William F. Ruddiman

Onset of Asian desertification by 22 Myr ago inferred from loess deposits in China

The initial desertification in the Asian interior is thought to be one of the most prominent climate changes in the Northern Hemisphere during the Cenozoic era. But the dating of this transition is uncertain, partly because desert sediments are usually scattered, discontinuous and difficult to date. Here we report nearly continuous aeolian deposits covering the interval from 22 to 6.2 million years ago, on the basis of palaeomagnetic measurements and fossil evidence. A total of 231 visually definable aeolian layers occur as brownish loesses interbedded with reddish soils. This new evidence indicates that large source areas of aeolian dust and energetic winter monsoon winds to transport the material must have existed in the interior of Asia by the early Miocene epoch, at least 14 million years earlier than previously thought. Regional tectonic changes and ongoing global cooling are probable causes of these changes in aridity and circulation in Asia.

The North Atlantic Ocean during the last deglaciation

Abstract The last deglacial warming of the high-latitude North Atlantic Ocean (40–65°) occurred in three discrete steps: in the southeast and central regions at 13 000 B.P.; in the central and northern sectors at 10 000 B.P.; and in the western (Labrador Sea) sector between 9000 and 6000 B.P. This regionally time-transgressive sequence was punctuated by a major cooling and polar front readvance from 11 000 to 1000 B.P.; this briefly returned most of the high-latitude North Atlantic to almost full-glacial temperatures. Carbonate productivity levels reached minimum values from 16 000 B.P. to 13 000 B.P. and then gradually rose, reaching maximum values in the Holocene at about 6000 B.P. We interpret these changes in carbonate productivity as indicating a major influx of products of glacial wastage (meltwater and icebergs) from 16 000 to 13 000 B.P., with considerably reduced influx after 13 000 B.P. Combined with evidence for meltwater influx to the Gulf of Mexico via the Mississippi River, this suggests that the bulk of volumetric deglaciation in the Northern Hemisphere occurred considerably earlier than the main areal retreat of ice-sheet limits. This implies that the still-extensive ice sheets in the Northern Hemisphere were relatively thin by 13 000 B.P. Because there is no palynologic evidence of unusual atmospheric warming on the southern margins of the ice sheets before 13 000 B.P., we infer that this early phase of rapid ice disintegration occurred largely by iceberg calving and marine downdraw. We also infer that from 16 000 to 13 000 B.P. low winter insolation combined with a low-salinity meltwater layer to form sea ice south to 50°N; after that date, the winter sea-ice cover diminished significantly. We infer from this that the critical flux of winter moisture to the ice sheets was largely cut off from 16 000 to 13 000 B.P. and still significantly suppressed after 13 000 B.P. Moisture starvation thus hastened the disintegration of Northern Hemisphere ice masses. The brief but strong oceanic cooling roughly coincident with the European Younger Dryas (11 000 to 10 000 B.P.) appears to mark a major influx of tabular icebergs from a disintegrating Arctic ice shelf, perhaps enhanced by external forcing at higher frequencies (2500 yr).

Influence of late Cenozoic mountain building on ocean geochemical cycles

In a steady-state ocean, input fluxes of dissolved salts to the sea must be balanced in mass and isotopic value by output fluxes. For the elements strontium, calcium, and carbon, rivers provide the primary input, whereas marine biogenic sedimentation dominates removal. Dissolved fluxes in rivers are related to rates of continental weathering, which in turn are strongly dependent on rates of uplift. The largest dissolved fluxes today arise in the Himalayan and Andean mountain ranges and the Tibetan Plateau. During the past 5 m.y., uplift rates in these areas have increased significantly; this suggests that weathering rates and river fluxes may have increased also. The oceanic records of carbonate sedimentation, level of the calcite compensation depth, and delta/sup 13/C and delta/sup 87/Sr in biogenic sediments are consistent with a global increase in river fluxes since the late Miocene. The cooling of global climate over the past few million years may be linked to a decrease in atmospheric CO/sub 2/ driven by enhanced continental weathering in these tectonically active regions.

