Transgressions: rethinking Beringian glaciation

The purpose of this investigation is to encourage a fresh look at Pleistocene Beringia. Heretofore, flooding of Bering Strait has been cited as the only barrier to migration, with marine sea transgressions being a “sea gate” that closed off migration during glacial interstadials and interglaciations. However, the possibility exists that glacial advances were also barriers, with marine ice transgressions being an “ice gate” that closed off migration during glacial stadials and glacial maxima. This possibility proceeds from the Marine Ice Transgression Hypothesis (MITH), which states that marine ice sheets form on the broad Arctic continental shelf of Northern Hemisphere continents when sea ice thickens, grounds and domes in shallow water, and then transgresses landward as continental ice sheets and seaward as floating ice shelves (Hughes, 1987). Landward transgression is onto coastal lowlands. During Pleistocene glaciations, a marine ice sheeet extending from Spitsbergen to Greenland may have transgressed the circumpolar continental landmass at its lowest and narrowest gap, central Beringia, and calved into the Pacific Ocean.

Four models of Beringian glaciation are presented, based on the distinction between marine glaciation and highland glaciation. Central Beringia was glaciated only in highlands in the traditional model (Hopkins et al., 1982), was also glaciated by a self-sustaining ice shelf floating over the deep ocean basins of the Bering Sea in the model by Grosswald and Vozovik (1984), was glaciated by a marine ice sheet that covered highlands, the continental shelf, and supplied the ice shelf in a model for maximum Pleistocene glaciation, and was glaciated by a marine ice sheet in the Chukchi Sea that merged with highland glaciers, transgressed the continental shelf of the western Bering Sea, and calved into the southern Bering Sea along the edge of the continental shelf in a model for the last glaciation. Field tests are suggested to assess the viability of these four models. The first model is already established for highland glaciation in Alaska, but less established in Siberia. The last model should be the easiest to evaluate for marine glaciation. The last model limits human migration across the Beringian land bridge to brief intervals between stadials and interstadials of the last glaciation cycle, when both the ice gate and the sea gate were opened to human migration. This model can influence the sea change now underway among Quaternary scientists studying peopling of the Americas, based on the archaeological, linguistic and ethnic diversity among native American populations.     

The “Dirty Ice” of the McMurdo Ice Shelf: Analogues for biological oases during the Cryogenian

The Cryogenian (~717–636 Ma) is characterized by widespread glaciation and dramatic fluctuations in biogeochemical cycling during the Sturtian and Marinoan glaciations. The Snowball Earth hypothesis posits that during this period, ice‐covered oceans of more or less global extent shut down or greatly diminished photosynthesis in the marine realm. However, rather than suffering a catastrophic loss of biodiversity, fossil evidence suggests that major eukaryotic lineages survived and, indeed, the end of the Cryogenian marks the onset of a rapid diversification of eukaryotic life. Persistence of diverse life forms through glaciations is thought to have occurred in supraglacial refugia although the exact nature and full extent of such habitats remain uncertain. We present further evidence for the diversity and characteristics of supraglacial ecosystems on the McMurdo Ice Shelf in Antarctica and suggest that refugia analogous to “dirty ice,” that is debris‐covered ice shelf ecosystems, potentially provided nutrient‐rich and long‐lasting biological Cryogenian oases. We also discuss how features of the McMurdo Ice Shelf indicate that mechanisms exist whereby material can be exchanged between the shallow sea floor and the surfaces of ice shelves along continental margins, providing vectors whereby ice shelf ecosystems can nourish underlying seafloor communities and vice versa.     

Transberingian dispersal of cestodes in mammals

During Pleistocene glaciations, eustatic lowering of sea-level exposed the continental shelf between northeastern Eurasia and northwestern North America. That land in combination with unglaciated areas on the adjacent continents formed a vast region open to the west but bounded on the east by continental ice. Organisms from Eurasia spread into the unglaciated Beringian refugium, which was biotically an eastward extension of the Palaearctic. With rising sea-levels following glacial periods, the Bering Strait was formed and organisms of Eurasian origin were left within the nearctic sector of Beringia. As the continental ice disappeared, plants and animals spread eastward and southward from Beringia, while organisms from beyond the southern margins of the ice extended their ranges northward. The significance of Beringia is discussed with reference to the dispersal of host-specific cestodes in mammals that attained holarctic status during the late Pleistocene.     

