Cenozoic Climate Change Research Articles

Cenozoic ice sheet history from East Antarctic Wilkes Land continental margin sediments

The long-term history of glaciation along the East Antarctic Wilkes Land margin, from the time of the first arrival of the ice sheet to the margin, through the significant periods of Cenozoic climate change is inferred using an integrated geophysical and geological approach. We postulate that the first arrival of the ice sheet to the Wilkes Land margin resulted in the development of a large unconformity (WL-U3) between 33.42 and 30 Ma during the early Oligocene cooling climate trend. Above WL-U3, substantial margin progradation takes place with early glacial strata (e.g., outwash deposits) deposited as low-angle prograding foresets by temperate glaciers. The change in geometry of the prograding wedge across unconformity WL-U8 is interpreted to represent the transition, at the end of the middle Miocene “climatic optimum” (14–10 Ma), from a subpolar regime with dynamic ice sheets (i.e., ice sheets come and go) to a regime with persistent but oscillatory ice sheets. The steep foresets above WL-U8 likely consist of ice proximal sediments (i.e., water-lain till and debris flows) deposited when grounded ice-sheets extended into the shelf. On the continental rise, shelf progradation above WL-U3 results in an up-section increase in the energy of the depositional environment (i.e., seismic facies indicative of more proximal turbidite and of bottom contour current deposition from the deposition of the lower WL-S5 sequence to WL-S7). Maximum rates of sediment delivery to the rise occur during the development of sequences WL-S6 and WL-S7, which we infer to be of middle Miocene age. During deposition of the two uppermost sequences, WL-S8 and WL-S9, there is a marked decrease in the sediment supply to the lower continental rise and a shift in the depocenters to more proximal areas of the margin. We believe WL-S8 records sedimentation during the final transition from a dynamic to a persistent but oscillatory ice sheet in this margin (14–10 Ma). Sequence WL-S9 forms under a polar regime during the Pliocene–Pleistocene, when most sediment delivered to the margin is trapped in the outer shelf and slope-forming steep prograding wedges. During the warmer but still polar, Holocene, biogenic sediment accumulates quickly in deep inner-shelf basins during the high-stand intervals. These sediments contain an ultrahigh resolution (annual to millennial) record of climate variability. Validation of our inferences about the nature and timing of Wilkes Land glacial sequences can be achieved by deep sampling (i.e., using IODP-type techniques). The most complete record of the long-term history of glaciation in this margin can be obtained by sampling both (1) the shelf, which contains the direct (presence or no presence of ice) but low-resolution record of glaciation, and (2) the rise, which contains the distal (cold vs. warm) but more complete record of glaciation. The Wilkes Land margin is the only known Antarctic margin where the presumed “onset” of glaciation unconformity (WL-U3) can be traced from shelf to the abyssal plain, allowing links between the proximal and the distal records of glaciation to be established. Additionally, the eastern segment of the Wilkes Land margin may be more sensitive to climate change because the East Antarctic Ice Sheet (EAIS) is grounded below sea level. Therefore, the Wilkes Land margin is not only an ideal location to obtain the long-term EAIS history but also to obtain the shorter-term record of ice sheet fluctuations at times that the East Antarctic Ice Sheet is thought to have been more stable (after 15 Ma-recent).

Climate-independent paleoaltimetry using stomatal density in fossil leaves as a proxy for CO2 partial pressure

Research Article| December 01, 2004 Climate-independent paleoaltimetry using stomatal density in fossil leaves as a proxy for CO2 partial pressure Jennifer C. McElwain Jennifer C. McElwain 1Department of Geology, Field Museum of Natural History, 1400 South Lake Shore Drive, Chicago, Illinois 60605, USA Search for other works by this author on: GSW Google Scholar Geology (2004) 32 (12): 1017–1020. https://doi.org/10.1130/G20915.1 Article history received: 21 Jun 2004 rev-recd: 01 Sep 2004 accepted: 04 Sep 2004 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jennifer C. McElwain; Climate-independent paleoaltimetry using stomatal density in fossil leaves as a proxy for CO2 partial pressure. Geology 2004;; 32 (12): 1017–1020. doi: https://doi.org/10.1130/G20915.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Existing methods for determining paleoelevation are primarily limited by (1) large errors (±450 m), (2) a reliance on incorrect assumptions that lapse rates in terrestrial temperature decrease with altitude in a globally predictable manner, and/or (3) are inherently climate dependent. Here I present a novel paleoelevation tool, based on a predictable, globally conserved decrease in CO2 partial pressure (pCO2) with altitude, as indicated by increased stomatal frequency of plant leaves. The approach was validated using historical populations of black oak (Quercus kelloggii). These analyses demonstrate highly significant inverse relationships between stomatal frequency and pCO2 (r2 > 0.73), independent of ecological or local climatic variability. As such, this is the first paleobotanical method to be globally applicable and independent of long-term Cenozoic climate change. Further, tests on modern leaves of known elevations indicate that species-specific application to the fossil record of Q. kelloggii (= Q. pseudolyrata) will yield paleoelevation estimates within average errors of ∼±300 m, representing a significant improvement in accuracy over the majority of existing methods. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Cenozoic climate change as a possible cause for the rise of the Andes.

