Since 65 million years ago (Ma), Earth's climate has undergone a significant and complex evolution, the finer details of which are now coming to light through investigations of deep-sea sediment cores. This evolution includes gradual trends of warming and cooling driven by tectonic processes on time scales of 10(5) to 10(7) years, rhythmic or periodic cycles driven by orbital processes with 10(4)- to 10(6)-year cyclicity, and rare rapid aberrant shifts and extreme climate transients with durations of 10(3) to 10(5) years. Here, recent progress in defining the evolution of global climate over the Cenozoic Era is reviewed. We focus primarily on the periodic and anomalous components of variability over the early portion of this era, as constrained by the latest generation of deep-sea isotope records. We also consider how this improved perspective has led to the recognition of previously unforeseen mechanisms for altering climate.

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Trends, Rhythms, and Aberrations in Global Climate 65 Ma to Present

Rising atmospheric carbon dioxide (CO2), primarily from human fossil fuel combustion, reduces ocean pH and causes wholesale shifts in seawater carbonate chemistry. The process of ocean acidification is well documented in field data, and the rate will accelerate over this century unless future CO2 emissions are curbed dramatically. Acidification alters seawater chemical speciation and biogeochemical cycles of many elements and compounds. One well-known effect is the lowering of calcium carbonate saturation states, which impacts shell-forming marine organisms from plankton to benthic molluscs, echinoderms, and corals. Many calcifying species exhibit reduced calcification and growth rates in laboratory experiments under high-CO2 conditions. Ocean acidification also causes an increase in carbon fixation rates in some photosynthetic organisms (both calcifying and noncalcifying). The potential for marine organisms to adapt to increasing CO2 and broader implications for ocean ecosystems are not well known; both are high priorities for future research. Although ocean pH has varied in the geological past, paleo-events may be only imperfect analogs to current conditions.

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Ocean Acidification: The Other CO 2 Problem

https://cyberleninka.org/article/n/352063.pdf

87Sr/86Sr, δ13C and δ18O evolution of Phanerozoic seawater

Negative carbon isotope anomalies in carbonate rocks bracketing Neoproterozoic glacial deposits in Namibia, combined with estimates of thermal subsidence history, suggest that biological productivity in the surface ocean collapsed for millions of years. This collapse can be explained by a global glaciation (that is, a snowball Earth), which ended abruptly when subaerial volcanic outgassing raised atmospheric carbon dioxide to about 350 times the modern level. The rapid termination would have resulted in a warming of the snowball Earth to extreme greenhouse conditions. The transfer of atmospheric carbon dioxide to the ocean would result in the rapid precipitation of calcium carbonate in warm surface waters, producing the cap carbonate rocks observed globally.

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A Neoproterozoic Snowball Earth

Equilibrium and nonequilibrium oxygen isotope effects in synthetic carbonates

The equator to high southern latitude sea surface and vertical temperature gradients are reconstructed from oxygen isotope values of planktonic and benthic foraminifers for the following five time intervals: late Paleocene, early Eocene, early middle Eocene, late Eocene, and early Oligocene. Paleotemperatures are calculated using standard oxygen isotope/temperature equations with adjustments to account for (1) variations in sea water δ18O related to changes in global ice volume over time and (2) latitudinal gradients in surface water δ18O. These reconstructions indicate that sea-surface temperatures (SST) of the Southern Oceans in the early Eocene were as high as 15°C, whereas temperatures during the late Paleocene and early middle Eocene reached maximum levels of 10°–12°C. By the late Eocene and early Oligocene high latitude SST had declined to 6 and 4°C, respectively. For most of the early Paleogene, low latitude sub-tropical temperatures remained constant and well within the range of Holocene temperatures (24°ndash;25°C) but by the late Eocene and early Oligocene declined to values in the range of 18° to 22°C. The late Paleogene apparent decline in tropical temperatures, however, might be artificial because of dissolution of near-surface foraminifera tests which biased sediment assemblages toward deeper-dwelling foraminifera. Moreover, according to recent plate reconstructions, it appears that the majority of sites upon which the late Eocene and early Oligocene tropical temperatures were previously established were located either in or near regions likely to have been influenced by upwelling. Global deepwater temperature on average paralleled southern ocean SST for most of the Paleogene. We speculate based on the overall timing and character of marine sea surface temperature variation during the Paleogene that some combination of both higher levels of greenhouse gases and increased heat transport was responsible for the exceptional high-latitude warmth of the early Eocene.