Phylogeography of a Patagonian lizard and frog: Congruent signature of southern glacial refuges

Pleistocene glaciations produced significant increases in continental ice cover in polar and mid‐latitude temperate areas, sea‐level declines and shifts and reshuffling of biomes, all of which promote either isolation, coalescence or fragmentation in the distribution of land biota. If populations of several taxa have been co‐distributed for a prolonged time, and if the periods between perturbation or vicariance processes have been more or less stable, it is expected that divergence patterns of closely related and ecologically similar species will be congruent because of their similar biological and demographic characteristics. Based on this premise, we analysed the phylogeographic structure (cytochrome b) of Liolaemus pictus and Batrachyla leptopus, two widely co‐distributed lizard and frog species, respectively, in the Chiloé Archipelago of southern Chile, to decipher their genetic structure in response to a common climatic and environmental history. Haplotype network analysis and Bayesian inference suggest an evolutionary pattern of genetic diversity for the two species that is consistent with the Quaternary glacial history of southern Chile, and suggests a complex phylogeographic history in the Liolaemus and Batrachyla species. High‐divergence levels among haplotypes in some island populations of the archipelago also suggest genetic connectivity between putative refuges from Chiloé Island and the mainland along the exposed continental shelf during sea level minima associated with the most recent Quaternary glaciations. Our results are consistent with our hypothesis that two species have responded to parallel historical events in which the historical process during the last glacial maximum (approximately 41°S) has been sufficient to influence their phylogeographic structure.     

Faunal evidence for a late quaternary trans‐Antarctic seaway

Collapse of the West Antarctic Ice Sheet (WAIS) would raise global sea level by ～3.3–5 m. Ice‐sheet models and geological data suggest at least one collapse has happened during the last 1.1 Ma, and some scenarios of future climate change predict a collapse within the next two centuries. A complete WAIS collapse would open shallow seaways across West Antarctica, potentially enabling exchange of animals between West Antarctic seas. We investigated biological evidence for past connectivity between different regions of Antarctica by comparing the composition of modern bryozoan assemblages from the continental margin around Antarctica. Surprisingly, we found most similarity between two areas which are not currently connected – the shelves of the Weddell Sea (WS) and Ross Sea (RS). We evaluated three hypotheses to explain this and conclude that bryozoans most likely dispersed through a trans‐Antarctic seaway that opened in response to a WAIS collapse and connected the WS and RS shelves. These bryozoans must have survived glaciations(s) during subsequent ice ages in refuges, whereas they were wiped out in most other regions of the Antarctic shelf. After the last glacial period, bryozoan assemblages could freely disperse between many of the regions we examined (e.g. Antarctic Peninsula and South Shetland Islands), which has allowed recolonization of areas in which bryozoans had been eradicated during the last ice age. For the bryozoans on the WS and RS shelves to be more similar than those which are in close proximity means the trans‐Antarctic seaway may have been as late as the last few interglacials. Current rates of warming are exceptional compared with the near past glacial cycles so our study, the strongest faunal evidence of WAIS collapse during the recent geological past, thus supports predictions of a near future WAIS collapse (with considerable global sea level implications) and resultant future major faunal exchanges.     

The Permo-Carboniferous Dwyka Formation of Southern Africa: Deposition by a predominantly subpolar marine ice sheet

The 800-m thick glacigene Dwyka Formation was deposited along the northern margin of the Permo-Carboniferous Dwyka Basin which covered an area of approximately 2 × 10⁶ km² in southwestern Gondwana. The palaeogeographic setting, geochemical data of the mudrocks and diamictites, and the palaeontology indicate marine conditions during sedimentation. Ice lobes from spreading centres in the north, east and south coalesced in the basin to form an extensive ice cover from the Westphalian to the late Sakmarian.

Lodgement, rain-out and subaqueous debris-flow diamictons, subaqueous and subglacial melt-water sands, suspended mud, and turbidity current sands and silts accumulated in the Dwyka Basin. Sedimentation started on the continental shelf during a grounded ice sheet stage (predominantly lodgement processes), then a floating ice stage (predominantly debris rain-out), and finally an ice sheet disintegration stage (debris rain-out, sand fall-out and suspension settling of mud). The palaeogeographic setting, presence of marine conditions in the basin and the scale of glaciation indicate deposition from a predominantly mid-latitudinal marine ice sheet. The overall characteristics of the glacial sequence are neither typical of a polar nor a temperate setting and for such ancient glaciations a subpolar setting with the presence of unstable ice shelves is suggested.     