Causal links between the rise of a large mountain range and climate have often been considered to work in one direction, with significant uplift provoking climate change. Here we propose a mechanism by which Cenozoic climate change could have caused the rise of the Andes. Based on considerations of the force balance in the South American lithosphere, we suggest that the height of, and tectonics in, the Andes are strongly controlled both by shear stresses along the plate interface in the subduction zone and by buoyancy stress contrasts between the trench and highlands, and shear stresses in the subduction zone depend on the amount of subducted sediments. We propose that the dynamics of subduction and mountain-building in this region are controlled by the processes of erosion and sediment deposition, and ultimately climate. In central South America, climate-controlled sediment starvation would then cause high shear stress, focusing the plate boundary stresses that support the high Andes.

Provenance patterns in a neotectonic basin: Pliocene and Quaternary sediment supply to the South Caspian

AbstractThe South Caspian Basin has accumulated a sedimentary succession ∼20 km thick. Roughly half of this was deposited in the last 5.5 Ma, mainly in the largely lower Pliocene, fluvio‐lacustrine Productive Series, which is also the principal hydrocarbon reservoir succession in the basin. Heavy mineral data identify different sediment sources for both Productive Series sandstones and modern river sands. Lesser Caucasus sediment was supplied by the Palaeo‐Kura into the western part of the South Caspian Basin. Productive Series strata in the north of the basin were supplied by the Palaeo‐Volga, and represent a mixture of sediment from the Greater Caucasus and Russian Platform/Urals. Greater Caucasus sand input to the Palaeo‐Volga increased at the start of deposition of the Pereriva Suite, which is an important reservoir subunit of the Productive Series. We interpret this provenance shift as indicating enhanced uplift and exhumation of the Greater Caucasus within the Pliocene, during regional re‐organization of the Arabia–Eurasia collision, although late Cenozoic climate changes may have played a role.

Application of benthic foraminiferal Mg/Ca ratios to questions of Cenozoic climate change

We investigate the evolution of Cenozoic climate and ice volume as evidenced by the oxygen isotopic composition of seawater (δ 18O sw) derived from benthic foraminiferal Mg/Ca ratios to constrain the temperature effect contained in foraminiferal δ 18O values. We have constructed two benthic foraminiferal Mg/Ca records from intermediate water depth sites (Ocean Drilling Program sites 757 and 689 from the subtropical Indian Ocean and the Weddell Sea, respectively). Together with the previously published composite record of Lear et al. [Science 287 (2002) 269–272] and the Neogene record from the Southern Ocean of Billups and Schrag [Paleoceanography 17 (2002) 10.1029/2000PA000567], we obtain three, almost complete representations of the δ 18O sw for the past 52 Myr. We discuss the sensitivity of early Cenozoic Mg/Ca-derived paleotemperatures (and hence the δ 18O sw) to assumptions about seawater Mg/Ca ratios. We find that during the middle Eocene (∼49–40 Ma), modern seawater ratios yield Mg/Ca-derived temperatures that are in good agreement with the oxygen isotope paleothermometer assuming ice-free conditions. Intermediate waters cooled during the middle Eocene reaching minimum temperatures by 40 Ma. The corresponding δ 18O sw reconstructions support ice growth on Antarctica beginning by at least 40 Ma. At the Eocene/Oligocene boundary, Mg/Ca ratios (and hence temperatures) from Weddell Sea site 689 display a well-defined maximum. We caution against a paleoclimatic significance of this result and put forth that the partitioning coefficient of Mg in benthic foraminifera may be sensitive to factors other than temperature. Throughout the remainder of the Cenozoic, the temporal variability among δ 18O sw records is similar and similar to longer-term trends in the benthic foraminiferal δ 18O record. An exception occurs during the Pliocene when δ 18O sw minima in two of the three records suggest reductions in global ice volume that are not apparent in foraminiferal δ 18O records, which provides a new perspective to the ongoing debate about the stability of the Antarctic ice sheet. Maximum δ 18O sw values recorded during the Pleistocene at Southern Ocean site 747 agree well with values derived from the geochemistry of pore waters [Schrag et al., Science 272 (1996) 1930–1932] further highlighting the value of the new Mg/Ca calibrations of Martin et al. [Earth Planet. Sci. Lett. 198 (2002) 193–209] and Lear et al. [Geochim. Cosmochim. Acta 66 (2002) 3375–3387] applied in this study. We conclude that the application of foraminiferal Mg/Ca ratios allows a refined view of Cenozoic ice volume history despite uncertainties related to the geochemical cycling of Mg and Ca on long time scales.

