Paleoecology

The desert and arid steppes of Mongolia and northern China were geographically central to the spread of pastoralism and the rise of pastoralist states, but research on the organizational strategies of pre-pastoralist hunter-gatherers and the spread of herding has been extremely limited. Until recently, catalogues of sites collected by Westerners in the 1920s and 1930s comprised the body of English-language publications on Gobi Desert prehistory. This article introduces a wealth of new site-specific and interpretive data, drawing on English-language sources as well as Russian-and Mongolian-language publications to create a synthesis for the prehistory of the Gobi Desert from the end of the Last Glacial Maximum to the adoption of herding. Special emphasis is placed on the relationship between a major shift in desert ecosystems, comparable to the 'greening of the Sahara', the establishment of an oasis-based broad-spectrum foraging strategy, and progressive desertification and deforestation after 2000 BC. We conclude that an oasis-based adaptation was contemporaneous with the expansion of forests and wetlands and persisted throughout the early stages of herding. A major decline in these economies occurs after 1000 BC, in conjunction with continuing trends towards heightened aridity and major societal changes across Northeast Asia. The persistent coexistence of Bronze Age burials and microblade-based habitation sites around oases, as well as similarities in material culture, suggest that these groups overlapped geographically or were the same entity.

Товчлол: Монгол болон Умард Хятадын говь, говь хээрийн бүс нутаг нь газарзүйн хувьд мал аж ахуй болон түүнд суурилсан нүүдэлчдийн төрт улсууд бүрэлдэн бий болоход нөлөөлсөн чухал бүс нутаг юм. Гэвч эдгээрийн суурь болсон эртний анчин, түүвэрлэгч нарын нийгмийн бүтэц, мал аж ахуйн тархалтын талаарх судалгаа туйлын бага хийгджээ. Саяхныг хүртэл 1920-1930-аад оны үед барууны судлаачдын говьд хийсэн түүхийн өмнөх үеийн судалгааны бүтээлүүд хожмын англи хэлээр бичигдсэн цөөн хэдэн эрдэм шинжилгээний ажлуудын  гол хэрэглэгдэхүүн болсоор иржээ. Харин энэхүү өгүүлэл нь сүүлийн жилүүдэд тухайн бүс нутагт хийгдсэн монгол, англи, орос хэл дээрх археологийн судалгааны эх хэрэглэгдэхүүнд тулгуурлан мөстлөгийн сүүл үеэс мал аж ахуйн үүсэл хүртэлх үеийн говь нутгийн түүхийн өмнөх үеийн асуудлуудыг нэгтгэн дүгнэсэн юм.

Мөн зохиогчид өнөөгийн “Сахаарын цөлийн ногоорол” хэмээх үзэгдэлтэй жишиж болохуйц эртний экосистемийн томоохон өөрчлөлт, хүмүүсийн “Баянбүрд” түшиглэсэн амьдралын хэв маяг болон НТӨ 2000 оноос хурдацтайгаар явагдаж эхэлсэн цөлжилт гэх зэрэгт онцгой анхаарал хандуулан тэдгээрийн хоорондын уялдаа холбоог судалсан байна.

Эртний хүмүүсийн баянбүрд түшиглэсэн амьдралын хэв маяг нь ойн болон чийглэг хөрст бүс тэлэн өргөжсөнтэй нэг цаг хугацаанд явагдаж мал аж ахуйн хөгжлийн эхний үе шат хүртэл үргэлжилсэн хэмээн зохиогчид үзжээ. Харин энэхүү амьдралын хэв маяг нь Зүүн хойд Азийг хамарсан хэт хуурайшилт болон тухайн үеийн нийгмийн бүтцэд томоохон өөрчлөлт гарч эхэлсэн зэрэг шалтгааны улмаас НТӨ 1000 оноос хойш аажмаар хумигдаж эхэлсэн байна. Зохиогчид говь хээрийн бүсэд өргөн тархсан баянбүрдүүдийн ойр орчимд хүрлийн үеийн булш оршуулга болон ижил хэлбэрийн бичил ялтас бүхий бууц суурингууд олноор тохиолдож байгаа нь эдгээр дурсгалуудыг үлдээсэн бүлгүүд нэг дор эсвэл хоорондоо холгүй амьдарч байсныг харуулахаас гадна нэг угсааны хүмүүс байх магадлалтайг цохон тэмдэглэжээ.

