in the 87Sr/86Sr of marine carbonates from low pre-Vendian ( > 610 Ma) values ( ∼ 0.7066) to high Middle Cambrian values ( ∼ 0.7090). This change is greater in magnitude than the significant increase in seawater 87Sr/86Sr through the Cenozoic. Both changes are attributed to high erosion rates associated with continent-continent collisions (Pan-African and Himalayan-Tibetan). In the latest Proterozoic these high erosion rates, probably coupled with high organic productivity and anoxic bottom-water conditions, contributed to a significant increase in the burial rate of organic C. Ice ages mark both the Neoproterozoic and Cenozoic, but different stratigraphic relationships between the Sr isotopic increase and continental glaciation indicate that uplift-driven models proposed to explain Cenozoic climatic change cannot account for the latest Proterozoic ice ages.