Northwestward decline of magnetic susceptibility for the red clay deposit in the Chinese Loess Plateau

The loess‐paleosol‐red clay sequence in the Chinese Loess Plateau has attracted attention for its potential in recording late Cenozoic climatic changes over northern China, although the details of the climatic implications of the proxies (for example, magnetic susceptibility) and the reconstructed summer monsoon changes remain unclear. Here we report on a new section from Baishui, in Gansu province and compare it with the Lingtai and Jingchuan sections. By correlating the magnetic susceptibility from these sections, we show that there is a clear systematic decrease of the magnetic susceptibility in the red clay deposits from the southeast (Lingtai section) to the northwest (Baishui section). This result suggests that the climatic parameters (probably precipitation) had a decreasing northwestward gradient during the red clay deposition. This climatic pattern is similar to the present condition, implying that the influence of the East Asian summer monsoon on the Chinese Loess Plateau has persisted from at least 6 Ma.

Teaching about relict, no-analog landscapes

Abstract Only a few very young landforms are the result of currently operating geomorphic processes. Because the time scale for landscape evolution is much longer than the time scale for late Cenozoic climate changes, almost all landscapes are palimpsests, written over repeatedly by various combinations of climate-determined processes. Relict glacial and periglacial landforms are widely identified in mid-latitude regions that have been traditionally described as having been shaped by the “normal” processes of fluvial erosion. Less confidently, deeply weathered regolith and associated relict landforms in the middle and high latitudes are attributed to early Tertiary warmth. However, assemblages of geomorphic processes specific to certain climatic regions, like faunal and floral assemblages, cannot be translated across latitude, so in spite of the many books about the geomorphology of specific modern climate regions, there are few sources that discuss former warm high-latitude, or cold low-latitude, low-altitude geomorphic processes that have no modern analogs. Students and teachers alike who attempt to interpret landforms by extrapolating modern climatic conditions to other latitudinal zones will find their outlook broadened, and they become better prepared to consider the geomorphic impacts of global climate change.

Rates of erosion and their implications for exhumation

AbstractAt time-scales of 102to 105years, erosion by rivers, landslides and glaciers can exceed 5 mm/y. Sustained denudation at these rates is sufficient to account for many of the rapid rates of unloading or cooling that are revealed by geobarometric or thermochronologic studies. Because feedbacks exist among many surface processes, determinations of erosion rates on a few geomorphic processes can be adequate to estimate mean rates across an entire landscape. Rates of fluvial and glacial incision exert a dominant control on landscape lowering, because these rates set the local base level and modulate the flux from adjacent hillslopes. When rates of deformation are sufficiently rapid and sustained, a collisional orogen approaches a dynamic equilibrium or topographic steady state. Due to variations in erosion rates as a function of Late Cenozoic climate changes, such a steady state should be defined at time-scales longer than one climate cycle. During dynamic equilibrium, the geomorphic system displays a predictable configuration of interacting rivers, hillslopes and glaciated process zones. A dynamic equilibrium appears to prevail near Nanga Parbat, Pakistan, and the Southern Alps of New Zealand, where spatial variations in geomorphic erosion rates mimic variations in bedrock cooling rates at time-scales of 106y.