Late Quaternary deposition of ice-rafted sand in the subpolar North Atlantic (lat 40° to 65°N)

A major change in the North Atlantic pattern of ice-rafting deposition, during the last interglacial-glacial cycle, occurred approximately 75,000 B.P. Prior to this time, deposition for a period of almost 50,000 yr during isotopic stage 5 was greatest in the northwest near Greenland and Newfoundland. The main glacial pattern was very different; the main depositional axis shifted abruptly to a zonal axis along lat 46° to 50°N, reflecting the passage of ice farther from the pole before reaching water warm enough in which to melt. This pattern remained essentially unchanged for 65,000 yr during the main Wurm glaciation. The peak interglacial depositional pattern can best be explained by analogy with the modern oceanic flow, except for the addition of a concentrated eastward component along lat 50°N. The glacial pattern is also best explained by counterclockwise flow. Laurentide and Greenlandic ice entering the western North Atlantic from the Labrador Sea moved to the east and southeast directly into the glacial depositional maximum. Scandinavian ice dropped part of its bed load near Norway, looped to the southwest into the North Atlantic in a counterclockwise passage south of Iceland, and finally melted along the primary depositional maximum. Total input rates of ice-rafted sediment to the Atlantic and Norwegian Sea increased slightly at 115,000 B.P. (the glacial inception), rose markedly at 75,000 B.P. (the major glacial transition), and continued to rise late in the Wurm toward the late-glacial maximum. North Atlantic ice-rafting deposition is thus positively correlated with ice-sheet size. During Quaternary time, roughly 70% of ice-rafted deposition of continental detritus in the world9s oceans has occurred in the subpolar North Atlantic south of Iceland. During the past 3 m.y., a mass of wet unconsolidated drift estimated at 200,000 km 3 has been moved from the continents to the deep Atlantic by ice-rafting alone. This is equivalent to a layer of drift 16 m thick over all parts of the continents thought to have supplied ice-rafted detritus to the North Atlantic.

Shaping of the Continental Rise by Deep Geostrophic Contour Currents

Geostrophic contour-following bottom currents involved in the deep thermohaline circulation of the world ocean appear to be the principal agents which control the shape of the continental rise and other sediment bodies.

The last interglacial ocean

The final effort of the CLIMAP project was a study of the last interglaciation, a time of minimum ice volume some 122,000 yr ago coincident with the Substage 5e oxygen isotopic minimum. Based on detailed oxygen isotope analyses and biotic census counts in 52 cores across the world ocean, last interglacial sea-surface temperatures (SST) were compared with those today. There are small SST departures in the mid-latitude North Atlantic (warmer) and the Gulf of Mexico (cooler). The eastern boundary currents of the South Atlantic and Pacific oceans are marked by large SST anomalies in individual cores, but their interpretations are precluded by no-analog problems and by discordancies among estimates from different biotic groups. In general, the last interglacial ocean was not significantly different from the modern ocean. The relative sequencing of ice decay versus oceanic warming on the Stage 6/5 oxygen isotopic transition and of ice growth versus oceanic cooling on the Stage 5e/5d transition was also studied. In most of the Southern Hemisphere, the oceanic response marked by the biotic census counts preceded (led) the global ice-volume response marked by the oxygen-isotope signal by several thousand years. The reverse pattern is evident in the North Atlantic Ocean and the Gulf of Mexico, where the oceanic response lagged that of global ice volume by several thousand years. As a result, the very warm temperatures associated with the last interglaciation were regionally diachronous by several thousand years. These regional lead-lag relationships agree with those observed on other transitions and in long-term phase relationships; they cannot be explained simply as artifacts of bioturbational translations of the original signals.

Matuyama 41,000-year cycles: North Atlantic Ocean and northern hemisphere ice sheets

Abstract During the middle Pleistocene, a change occurred in the climatic response of northern hemisphere ice sheets and the high-latitude North Atlantic Ocean to orbitally controlled variations in insolation. The dominant periodicity during the late Pliocene and early Pleistocene (2.47 Myr B.P. to about 0.735 Myr B.P.) was 41,000 years; thereafter, the dominance shifted to a period near 100,000 years. Although orbital forcing is the primary cause of each of these rhythmic responses, it does not explain the mid-Pleistocene shift of power. Changes in the solid boundary conditions of the ocean-air-ice system, and particularly in mountain elevation, are implicated.

