Response of Late Ordovician paleoceanography to changes in sea level, continental drift, and atmospheric pCO2: potential causes for long-term cooling and glaciation

We performed sensitivity experiments using an ocean general circulation model at two stages of the Late Ordovician (Caradoc, 454 Ma; Ashgill, 446 Ma) under a range of atmospheric pCO₂ values (8–18× PAL; pre-industrial atmospheric level) at high and low sea level.

The model results indicate that the long-term cooling trend during the Late Ordovician can be explained by progressive cooling of the global ocean in response to falling levels of atmospheric pCO₂, sea level change, and paleogeographic change. These results also explain the occurrence of low latitude cool-water carbonates in North America.

In all simulations, a drop in sea level led to a reduction in poleward ocean heat transport. This indicates a possible positive feedback that could have enhanced global cooling in response to sea level drop during the Late Ordovician. Alterations in poleward ocean heat transport linked to changes of atmospheric pCO₂ also indicate that there is a threshold of 10× PAL, above which ocean current change cannot be responsible for glaciation in the Late Ordovician. Continental drift could explain the observed global cooling trend in the Late Ordovician through a combined poleward ocean heat transport feedback and increased ice-albedo effect if atmospheric pCO₂ was low during the entire Late Ordovician.

The model results further indicate that the response of meridional overturning to changes in paleogeography, atmospheric pCO₂, and sea level is stronger than the response of surface circulation to these perturbations. Because the overturning circulation is so strong, meridional overturning was the dominant mechanism for described changes in heat transport in the Late Ordovician.     

Testing dispersal and cryptic diversity in a widely distributed groundwater amphipod (Niphargus rhenorhodanensis)

Theories about colonization and evolution in groundwater have assumed that the fragmented structure of groundwater strongly limits dispersal. The high number of endemic and allopatric species in groundwater supports this hypothesis, but the occurrence of widely distributed groundwater taxa calls into question its universality. These widely distributed taxa might also be sets of cryptic species because extreme conditions of life in groundwater promote cryptic diversity by inducing convergent morphological evolution. Niphargus rhenorhodanensis is a widely distributed and ubiquitous groundwater amphipod which supposedly colonized the Alps after Quaternary glaciations. We tested the dispersal and the cryptic species hypotheses within this species using a phylogeographic approach based on two mitochondrial genes (COI and 16S) and a nuclear gene (28S). Results support the view that poor dispersal is a main evolutionary factor in groundwater. All genes independently supported the existence of numerous cryptic and mostly allopatric units within N. rhenorhodanensis, indicating that its apparently wide distribution range is an artefact generated by cryptic diversity. We reject the hypothesis of a recent and global colonization of the Alps and argue that some N. rhenorhodanensis lineages probably survived glaciations near or within the Alps.     

论南京直立人化石的年代

根据南京直立人洞穴钙板方解石的ＴＩＭＳ测年,南京直立人化石的年代应早于距今５０万年。南京猿人洞穴堆积所含哺乳动物化石和孢粉分析,表明含人类化石的堆积形成于冷期;哺乳动物化石的比较分析和聚类分析认为含南京直立人化石层可对比于北京猿人洞穴堆积的第６层和深海岩芯氧同位素记录的１４阶段,从而进一步支持南京直立人化石时代早于５０万年的Ｔ＾ＩＭＳ测年结果。     

Evidence for historical introgression along a contact zone between two species of char (Pisces: Salmonidae) in northwestern North America

Phylogeographic analyses can yield valuable insights into the geographic and historical contexts of contact and hybridization between taxa. Two species of char (Salmonidae), Dolly Varden (Salvelinus malma) and bull trout (S. confluentus) have largely parapatric distributions in watersheds of northwestern North America. They are, however, sympatric in several localities and hybridization and some introgression occurs across a broad area of contact. We conducted a comparative phylogenetic analysis of Dolly Varden and bull trout to gain a historical perspective of hybridization between these species and to test for footprints of historical introgression. We resolved two major Dolly Varden mitochondrial DNA (mtDNA) clades (with 1.4-2.2% sequence divergence between haplotypes) that had different geographical distributions. Clade N is distributed across most of the range of Dolly Varden, from southern British Columbia through to the Kuril Islands in Asia. Clade S had a much more limited distribution, from Washington state, at the southern limit of the Dolly Varden range, to the middle of Vancouver Island. The distribution and inferred ages of the mtDNA clades suggested that Dolly Varden survived the Wisconsinan glaciation in a previously unsuspected refuge south of the ice sheet, and that Dolly Varden and bull trout were probably in continuous contact over most of the last 100,000 years. When bull trout were included in the phylogenetic analysis, however, the mtDNA of neither species was monophyletic: Clade S Dolly Varden clustered within the bull trout mtDNA clade. This pattern was discordant with two nuclear phylogenies produced (growth hormone 2 and rRNA internal transcribed sequence 1), in which Dolly Varden and bull trout were reciprocally monophyletic. This discordance between mtDNA- and nDNA-based phylogenies indicates that historical introgression of bull trout mtDNA into Dolly Varden occurred. Percent sequence divergence within these introgressed Dolly Varden (clade S) was 0.2-0.6%, implying that the introgression occurred prior to the most recent glaciation. Our analysis and other evidence of contact between divergent lineages in northwestern North America strongly suggests that the area may be the site of previously unsuspected suture zones of aquatic biotas.     