—A macrofloral assemblage dominated by elements of the Euramerican dryland biome is described from the Brazil Formation in Clay County, Indiana (Illinois Basin). Fossils were recovered from a thin heterolithic unit between a shallow-marine bed and the paleosol beneath the Minshall Coal, a Middle Pennsylvanian succession deposited near the Atokan-Desmoinesian and Bolsovian-Asturian boundaries. Sedimentological indicators imply accumulation under a seasonal climate, including interbedded siltstone and sandstone deposited during flashfloods, intraclasts eroded from local sources, and charcoal produced by wildfires. The macrofloral assemblage is consistent with a dryland setting, being dominated by large, coriaceous gymnosperm leaves with mesic to xeric traits, including Cordaites spp. indet., Lesleya sp. indet., and Taeniopteris sp. cf. T. multinervia. Sphenopsids and ferns typical of the wetland biome are rare. In contrast , the microfloral assemblage is dominated by fern spores, with lesser lycopsid spores and cordaitalean pollen. The succession indicates that the dryland biome predominated during late regression, prior to the onset of perhumid conditions that resulted in peat accumulation at late lowstand. However, the abundance of palynomorphs from wetland vegetation implies gradual fragmentation of the prevailing dryland flora and replacement by the wetland biome in the transition to glacial maximum. The taphonomic and paleobiogeographic context confirms that floras adapted to seasonal moisture deficit periodically dispersed into tropical lowlands, rather than being transported from 'extrabasinal' or 'upland' environments. The precocious occurrence of Taeniopteris, more typical of Late Pennsylvanian and Permian floras, may be the earliest record of the fossil-genus, and exemplifies the association of derived plant taxa with dryland habitats. The predominance of broad-leaved gymnosperms with mesic to xeric characters suggests that dryland communities contained more slow-growing and long-lived plants than contemporaneous wetland floras.

Trace fossils in estuarine deposits of different ages have been compared to evaluate colonization history of brackish-water ecosystems and to calibrate trace-fossil,brackish-water models with respect to geologic time. This comparative analysis reveals that, although the colonization of marginal-marine, brackish-water environments was a long-term process that spanned most of the Phanerozoic, this process of invasion of fully marine organisms into restricted, marginal-marine habitats did not occur at a constant rate. Five major colonization phases can be distinguished. The first phase (Ediacaran–Ordovician) represents a prelude to the major invasion that occurred during the rest of the Paleozoic. While Ediacaran–Cambrian ichnofaunas seem to be restricted to the outermost zones of marginal-marine de- positional systems, Ordovician assemblages show some degree of landward expansion within brackish-water ecosystems. Intensity of bioturbation and ichnodiversity levels was relatively low during this phase. The second phase (Silurian–Carboniferous) is marked by the appearance of more varied morphologic patterns and behavioral strategies, resulting in a slight increase in ichnodiversity. While previous assemblages were arthropod dominated, brackish-water Si- lurian–Carboniferous ichnofaunas include structures produced by bivalves, ophiuroids, and polychaetes. Ichnofaun- as from the third phase (Permian–Triassic) seem to be characterized by the presence of crustacean burrows, reflecting the late Paleozoic crustacean radiation and adaptation of some groups to brackish-water conditions. The fourth phase (Jurassic–Paleogene) is typified by a remarkable increase in ichnodiversity and intensity of bioturbation of estuarine facies. Colonization occurred not only in softgrounds and firmgrounds, but also in hardgrounds and xylic substrates. The fifth phase (Neogene–Recent) records the onset of modern brackish-water benthos. Although still impoverished with respect to their fully marine counterparts, brackish- water ichnofaunas may reach moderately high diversities, particularly in middle- and outer-estuarine regions, and degree of bioturbation may be high in certain estuarine sub-environments. Comparative analysis of brackish-water ichnofaunas through geologic time provides valuable evidence to understand colonization of marginal-marine environments through the Phanerozoic, and allows for calibration of ichnologic models that may aid in the recognition of estuarine valley-fill deposits in the stratigraphic record.