Molecular evidence for glacial refugia of mountain plants in the European Alps

Many mountain ranges have been strongly glaciated during the Quaternary ice ages, and the locations of glacial refugia of mountain plants have been debated for a long time. A series of detailed molecular studies, investigating intraspecific genetic variation of mountain plants in the European Alps, now allows for a first synopsis. A comparison of the phylogeographic patterns with geological and palaeoenvironmental data demonstrates that glacial refugia were located along the southwestern, southern, eastern and northern border of the Alps. Additional glacial refugia were present in central Alpine areas, where high-elevation plants survived the last glaciation on ice-free mountain tops. The observed intraspecific phylogeographies suggest general patterns of glacial survival, which conform to well-known centres of Alpine species diversity and endemism. This implies that evolutionary or biogeographic processes induced by climatic fluctuations act on gene and species diversity in a similar way.     

Persistent genetic signatures of historic climatic events in an Antarctic octopus

Repeated cycles of glaciation have had major impacts on the distribution of genetic diversity of the Antarctic marine fauna. During glacial periods, ice cover limited the amount of benthic habitat on the continental shelf. Conversely, more habitat and possibly altered seaways were available during interglacials when the ice receded and the sea level was higher. We used microsatellites and partial sequences of the mitochondrial cytochrome oxidase 1 gene to examine genetic structure in the direct‐developing, endemic Southern Ocean octopod Pareledone turqueti sampled from a broad range of areas that circumvent Antarctica. We find that, unusually for a species with poor dispersal potential, P. turqueti has a circumpolar distribution and is also found off the islands of South Georgia and Shag Rocks. The overriding pattern of spatial genetic structure can be explained by hydrographic (with ocean currents both facilitating and hindering gene flow) and bathymetric features. The Antarctic Peninsula region displays a complex population structure, consistent with its varied topographic and oceanographic influences. Genetic similarities between the Ross and Weddell Seas, however, are interpreted as a persistent historic genetic signature of connectivity during the hypothesized Pleistocene West Antarctic Ice Sheet collapses. A calibrated molecular clock indicates two major lineages within P. turqueti, a continental lineage and a sub‐Antarctic lineage, that diverged in the mid‐Pliocene with no subsequent gene flow. Both lineages survived subsequent major glacial cycles. Our data are indicative of potential refugia at Shag Rocks and South Georgia and also around the Antarctic continent within the Ross Sea, Weddell Sea and off Adélie Land. The mean age of mtDNA diversity within these main continental lineages coincides with Pleistocene glacial cycles.     

Pattern of glacial—interglacial vegetation in subtropical Chile

Pleistocene vegetation in the Mediterranean climate region of subtropical Chile during >40,000 yr of the last glaciation stands in contrast with vegetation of the past 10,000 yr of the present interglaciation. During the last ice age, open woodland of southern beech (Nothofagus dombeyi and N. obliqua types) and Andean podocarp (Prumnopitys andina) mixed with grasses and composites, was established on unglaciated, low‐lying terrain now occupied by broad sclerophyllous plant communities. This marked vegetation change during a glacial‐interglacial cycle, inferred from pollen contained in a core from Laguna de Tagua Tagua (34°30'S), suggests a pattern applicable to the vegetational setting during repeated cycles throughout the Quaternary.

Ice‐age vegetation can be accounted for by year‐long domination of the polar maritime air mass at lower, middle latitudes, unlike modern interglacial type plant formations, which are regulated by seasonal interaction between subtropical and polar maritime air. Glacial‐interglacial shifting of air mass centers, as a cause for the changing distribution of species and plant communities, appears geared to events in the atmosphere/ocean system.     