冲绳海槽末次冰期以来黑潮流游移在沉积硅藻中的记录

文中描述冲绳海槽中段2个柱样沉积物中优势硅藻种相对含量,热带远洋种相对含量和硅藻组合的分布特征;讨论地层的划分和对比、冲绳海槽末次冰期鼎盛期的古黑潮流、全新世黑潮流游移等科学问题。结果表明,冲绳海槽末次冰期鼎盛期的古黑潮流在现今1000m等深线之间流过,当时的黑潮流宽度应小于50km;16ka—11kaB．P．随着海面上升,黑潮流越过西侧1000m等深线,向东海陆架偏移;全新世黑潮流分别在大约7ka—8kaB．P．1ka—4kaB．P．出现过两次黑潮流宽度变窄、强度变弱的游移。     

Genetic differentiation of an endangered capercaillie (<Emphasis Type="Italic">Tetrao urogallus</Emphasis>) population at the Southern edge of the species range

The low-latitude limits of species ranges are thought to be particularly important as long-term stores of genetic diversity and hot spots for speciation. The Iberian Peninsula, one of the main glacial refugia in Europe, houses the southern distribution limits of a number of boreal species. The capercaillie is one such species with a range extending northwards to cover most of Europe from Iberia to Scandinavia and East to Siberia. The Cantabrian Range, in North Spain, constitutes the contemporary south-western distribution limit of the species. In contrast to all other populations, which live in pure or mixed coniferous forests, the Cantabrian population is unique in inhabiting pure deciduous forests. We have assessed the existence of genetic differentiation between this and other European populations using microsatellite and mitochondrial DNA (mtDNA) extracted from capercaillie feathers. Samples were collected between 2001 and 2004 across most of the current distribution of the Cantabrian population. Mitochondrial DNA analysis showed that the Cantabrian birds form a distinct clade with respect to all the other European populations analysed, including the Alps, Black Forest, Scandinavia and Russia, which are all members of a discrete clade. Microsatellite DNA from Cantabrian birds reveals the lowest genetic variation within the species in Europe. The existence of birds from both mtDNA clades in the Pyrenees and evidence from microsatellite frequencies for two different groups, points to the existence of a Pyrenean contact zone between European and Cantabrian type birds. The ecological and genetic differences of the Cantabrian capercaillies qualify them as an Evolutionarily Significant Unit and support the idea of the importance of the rear edge for speciation. Implications for capercaillie taxonomy and conservation are discussed.     

Cherty carbonate facies of the Montoya Group, southern New Mexico and western Texas and its regional correlatives: a record of Late Ordovician paleoceanography on southern Laurentia

The Upper Ordovician Montoya Group in southern New Mexico and westernmost Texas records predominantly subtidal deposition on a gently dipping carbonate ramp that was subsequently nearly entirely dolomitized. The medial unit of the Montoya Group, the Aleman Formation is unique because it contains abundant chert (10–70% by volume). The chert occurs as: (1) thin continuous beds of sponge spicules within mudstone or calcisiltite; (2) discontinuous, lenses or nodules within skeletal wackestones and packstones; or (3) as a replacement of skeletal grains and burrows. Coeval skeletal grainstones and muddy peritidal facies contain little chert. Phosphate (up to 5 wt.%) occurs within the underlying Upham Formation and the Aleman Formation as replacement of fossils and peloids. The abundance of chert and phosphate in these subtidal facies indicates they formed within a region of strong upwelling. Regional correlation with Upper Ordovician cherty units along the periphery of southern Laurentia and other low latitude continents suggests that upwelling was widespread and long-lived during the Late Ordovician. The upwelling is interpreted to record vigorous oceanic circulation produced by the onset of glaciation on Gondwana during this period. Late Ordovician relative sea-level curves around the periphery of Laurentia indicate correlative third-order (1–3 my duration) fluctuations that may provide a means for high-resolution global correlations. However, the mechanism(s) that produced these long-term fluctuations are unclear.     