Fluvial incision and tectonic uplift across the Himalayas of central Nepal

The pattern of fluvial incision across the Himalayas of central Nepal is estimated from the distribution of Holocene and Pleistocene terraces and from the geometry of modern channels along major rivers draining across the range. The terraces provide good constraints on incision rates across the Himalayan frontal folds (Sub‐Himalaya or Siwaliks Hills) where rivers are forced to cut down into rising anticlines and have abandoned numerous strath terraces. Farther north and upstream, in the Lesser Himalaya, prominent fill terraces were deposited, probably during the late Pleistocene, and were subsequently incised. The amount of bedrock incision beneath the fill deposits is generally small, suggesting a slow rate of fluvial incision in the Lesser Himalaya. The terrace record is lost in the high range where the rivers are cutting steep gorges. To complement the terrace study, fluvial incision was also estimated from the modern channel geometries using an estimate of the shear stress exerted by the flowing water at the bottom of the channel as a proxy for river incision rate. This approach allows quantification of the effect of variations in channel slope, width, and discharge on the incision rate of a river; the determination of incision rates requires an additional lithological calibration. The two approaches are shown to yield consistent results when applied to the same reach or if incision profiles along nearby parallel reaches are compared. In the Sub‐Himalaya, river incision is rapid, with values up to 10–15 mm/yr. It does not exceed a few millimeters per year in the Lesser Himalaya, and rises abruptly at the front of the high range to reach values of ∼4–8 mm/yr within a 50‐km‐wide zone that coincides with the position of the highest Himalayan peaks. Sediment yield derived from the measurement of suspended load in Himalayan rivers suggests that fluvial incision drives hillslope denudation of the landscape at the scale of the whole range. The observed pattern of erosion is found to closely mimic uplift as predicted by a mechanical model taking into account erosion and slip along the flat‐ramp‐flat geometry of the Main Himalayan Thrust fault. The morphology of the range reflects a dynamic equilibrium between present‐day tectonics and surface processes. The sharp relief together with the high uplift rates in the Higher Himalaya reflects thrusting over the midcrustal ramp rather than the isostatic response to reincision of the Tibetan Plateau driven by late Cenozoic climate change, or late Miocene reactivation of the Main Central Thrust.

Insect herbivory, plant defense, and early Cenozoic climate change.

Insect damage on fossil leaves from the Central Rocky Mountains, United States, documents the response of herbivores to changing regional climates and vegetation during the late Paleocene (humid, warm temperate to subtropical, predominantly deciduous), early Eocene (humid subtropical, mixed deciduous and evergreen), and middle Eocene (seasonally dry, subtropical, mixed deciduous and thick-leaved evergreen). During all three time periods, greater herbivory occurred on taxa considered to have short rather than long leaf life spans, consistent with studies in living forests that demonstrate the insect resistance of long-lived, thick leaves. Variance in herbivory frequency and diversity was highest during the middle Eocene, indicating the increased representation of two distinct herbivory syndromes: one for taxa with deciduous, palatable foliage, and the other for hosts with evergreen, thick-textured, small leaves characterized by elevated insect resistance. Leaf galling, which is negatively correlated with moisture today, apparently increased during the middle Eocene, whereas leaf mining decreased.

Quantifying the role of geographic change in Cenozoic ocean heat transport using uncoupled atmosphere and ocean models

A series of five Cenozoic atmospheric general circulation model (AGCM) experiments has been performed using a new set of paleogeographic reconstructions for 55, 40, 33, 20 and 14 Ma. The five continental reconstructions incorporate the tectonic evolution of early Eocene to middle Miocene continental positions and topography. With all other model boundary conditions and forcings held constant, the series of AGCM experiments captures a <1°C decrease in annual mean temperature through the Paleogene and early Neogene. Regional and seasonal differences among the five experiments are small in magnitude, but are consistent with the imposed paleogeographic changes. From the AGCM experiments alone, it might be concluded that changes in continental positions had little impact on Cenozoic climate change. However, ocean configuration changes between 55 and 14 Ma, especially gateway openings and closures, are expected to produce significant changes in ocean thermohaline circulation, a system that cannot be simulated by the slab ocean model component of an AGCM. The nature of changes in ocean heat transport and thermohaline circulation arising from the evolution of early Eocene through middle Miocene ocean basin configurations is examined in a series of five global, three-dimensional ocean model experiments forced by output from the AGCM. The ocean model suggests that paleogeographic change throughout the Cenozoic has caused changes in the interhemispheric partitioning of heat transport and that the modern shape of the ocean heat transport curve has evolved in response to ocean basin evolution. The prediction of very low ocean heat transport in the Northern Hemisphere of the early and middle Eocene suggests a much more important role for atmospheric heat transport in the temperate polar climates of the Eocene than is generally acknowledged. Results suggest that Southern Hemisphere ocean heat transport decreased throughout the interval 55–14 Ma. The results also show that, in the absence of reliable coupled models for paleoclimate studies, full three-dimensional ocean models must be used in parallel with slab ocean AGCMs if we wish to understand the true effects of paleogeographic change on climate and the true nature of past ocean heat transport.