Animal survival strategies in Neoproterozoic ice worlds

The timing of the first appearance of animals is of crucial importance for understanding the evolution of life on Earth. Although the fossil record places the earliest metazoans at 572–602 Ma, molecular clock studies suggest a far earlier origination, as far back as ~850 Ma. The difference in these dates would place the rise of animal life into a time period punctuated by multiple colossal, potentially global, glacial events. Although the two schools of thought debate the limitations of each other's methods, little time has been dedicated to how animal life might have survived if it did arise before or during these global glacial periods. The history of recent polar biota shows that organisms have found ways of persisting on and around the ice of the Antarctic continent throughout the Last Glacial Maximum (33–14 Ka), with some endemic species present before the breakup of Gondwana (180–23 Ma). Here we discuss the survival strategies and habitats of modern polar marine organisms in environments analogous to those that could have existed during Neoproterozoic glaciations. We discuss how, despite the apparent harshness of many ice covered, sub-zero, Antarctic marine habitats, animal life thrives on, in and under the ice. Ice dominated systems and processes make some local environments more habitable through water circulation, oxygenation, terrigenous nutrient input and novel habitats. We consider how the physical conditions of Neoproterozoic glaciations would likely have dramatically impacted conditions for potential life in the shallows and erased any possible fossil evidence from the continental shelves. The recent glacial cycle has driven the evolution of Antarctica's unique fauna by acting as a “diversity pump,” and the same could be true for the late Proterozoic and the evolution of animal life on Earth, and the existence of life elsewhere in the universe on icy worlds or moons.     

Late pleistocene paleoclimates of the South Pacific based on statistical analysis of planktonic foraminifers

Statistical analysis applied to foraminiferal data from 78 South Pacific core tops enables the derivation of a transfer function that relates sea surface temperature to foraminiferal assemblages. Application of this transfer function to eight cores from the southern part of the East Pacific Rise yields estimates of the sea surface temperatures of the last glacial maximum, as well as the paleotemperature record of the past 150,000 years. Comparison of the last glacial temperature estimates with the recent sea surface temperature shows that the greatest change between glacial and present conditions (about 5°C) occurs in a climatically sensitive area near 50°S. Stratigraphic correlation of two cores from this area suggests that the last glacial started in this area with rapid cooling, and that the climate stayed generally cold until the end of the glacial. Similar general shape of the climatic record is found in the high latitudes of the North Atlantic as well as in the ice sheets of Greenland and Antarctica. In contrast to the similarity in the shape of these high-latitude records, they differ distinctly from the foraminiferal oxygen isotope record of several deep-sea cores which indicates a general gradual increase of ice volume from the beginning of the last glacial to the maximum glaciation which occurred about 18,000 years B.P.In the study area the rate of sediment accumulation during the last glacial is about two to three times less than in the last interglacial. There is no indication of increased carbonate solution during the glacial, and it is suggested that the change in the accumulation rate results from a reduction in the supply of calcareous shells to the sediment. It seems that with cooling, the environment becomes less favorable to organisms producing calcium carbonate tests, and therefore carbonate production decreases during the glacial.     

Climate control on ancestral population dynamics: insight from Patagonian fish phylogeography

Changes in lake and stream habitats during the growth and retreat of Pleistocene glaciers repeatedly altered the spatial distributions and population sizes of the aquatic fauna of the southern Andes. Here, we use variation in mtDNA control region sequences to infer the temporal dynamics of two species of southern Andean fish during the past few million years. At least five important climate events were associated with major demographic changes: (i) the widespread glaciations of the mid-Pliocene (c. 3.5 Ma); (ii) the largest Patagonian glaciation (1.1 Ma); (iii) the coldest Pleistocene glaciation as indicated by stacked marine delta(18)O (c. 0.7 Ma); (iv) the last southern Patagonian glaciation to reach the Atlantic coast (180 ka); and (v) the last glacial maximum (LGM, 23-25,000 years ago). The colder-water inhabitant, Galaxias platei, underwent a strong bottleneck during the LGM and its haplotype diversity coalesces c. 0.7 Ma. In contrast, the more warm-adapted and widely distributed Percichthys trucha showed continuous growth through the last two glacial cycles but went through an important bottleneck c. 180,000 years ago, at which time populations east of the Andes may have been eliminated. Haplotype diversity of the most divergent P. trucha populations, found west of the Andes, coalesces c. 3.2 Ma. The demographic timelines obtained for the two species thus illustrate the continent-wide response of aquatic life in Patagonia to climate change during the Pleistocene, but also show how differing ecological traits and distributions led to distinctive responses.     