Geochemistry and palaeogeography of upper Ordovician glaciogenic sedimentary rocks in the Table Mountain Group, South Africa

Evidence for a late Ordovician glaciation is best known from northern Africa and locally in South Africa, and western South America. The South African glacial deposits (Pakhuis Formation of the Table Mountain Group) are present in the western part of the Cape Fold Belt. Two thin diamictites are succeeded by a coarsening upward shale-siltstone unit known as the Cedarberg Formation. These glaciogenic rocks are associated with one of the world's greatest accumulations of mature sandstones. The diamictites are under- and overlain by shales and sandstones that have undergone extensive alteration, presumably as a result of chemical weathering. Some diamictite samples have low values for a Chemical Index of Alteration (CIA), in keeping with their inferred glacial origin. Immediately above the glaciogenic diamictites, black shales, comprising the basal Soom Member of the Cedarberg Formation preserve an unusual soft bodied fauna and are enriched in Mo, U and other trace elements. The unusual chemical composition of these shales is attributed to starved basin conditions and a reducing environment during a rapid sea level rise that accompanied demise of the late Ordovician glaciers. The late Ordovician glacial deposits of western South Africa are widely regarded as the distal deposits of a large continental ice sheet centred on North Africa but they may have been partly derived from piedmont glaciers that descended from highlands formed as a result of late Ordovician orogeny.     

Genetic homogeneity between Korean and Japanese populations of the broad-leaved evergreen tree Machilus thunbergii (Lauraceae): A massive post-glacial immigration through the Korea Strait or something else?

The warm-temperate vegetation of Korea, currently limited to southern coastal areas, shifted during the Last Glacial Maximum (LGM) towards glacial refugia, putatively located in southern Japan. We hypothesized two scenarios of post-glacial re-colonization of warm-temperate species in relation to current levels of genetic diversity within their populations. If extant Korean populations originated from a single source (a single glacial refugium), we expect significantly lower levels of genetic diversity relative to those from Japan due to founder effects. Alternatively, if they were derived from multiple source populations, levels of genetic diversity within Korean populations will not be significantly reduced compared to those of Japanese ones. To test which of these scenarios is more likely, we investigated the patterns of genetic diversity in 14 populations (seven from southern Korea and seven from southern Japan) of the broad-leaved evergreen tree Machilus thunbergii, employing 11 allozyme loci. High levels of genetic variation in M. thunbergii were found both in southern Korea and southern Japan, with a considerable genetic homogeneity not only between the two regions but also between populations within the two regions. These results suggest a pattern of re-colonization after the LGM fitting the second scenario (immigration from multiple refugia), probably through multiple waves and/or with large founder populations.     

Cold-climate rodent indicators for the Late Pleistocene of Central Iberia: New data from the Buena Pinta Cave (Pinilla del Valle,Madrid Region,Spain)

In the Iberian Peninsula, the Late Pleistocene record of small mammal indicators of cold climates is largely restricted to two sets of sites at the eastern and western ends of the Pyrenees. Some assemblages from other sites at the Peninsular centre have, however, recently yielded such taxa. This work describes the remains of three such rodent species from the Buena Pinta Cave, a site in the Sierra de Guadarrama mountains in the Spanish Central System. Excavation campaigns in the cave have taken place every summer since 2003. Thermoluminescence analyses of sediments from levels 2–5 of the site suggest an age corresponding to the middle of the Late Pleistocene, within Marine Isotope Stage 4 or the beginning of Marine Isotope Stage 3. Sieve-washing and picking out of the small fossils contained in the sediments of levels 2–5 yielded several thousand small mammal teeth and other remains, dominated by Microtus arvalis. Smaller numbers of remains belonging to other rodents typical of cold climates were also identified, such as Microtus oeconomus, Microtus gregalis and Chionomys nivalis. Thus, the small mammal record of the Buena Pinta Cave shows that rodent indicators of cold climates reached the centre of the Iberian Peninsula during the mid-Late Pleistocene, i.e., well before the Last Glacial Maximum. These findings represent one of the southernmost Pleistocene records for M. oeconomus in Europe, and the most southerly for M. gregalis.