Distribution of calcareous dinoflagellates from the Maastrichtian to early Miocene of DSDP Site 357 (Rio Grande Rise, western South Atlantic Ocean)

The evolution of calcareous dinoflagellate communities has been investigated for the latest Cretaceous to earliest Neogene interval of the mid-latitude South Atlantic. In doing so, the response of calcareous dinoflagellates to Cenozoic climatic change has been addressed for the first time. Trends in species composition and distribution patterns of wall types indicate significant changes which correlate with major palaeoenvironmental modifications. A first major shift concerning the relative abundance of species and wall types occurred across the Cretaceous–Tertiary boundary. The associations remained stable during the entire Paleocene and Eocene. Only in the late Eocene did a dramatic decrease in temperature cause a slight diversification. A second major shift in the abundance patterns occurred across the Eocene–Oligocene boundary. The early Miocene warming is possibly reflected in the distinct increase in relative abundance of one species. The assemblages of calcareous dinoflagellates evidently react to major climatic changes during the Cenozoic. These poorly investigated organisms may thus provide an important contribution to the understanding of earth's palaeoclimatic evolution.

Comparison of cosmogenic radionuclide production and geomagnetic field intensity over the last 200 000 years

The production rate of cosmogenic radionuclides such as 10Be or 14C is known to vary as a function of the geomagnetic field intensity. It should, therefore, be possible to extract a record of palaeofield intensity from the deposition record of these radionuclides in marine or terrestrial sediments and ice cores. Field intensity variations, however, are not the only factor that has influenced the cosmogenic radionuclide records. In the case of 14C, variations of the global carbon cycle, caused by reorganization of the ocean circulation patterns from the last glacial to the present interglacial, are superimposed. 10Be is not affected by these variations because it is not part of the carbon cycle, but its deposition rates in marine sediments vary as a function of lateral sediment redistribution and boundary scavenging intensity. A global stacked record of 10Be deposition rates, corrected for sediment redistribution by normalizing to 230Thex, was shown to remove most of the disturbances, and provides a record of 10Be production rate variations over the last 200 000 years, which translates into geomagnetic field intensity variations. This dataset is compared with palaeofield intensities reconstructed from marine sediments by palaeomagnetic methods, from variations in atmospheric 14C/12C derived from independent calibrations of 14C ages, such as U/Th dating and tree ring chronology, and from 36Cl and 10Be fluxes in polar ice cores. Potential influences of the Earth’s orbital parameters and insufficient correction for orbitally triggered climate variations on the palaeointensity reconstructions are assessed. It is argued that the palaeointensity records derived from marine sediments are not significantly affected by these factors.

Cenozoic climatic change in Antarctica recorded by volcanic activity and landscape evolution

A long-lasting Cenozoic record (∼50 m.y.) of alkaline igneous rocks characterizes northern Victoria Land, Antarctica. Landscape analysis allows distinction between older volcanic and intrusive rocks with well-developed alpine topography sculptured by wet-based glaciers, and younger volcanic cones lacking these features. Many K-Ar and Rb-Sr dates testify that the erosion that formed the alpine landscape ceased between ca. 8.2 and 7.5 Ma. Since ca. 8 Ma, morphological evolution has been driven by cold-based glaciers; warm-based glaciers were no longer active. That this change affected a 300-km-long coastal area suggests a persistent cause of global significance. Glacier dynamics control landscape shaping as a function of ice thickness and temperature, which are driven by climatic conditions. In this view, a significant climatic change occurred in northern Victoria Land between 8.2 and 7.5 Ma. The perfectly preserved serrated alpine ridges, with their delicate spires, testify that no warm-based ice sheets overrode the region after 7.5 Ma and that polar conditions held sway in the Pliocene and Pleistocene Epochs.