Climate-induced changes to the ancestral population size of two Patagonian galaxiids: the influence of glacial cycling

Patagonia is one of the few areas in the Southern Hemisphere to have been directly influenced by Quaternary glaciers. In this study, we evaluate the influence that Quaternary glacial ice had on the genetic diversity of two congeneric fish species, the diadromous Galaxias maculatus and the nondiadromous Galaxias platei, using multilocus estimates of effective population size through time. Mid-Quaternary glaciations had far-reaching consequences for both species. Galaxias maculatus and G. platei each experienced severe genetic bottlenecks during the period when Patagonia ice sheet advance reached its maximum positions c. 1.1-0.6 Ma. Concordant drops in effective size during this time suggest that range sizes were under similar constraints. It is therefore unlikely that coastal (brackish/marine) environments served as a significant refuge for G. maculatus during glacial periods. An earlier onset of population declines for G. platei suggests that this species was vulnerable to modest glacial advances. Declines in effective sizes were continuous for both species and lasted into the late-Pleistocene. However, G. maculatus exhibited a strong population recovery during the late-Quaternary (c. 400,000 bp). Unusually long and warm interglacials associated with the late-Quaternary may have helped to facilitate a strong population rebound in this primarily coastal species.     

The biogeography of aquatic macrophytes in North America since the Last Glacial Maximum

Aim To document the post‐glacial migration of the major aquatic macrophytes of North America. Location North America north of Mexico. Methods Aquatic macrophyte pollen were extracted from the North American Pollen Database. The modern pollen distribution was mapped and related to the climate to document the geographical and climatic constraints on these taxa. The fossil pollen were mapped at 2‐ka intervals for the past 21 ka. Results Numerous genera were present in ice‐free Alaska during the Last Glacial Maximum, and south of the Laurentide Ice Sheet in the southeast. Those taxa with the widest modern climatic ranges migrated rapidly into ice‐marginal areas, first in the west and then in the east of North America. Subsequent changes in the range and abundance were smaller. Main conclusions There were four migration routes of aquatic macrophytes during the late‐glacial and post‐glacial periods: a southward migration from Alaska between 14–13 and ka, a northern migration in the west at the same time into the ice‐free Cordilleran region, and movements east and west of Appalachia as early as 19 ka for some taxa into the lower Mississippi and into the upper Mississippi and Great Lakes by 11 ka. As the Laurentide ice sheet wasted, aquatic taxa with the broadest contemporary temperature tolerances rapidly occupied ice‐marginal environments.     

生物冰期避难所与冰期后的重新扩散

冰期（尤其是更新世冰期）对当今生物的空间分布格局和遗传结构产生了深远影响,研究生物冰期避难所对于了解不同生物区系间的关系、物种形成以及生物多样性保护具有十分重要的意义。生物冰期避难所的确定员韧是根据特有种的分布、化石、抱粉等证据推测而来,分子遗传标记为冰期生物避难所以及冰期后重新扩散路线的研究提供了有力的工具。本文以北美和欧洲为例,介绍了分子遗传标记在生物避难所以及冰期后再扩散路线研究中的应用和一些结论。我国存在许多东西走向的大山,减缓了冰期时低温对生物的影响,为许多生物提供了避难场所,但我国有关研究开展得很少。员后,对我国该领域提出了应优先开展的研究方向。     

Impact of ice ages on circumpolar molecular diversity: insights from an ecological key species

We address the impact of the ice age cycles on intraspecific cpDNA diversity, for the first time on the full circumboreal-circumarctic scale. The bird-dispersed bog bilberry (or arctic blueberry, Vaccinium uliginosum) is a key component of northern ecosystems and is here used to assess diversity in previously glaciated vs. unglaciated areas and the importance of Beringia as a refugium and source for interglacial expansion. Eighteen chloroplast DNA haplotypes were observed in and among 122 populations, grouping into three main lineages which probably diverged before, and thus were affected more or less independently by, all major glaciations. The boreal 'Amphi-Atlantic lineage' included one haplotype occurring throughout northern Europe and one occurring in eastern North America, suggesting expansion from at least two bottlenecked, glacial refugium populations. The boreal 'Beringian lineage' included seven haplotypes restricted to Beringia and the Pacific coast of USA. The 'Arctic-Alpine lineage' included nine haplotypes, one of them fully circumpolar. This lineage was unexpectedly diverse, also in previously glaciated areas, suggesting that it thrived on the vast tundras during the ice ages and recolonized deglaciated terrain over long distances. Its largest area of persistence during glaciations was probably situated in the north, stretching from Beringia and far into Eurasia, and it probably also survived the last glaciation in southern mountain ranges. Although Beringia apparently was important for the initial divergence and expansion of V. uliginosum as well as for continuous survival of both the Beringian and Arctic-Alpine lineages during all ice ages, this region played a minor role as a source for later interglacial expansions.     