Evolution of Early Cenozoic marine temperatures

The isotopic and cation chemistry of meteoric waters changes in response to the effects of rock—water interaction, uptake of organically derived CO2, and primary mineralogic differences among carbonate terranes. Moreover, variations in the dominance of these factors produce diverse chemical conditions within the meteoric systems which allow the sub- environments of vadose-phreatic, mixed-water, and spelean diagenesis to be distinguished. Therefore, geochemical patterns within the meteoric water system are examined to provide criteria for recognition of these subenvironments of meteoric diagenesis in ancient carbonate sequences.

Geochemical Patterns of Meteoric Diagenetic Systems and Their Application to Studies of Paleokarst

Detailed investigations of high latitude sequences recently collected by the Ocean Drilling Program (ODP) indicate that periods of rapid climate change often culminated in brief transient climates, with more extreme conditions than subsequent long term climates. Two examples of such events have been identified in the Paleogene; the first in latest Paleocene time in the middle of a warming trend that began several million years earlier: the second in earliest Oligocene time near the end of a Middle Eocene to Late Oligocene global cooling trend. Superimposed on the earlier event was a sudden and extreme warming of both high latitude sea surface and deep ocean waters. Imbedded in the latter transition was an abrupt decline in high latitude temperatures and the brief appearance of a full size continental ice-sheet on Antarctica. In both cases the climate extremes were not stable, lasting for less than a few hundred thousand years, indicating a temporary or transient climate state. Geochemical and sedimentological evidence suggest that both Paleogene climate events were accompanied by reorganizations in ocean circulation, and major perturbations in marine productivity and the global carbon cycle. The Paleocene-Eocene thermal maximum was marked by reduced oceanic turnover and decreases in global delta 13C and in marine productivity, while the Early Oligocene glacial maximum was accompanied by intensification of deep ocean circulation and elevated delta 13C and productivity. It has been suggested that sudden changes in climate and/or ocean circulation might occur as a result of gradual forcing as certain physical thresholds are exceeded. We investigate the possibility that sudden reorganizations in ocean and/or atmosphere circulation during these abrupt transitions generated short-term positive feedbacks that briefly sustained these transient climatic states.

Abrupt climate change and transient climates during the Paleogene: a marine perspective.

Controls on the stable isotope composition of seasonal growth bands in aragonitic fresh-water bivalves (unionidae)

We report a quantitative analysis of regional differences in the the oxygen isotope composition of river water and precipitation across the USA because data are now available to undertake a more geographically and temporally extensive analysis than was formerly possible. Maps of modern, mean annual δ 18 O values for both precipitation (δ 18 O PPT ) and river water (δ 18 O RIV ) across the 48 contiguous states of the USA have been generated using latitude and elevation as the primary predictors of stable isotope composition while also incorporating regional and local deviations based on available isotopic data. The difference between these two maps was calculated to determine regions where δ 18 O RIV is significantly offset from local δ 18 O PPT . Additional maps depicting seasonal and extreme values for δ 18 O RIV and δ 18 O PPT were also constructed. This exercise confirms the presence of regions characterized by differences in δ 18 O RIV and δ 18 O PPT and specifically identifies the magnitude and regional extent of these offsets. In particular, the Great Plains has δ 18 O RIV values that are more positive than precipitation, while much of the western USA is characterized by significantly lower δ 18 O RIV values in comparison with local δ 18 O PPT . The most salient feature that emerged from this comparison is the 'catchment effect' for the rivers. Because river water is largely derived from precipitation that fell upstream of the sample locality (i.e. at higher elevations) δ 18 O RIV values are often lower than local δ 18 O PPT values, particularly in catchments with high-elevation gradients. Seasonal patterns in the isotopic data substantiate the generally accepted notion that amplitudes of δ 18 O variation are greatly dampened in river water relative to those of local precipitation.

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Kyger C. Lohmann

Papers

Evolution of Early Cenozoic marine temperatures

Geochemical Patterns of Meteoric Diagenetic Systems and Their Application to Studies of Paleokarst

Abrupt climate change and transient climates during the Paleogene: a marine perspective.

Controls on the stable isotope composition of seasonal growth bands in aragonitic fresh-water bivalves (unionidae)

Spatial distribution and seasonal variation in 18O/16O of modern precipitation and river water across the conterminous USA