40Ar/39Ar Dating of African Manganese Pisolites: Implication for Cenozoic Climatic Changes

Chemical and mechanical weathering-derived processes are among the most efficient factors in modifying the Earth surface. Weathering gradients are strongly dependent upon temperature, rainfall and tectonic stability over geological time. In this way, the upper part of tropical and sub-tropical cratonic lithosphere has been transformed into thick supergene lateritic mantles with frequent metal accumulations. In West Africa, the lateritic weathering episodes have developed from early Tertiary successive crusts that have controlled the local geomorphology. These indurated formations are evidence of the successive weathering and erosion cycles. In this way, laterites that constitutes the rain forest substratum, may trace the palaeoclimate evolution. One of the best opportunities to date weathering events is to apply the 4~ method on isotopically closed supergene K-bearing minerals such as K-Mn oxides from the hollandite group (AxBsO16, nH20 , where A represents K +, Ba 2+, pb2+or Na + and B represents Mn 3§ Mn 4+, Fe3+,). Initiated by Chukhrov et aI. (1966), using the K-Ar dating method, direct dating of potassium-rich manganese oxides was widely facilitated by the recent development of the 4~ laser probe method as shown by the studies of Vasconcelos et al. (1992 and 1994) and Ruffet et al. (1996). Cryptomelane is well represented in numerous lateritic manganese weathering profiles over the tropical belt. Then radiochronological study may help to understand the genesis of the Mn-lateritic systems and to correlate them to the development of Fe-laterites. This work presents analytical performances of the 4~ argon-ion laser probe method in 'dating' K-rich samples of the pisolitic manganese formation and the underlying manganese crust of the Tambao deposit (northern Burkina Faso).

Paleophysiographic and paleoenvironmental histories in northern Latin America - Possible impact on modes of speciation

Although land surfaces between central and southern Mexico and northern South America are presently continuous, the area encompasses three zones that have had different physiographic and environmental histories. In central and southern Mexico highlands were present in the Late Cretaceous, and significant physiographic relief and habitat diversity have existed since the Late Tertiary. Continuous land connections existed to the north throughout the Cenozoic, and to the south after about 3.5 Ma, and these connections provided source areas for the immigration and emigration of temperate and tropical elements, as well as introductions throughout the Cenozoic by long-distance dispersal. The region experienced the global climatic cooling of the Late Cenozoic, which was augmented by coastal upwelling with closure of the Panama land bridge. In contrast, central America has never had the extensive high elevations or habitat diversity of central to southern Mexico; the region consisted of islands and peninsulas for most of the Cenozoic with limited access to source areas both to the north and south until about 3.5 Ma; and the impact of Late Cenozoic climatic change was muted, in part, because of the lower latitude, low physiographic relief, and the insular environment. The Amazon Basin of northern South America has had low physiographic relief throughout the Cenozoic, but it is surrounded by the northern Andes that had attained substantial relief and habitat diversity by the Miocene. Biotas in the Amazon Basin experienced some effects of Late Cenozoic climatic change, and these were augmented by fluctuations in the water table resulting from sea-level changes caused by the expansion and contraction of polar glaciers. The effect was the periodic fragmentation of the lowland biota, accompanied by expansion of drier-habitat communities during intervals of cool glacial climates and lowered water tables. This was followed by periods of warmer temperatures, higher water tables, coalescence of rain forest communities, and restriction of drier-habitat species to the bordering uplands they occupy at present. The pace of these changes produced a dynamism that was not experienced, at least to the same degree, by communities to the north. These differences in physiographic and climatic histories likely had an effect on the nature of speciation mechanisms operating in the three regions through a reordering of the components of the speciation process. Some residue of this reordering may still reside in large speciose genera distributed through northern Latin America in the form of morphological (leaf epidermal cell size, pollen aperture number, pollen sterility) and cytological (polyploidy) characters. If so, recognition of the separate histories may be useful in explaining features in modern taxa that are correlated with geography.