Bacteria beneath the West Antarctic Ice Sheet

Subglacial environments, particularly those that lie beneath polar ice sheets, are beginning to be recognized as an important part of Earth's biosphere. However, except for indirect indications of microbial assemblages in subglacial Lake Vostok, Antarctica, no sub-ice sheet environments have been shown to support microbial ecosystems. Here we report 16S rRNA gene and isolate diversity in sediments collected from beneath the Kamb Ice Stream, West Antarctic Ice Sheet and stored for 15 months at 4°C. This is the first report of microbes in samples from the sediment environment beneath the Antarctic Ice Sheet. The cells were abundant (∼10⁷ cells g⁻¹) but displayed low diversity (only five phylotypes), likely as a result of enrichment during storage. Isolates were cold tolerant and the 16S rRNA gene diversity was a simplified version of that found in subglacial alpine and Arctic sediments and water. Although in situ cell abundance and the extent of wet sediments beneath the Antarctic ice sheet can only be roughly extrapolated on the basis of this sample, it is clear that the subglacial ecosystem contains a significant and previously unrecognized pool of microbial cells and associated organic carbon that could potentially have significant implications for global geochemical processes.     

Evolutionary dynamics at high latitudes: speciation and extinction in polar marine faunas

Ecologists have long been fascinated by the flora and fauna of extreme environments. Physiological studies have revealed the extent to which lifestyle is constrained by low temperature but there is as yet no consensus on why the diversity of polar assemblages is so much lower than many tropical assemblages. The evolution of marine faunas at high latitudes has been influenced strongly by oceanic cooling during the Cenozoic and the associated onset of continental glaciations. Glaciation eradicated many shallow-water habitats, especially in the Southern Hemisphere, and the cooling has led to widespread extinction in some groups. While environmental conditions at glacial maxima would have been very different from those existing today, fossil evidence indicates that some lineages extend back well into the Cenozoic. Oscillations of the ice-sheet on Milankovitch frequencies will have periodically eradicated and exposed continental shelf habitat, and a full understanding of evolutionary dynamics at high latitude requires better knowledge of the links between the faunas of the shelf, slope and deep-sea. Molecular techniques to produce phylogenies, coupled with further palaeontological work to root these phylogenies in time, will be essential to further progress.     

Multiple glacial refugia in the North American Arctic: inference from phylogeography of the collared lemming (Dicrostonyx groenlandicus)

Cryptic northern refugia beyond the ice limit of the Pleistocene glaciations may have had significant influence on the current pattern of biodiversity in Arctic regions. In order to evaluate whether northern glacial refugia existed in the Canadian Arctic, we examined mitochondrial DNA phylogeography in the northernmost species of rodents, the collared lemming (Dicrostonyx groenlandicus) sampled across its range of distribution in the North American Arctic and Greenland. The division of the collared lemming into the Canadian Arctic and eastern Beringia phylogroups does not support postglacial colonization of the North American Arctic from a single eastern Beringia refugium. Rather, the phylogeographical structure and sparse fossil records indicate that, during the last glaciation, some biologically significant refugia and important sources of postglacial colonization were located to the northwest of the main ice sheet in the Canadian Arctic.     

末次冰期南海巽他陆架上的植被和气候——17962孔孢粉记录

提供了南海大陆坡上17962孔深海沉积物的孢粉记录。该孔长8m,底部^14C年龄为30000aBP,跨越了深海氧同位素3期,末次初期,冰消期和全新世几个地质历史阶段。孢偻记录显示末次冰期低海面时南海南部出露的陆架上了低地雨林。同时,在邻近的岛屿上山地雨林扩张,表明气候比较在凉,但是没有变干的迹象,冰消期植被和气候都经历了快速变暖的变凉的波动。全新世时,红树林和低地雨林的扩及孢粉沉积率的明显降低均表明气候变暖,海面上升,大陆架被淹没。