13C Record of benthic foraminifera in the last interglacial ocean: Implications for the carbon cycle and the global deep water circulation

The 13C/12C ratios of Upper Holocene benthic foraminiferal tests (genera Cibicides and Uvigerina) of deep sea cores from the various world ocean basins have been compared with those of the modern total carbon dioxide (TCO2) measured during the GEOSECS program. The δ13C difference between benthic foraminifera and TCO2 is 0.07 ± 0.04‰ for Cibicides and −0.83 ± 0.07‰ for Uvigerina at the 95% confidence level. δ13C analyses of the benthic foraminifera that lived during the last interglaciation (isotopic substage 5e, about 120,000 yr ago) show that the bulk of the TCO2 in the world ocean had a δ13C value 0.15 ± 0.12‰ lower than the modern one at the 95% confidence level, reflecting a depletion, compared to the present value, of the global organic carbon reservoir. Regional differences in δ13C between the various oceanic basins are explained by a pattern of deep water circulation different from the modern one: the Antarctic Bottom Water production was higher than today during the last interglaciation, but the eastward transport in the Circumpolar Deep Water was lower.

Evolution of Atlantic―Pacific δ13C gradients over the last 2.5 m.y.

The evolution of interocean carbon isotopic gradients over the last 2.5 m.y. is examined using high-resolution δ13C records from deep sea cores in the Atlantic and Pacific Oceans. Over much of the Northern Hemisphere ice ages, relative reductions in North Atlantic Deep Water production occur during ice maxima. From 2.5 to 1.5 Ma, glacial reductions in NADW are less than those observed in the late Pleistocene. Glacial suppression of NADW intensified after 1.5 Ma, earlier than the transition to larger ice sheets around 0.7 Ma. At a number of times during the Pleistocene, δ13C values at DSDP Site 607 in the North Atlantic were indistinguishable from eastern equatorial Pacific δ13C values from approximately the same depth (ODP Site 677), indicating significant incursions of low δ13C water into the deep North Atlantic. Atlantic/Pacific δ13C values converge during glaciations between ∼ 1.13-1.05 m.y., 0.83-0.70 m.y., and 0.46-0.43 m.y. This represents a pseudo-periodicity of approximately 300 kyr which cannot easily be ascribed to global ice volume or orbital forcing. This partial decoupling, at low frequencies, of the δ18O and δ13C signals at Site 607 indicates that variations in North Atlantic deep water circulation cannot be viewed simply as a linear response to ice sheet forcing.

Holocene carbon emissions as a result of anthropogenic land cover change

Humans have altered the Earth’s land surface since the Paleolithic mainly by clearing woody vegetation first to improve hunting and gathering opportunities, and later to provide agricultural cropland. In the Holocene, agriculture was established on nearly all continents and led to widespread modification of terrestrial ecosystems. To quantify the role that humans played in the global carbon cycle over the Holocene, we developed a new, annually resolved inventory of anthropogenic land cover change from 8000 years ago to the beginning of large-scale industrialization (ad 1850). This inventory is based on a simple relationship between population and land use observed in several European countries over preindustrial time. Using this data set, and an alternative scenario based on the HYDE 3.1 land use data base, we forced the LPJ dynamic global vegetation model in a series of continuous simulations to evaluate the impacts of humans on terrestrial carbon storage during the preindustrial Holocene. Our model setup allowed us to quantify the importance of land degradation caused by repeated episodes of land use followed by abandonment. By 3 ka BP, cumulative carbon emissions caused by anthropogenic land cover change in our new scenario ranged between 84 and 102 Pg, translating to c. 7 ppm of atmospheric CO2. By ad 1850, emissions were 325–357 Pg in the new scenario, in contrast to 137–189 Pg when driven by HYDE. Regional events that resulted in local emissions or uptake of carbon were often balanced by contrasting patterns in other parts of the world. While we cannot close the carbon budget in the current study, simulated cumulative anthropogenic emissions over the preindustrial Holocene are consistent with the ice core record of atmospheric δ13CO2 and support the hypothesis that anthropogenic activities led to the stabilization of atmospheric CO2 concentrations at a level that made the world substantially warmer than it otherwise would be.