Tibet uplift and erosion

The 5-km-high Tibetan plateau is an outstanding topographic feature on the Earth today. Its horziontal extent, elevation, and location cause significant effects on modern atmospheric circulation and climate, so the history of uplift of the surface of Tibet is linked to Cenozoic climate changes, at local, regional, and perhaps global scales. Geological and geophysical studies of the plateau are contributing data on the present and past deformation of the Tibetan lithosphere that has formed the plateau, primarily during the Cenozoic. The principal of isostasy then can be used to estimate the elevation history of the surface for a given deformation history. Different parts of Tibet probably had different uplift histories, but presently available data are not sufficient for distinguishing many contrasts. In one scenario, Cretaceous and Early Cenozoic north-south distributed shortening of Tibetan crust and mantle lithosphere probably caused significant uplift of the surface relative to sea level to perhaps half of the present elevation by the early Miocene. Thinning of the high-density mantle portion of the lithosphere during the Miocene may then have allowed the thick Tibetan crust to rise close to its present elevation (perhaps higher) before ∼8 Ma. Since then, slow east-west extension of Tibet probably reduced the crustal thickness slightly and may have caused the elevation of the plateau to decrease during the late Cenozoic.Erosion of Tibet, unlike narrow mountain belts, has been unable to match the uplift of a broad plateau. Orographic precipitation and efficient river networks concentrate erosion on the edges, while the interior is protected from significant erosion despite its lofty elevation. The southern edge of the plateau, the Himalaya, has suffered a minimum of 25 km of denudation since the Miocene, while central Tibet shows little or no sign of major erosion since that time. The Gangdese arc in southern Tibet was rapidly eroded during the mid-Miocene when >4 km of rock apparently were removed from the surface, as shown by mineral cooling ages. This pulse of erosion was probably caused by a combination of local thrust-system movement and changes in base level and precipitation due to relative elevation changes between the Gangdese and the Himalaya to the south. The modern long-wavelength flatness of Tibet is unlikely to have been caused by erosion and indicates viscous flow at some level of the lithosphere has been acting to level the plateau surface.

Deposition of Organic Matter in the Norwegian-Greenland Sea during the Past 2.7 Million Years

Variations in the amount and composition of sedimentary organic matter in glacial and interglacial deposits of the last 2.7 myr correlate with late Cenozoic climatic and oceanographic changes in the Norwegian-Greenland Sea. These variations are predominantly caused by the changing supply of terrestrial and reworked organic matter. The highest amounts of terrestrial organic particles (macerals) and of reworked coal clasts in glacial and early deglacial diamictons are closely related to glacial erosion of Mesozoic strata that crop out along the Scandinavian Shelf. The first occurrence of coal clasts at 2.53 myr demonstrates an initial advance of continental ice margins to these source areas. The establishment of anoxic conditions at the sea floor during diamicton deposition probably reflects an increased vertical flux of labile organic matter due to lithogenic adsorbtion followed by an almost complete mineralization at the water/sediment interface. The content of marine organic matter was continuously low over the past 2.7 myr except for past interglacial highstands (i.e., isotopic event 5.5.1), suggesting persistent diagenetic degradation of the labile organic fraction. The marine organic matter exclusively consists of residual dinoflagellate cysts and fragments of them.

Oceanic crust production and climate during the last 100 Myr

ABSTRACTIn order to evaluate the possible influence of oceanic crust production on climatic changes during the past 100Myr variations in total oceanic crust for this period including production at mid‐ocean ridges, oceanic plateaus, and back‐arc basins were calculated using the most recent and accurate time‐scales. The rates presented here differ from those of Larson (1991a, b) on Cenozoic fluctuations and show that (1) maximum production values occurred during the Cenomanian, Palaeocene, and late Oligocene‐early Miocene and (2) minimum values occurred in Campanian‐Maastrichtian, late Eocene, and middle Miocene. Significantly, variations of oceanic crust production correspond with variations in the δ18O of deep‐water benthic foraminifera: maximum values of oceanic crust production correspond with minimum values of δ18O, and minimum production values with maximum values of δ18O. This latter synchronism suggests that changes in land‐sea relationships and atmospheric CO2 related to major fluctuations in oceanic crust production were the main cause of mid‐Cretaceous warming and Late Cretaceous cooling, and of climatic quasi‐cycles having a periodicity of 33–38 million years over the last 100 Myr. This is the first report showing variations of ocean crust production synchronized with the Cenozoic climate changes.

The Vendian record of Sr and C isotopic variations in seawater: Implications for tectonics and paleoclimate

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.