The interpretation of δ18O and δ13C analyses of extinct foraminifera is hampered by insufficient knowledge on the contribution of environmental versus biological factors on the measured values. We present single-species stable isotope data for 18 benthic and 11 planktic foraminiferal species and one calcareous dinoflagellate cyst species from a Turonian equatorial record measured on samples collected in SE Tanzania. All microfossils analyzed were excellently preserved and results are compared to isotopic measurements from bulk sediment of the same material. Remarkably low intra-specific variability (standard deviations <0.2‰) across the studied interval and consistently large benthic / planktic offsets in δ18O (~2.8‰) and δ13C (~1.5‰) indicate absence of major long-term environmental fluctuations and presence of a well stratified water column. Stratigraphic isotopic trends show small shifts that are parallel in all studied benthic species (with the exception of the δ13C record of Lenticulina species), but within-sample inter-specific offsets reflect a combination of habitat differences and species-specific isotopic disequilibrium fractionation. Among planktic taxa closely related taxa tend to have similar isotopic values. Biserial taxa have the lowest δ18O values (~-4.7‰) but also have relatively low δ13C values (~1.6‰) indicating that, for the assumption of a surface water habitat, additional metabolic effects need to be involved, which supports the concept of their opportunistic life style. Among trochospiral planktics the keeled taxa with compressed chambers have slightly higher δ18O (~-4.2‰) and lower δ13C (~2.3‰) values than taxa with globular chambers (~-4.4‰ and ~2.4‰, respectively), suggesting differences in habitat or in peak seasonal abundance. Benthic taxa are subdivided into three isotopically distinct groups with differences reflecting mineralogy, life strategy and habitat; Group I: aragonitic taxa (Epistomina, Colomia africana), Group II: calcitic trochospiral taxa with isotopic values seemingly close to equilibrium (Berthelina berthelini, Oridorsalis umbonatus, Lingulogavelinella globosa, Lingulogavelinella convexa) and Group III: calcitic taxa with low δ18O values and large scatter in δ13C values (Lenticulina). Four other benthic species show values between Groups II and III.

The wealth and consistency of data, especially for benthic taxa, permits refined conclusions. The five studied species of Epistomina have a similar isotopic signature and are inferred to form their aragonitic shells with close to equilibrium values. Small isotopic offsets are seen among species that differ in orientation of the septal foramen, but no offsets are seen that seem to co-vary with test shape or ornamentation. The conical species C. africana has δ18O values ~0.2‰ lower and δ13C values ~0.7‰ lower than species of Epistomina. The latter offset probably reflects an infaunal habitat for C. africana. Epistomina and Group II species are recommended for paleoenviromental studies. Mono-specific Lenticulina δ18O values may be used if increased by correction factors of 0.4‰ to 0.9‰, but this taxon’s low and variable δ13C values are interpreted to reflect incorporation of respiratory CO2, possibly combined with fast growth rates related to opportunism, rather than equilibrium with benthic dissolved inorganic carbon. The observed stability of inter-specific isotopic offsets suggest disequilibrium isotopic effects are relatively consistent for many taxa in this section, and correction factors are proposed that may help to generate more robust Upper Cretaceous foraminiferal isotopic records. However, δ18O / δ13C covariance in some benthic inter-specific comparisons suggest offsets might vary with changes in temperature, food supply or other factors so the appropriateness of the correction factors should be confirmed before being applied at different sites. Similarities between the observed single-species isotopic patterns from this and other studies are used to present a summarizing scheme of influences on the stable isotopic composition in foraminiferal tests. By applying the new insights from our study to data from other Cretaceous studies we demonstrate that the selection of species for isotopic analysis and understanding these signals are critical to the outcome of paleoceanographic interpretations.

The development of isotopic baselines for comparison with paleodietary data is crucial, but often overlooked. We review the factors affecting the carbon (δ13C) and nitrogen (δ15N) isotopic compositions of plants, with a special focus on the carbon and nitrogen isotopic compositions of twelve different species of cultivated plants (n = 91) and 139 wild plant species collected in northern Peru. The cultivated plants were collected from nineteen local markets. The mean δ13C value for maize (grain) was −11.8±0.4 ‰ (n = 27). Leguminous cultigens (beans, Andean lupin) were characterized by significantly lower δ15N values and significantly higher %N than non-leguminous cultigens. Wild plants from thirteen sites were collected in the Moche River Valley area between sea level and ~4,000 meters above sea level (masl). These sites were associated with mean annual precipitation ranging from 0 to 710 mm. Plants growing at low altitude sites receiving low amounts of precipitation were characterized by higher δ15N values than plants growing at higher altitudes and receiving higher amounts of precipitation, although this trend dissipated when altitude was >2,000 masl and MAP was >400 mm. For C3 plants, foliar δ13C was positively correlated with altitude and precipitation. This suggests that the influence of altitude may overshadow the influence of water availability on foliar δ13C values at this scale.