Search

The scarcity of well-dated, highly temporally resolved, paleoclimate records since the last glacial maximum (LGM) in Northeast China limits our understanding of past climatic variations and the prediction of future hydrological changes in the context of anthropogenic global warming. A high-resolution n -alkane record covering the past ∼25 kyr was retrieved from Lake Tuofengling, a hydrologically closed crater lake in the central Great Khingan Mountains. The sediments contained a suite of mid- and long-chain n -alkanes with a strong odd-to even-carbon number predominance. The relative proportion of mid-to long-chain n -alkane homologues (P aq) was proposed to evaluate the input of submerged/floating plants into lake sediments relative to that from emergent/terrestrial plants, which could be a useful indicator of lake level in lacustrine settings. The P aq proxy demonstrated that lake level increased stably from the shallowest level during the LGM to a maximum depth at 7.5–3.0 cal ka BP, with a minor decreasing trend during the late Holocene. The asynchronous changes in effective moisture in the East Asian summer monsoon (EASM) region since the LGM might be directly or indirectly mediated by the shift in the rainfall belt, the position and intensity of the Western Pacific Subtropical High and Okhotsk High at different timescales. Our study provides new insights into climatic evolution in Northeast China, where the main climate pattern may not always be similar to the typical EASM pattern. • Changes in lake level reconstructed using Paq ratio since LGM. • Younger Dryas was characterized by relatively humid conditions in Northeast China. • Pacific Ocean was the main regulator of hydroclimatic pattern in the EASM region. [ABSTRACT FROM AUTHOR]

Tephras are important for the NZ-INTIMATE project because they link all three records comprising the composite inter-regional stratotype developed for the New Zealand climate event stratigraphy (NZ-CES). Here we firstly report new calendar ages for 24 widespread marker tephras erupted since 30,000 calendar (cal.) years ago in New Zealand to help facilitate their use as chronostratigraphic dating tools for the NZ-CES and for other palaeoenvironmental and geological applications. The selected tephras comprise 12 rhyolitic tephras from Taupo, nine rhyolitic tephras from Okataina, one peralkaline rhyolitic tephra from Tuhua, and one andesitic tephra each from Tongariro and Egmont/Taranaki volcanic centres. Age models for the tephras were obtained using three methods: (i) 14C-based wiggle-match dating of wood from trees killed by volcanic eruptions (these dates published previously); (ii) flexible depositional modelling of a high-resolution 14C-dated age–depth sequence at Kaipo bog using two Bayesian-based modelling programs, Bacon and OxCal's P_Sequence function, and the IntCal09 data set (with SH offset correction −44 ± 17 yr); and (iii) calibration of 14C ages using OxCal's Tau_Boundary function and the SHCal04 and IntCal09 data sets. Our preferred dates or calibrated ages for the 24 tephras are as follows (youngest to oldest, all mid-point or mean ages of 95% probability ranges): Kaharoa AD 1314 ± 12; Taupo (Unit Y) AD 232 ± 10; Mapara (Unit X) 2059 ± 118 cal. yr BP; Whakaipo (Unit V) 2800 ± 60 cal. yr BP; Waimihia (Unit S) 3401 ± 108 cal. yr BP; Stent (Unit Q) 4322 ± 112 cal. yr BP; Unit K 5111 ± 210 cal. yr BP; Whakatane 5526 ± 145 cal. yr BP; Tuhua 6577 ± 547 cal. yr BP; Mamaku 7940 ± 257 cal. yr BP; Rotoma 9423 ± 120 cal. yr BP; Opepe (Unit E) 9991 ± 160 cal. yr BP; Poronui (Unit C) 11,170 ± 115 cal. yr BP; Karapiti (Unit B) 11,460 ± 172 cal. yr BP; Okupata 11,767 ± 192 cal. yr BP; Konini (bed b) 11,880 ± 183 cal. yr BP; Waiohau 14,009 ± 155 cal. yr BP; Rotorua 15,635 ± 412 cal. yr BP; Rerewhakaaitu 17,496 ± 462 cal. yr BP; Okareka 21,858 ± 290 cal. yr BP; Te Rere 25,171 ± 964 cal. yr BP; Kawakawa/Oruanui 25,358 ± 162 cal. yr BP; Poihipi 28,446 ± 670 cal. yr BP; and Okaia 28,621 ± 1428 cal. yr BP. Secondly, we have re-dated the start and end of the Lateglacial cool episode (climate event NZce-3 in the NZ-CES), previously referred to as the Lateglacial climate reversal, as defined at Kaipo bog in eastern North Island, New Zealand, using both Bacon and OxCal P_Sequence modelling with the IntCal09 data set. The ca 1200-yr-long cool episode, indicated by a lithostratigraphic change in the Kaipo peat sequence to grey mud with lowered carbon content, and a high-resolution pollen-derived cooling signal, began 13,739 ± 125 cal. yr BP and ended 12,550 ± 140 cal. yr BP (mid-point ages of the 95% highest posterior density regions, Bacon modelling). The OxCal modelling, generating almost identical ages, confirmed these ages. The Lateglacial cool episode (ca 13.8–12.6 cal. ka BP) thus overlaps a large part of the entire Antarctic Cold Reversal chronozone (ca 14.1–12.4 cal. ka BP or ca 14.6–12.8 cal. ka BP), and an early part of the Greenland Stadial-1 (Younger Dryas) chronozone (ca 12.9–11.7 cal. ka BP). The timing of the Lateglacial cool episode at Kaipo is broadly consistent with the latitudinal patterns in the Antarctic Cold Reversal signal suggested for the New Zealand archipelago from marine and terrestrial records, and with records from southern South America. [ABSTRACT FROM AUTHOR]

Abstract: We present pollen records from three sites in south Westland, New Zealand, that document past vegetation and inferred climate change between approximately 30,000 and 15,000 cal. yr BP. Detailed radiocarbon dating of the enclosing sediments at one of those sites, Galway tarn, provides a more robust chronology for the structure and timing of climate-induced vegetation change than has previously been possible in this region. The Kawakawa/Oruanui tephra, a key isochronous marker, affords a precise stratigraphic link across all three pollen records, while other tie points are provided by key pollen-stratigraphic changes which appear to be synchronous across all three sites. Collectively, the records show three episodes in which grassland, interpreted as indicating mostly cold subalpine to alpine conditions, was prevalent in lowland south Westland, separated by phases dominated by subalpine shrubs and montane-lowland trees, indicating milder interstadial conditions. Dating, expressed as a Bayesian-estimated single ‘best’ age followed in parentheses by younger/older bounds of the 95% confidence modelled age range, indicates that a cold stadial episode, whose onset was marked by replacement of woodland by grassland, occurred between 28,730 (29,390–28,500) and 25,470 (26,090–25,270) cal. yr BP (years before AD, 1950), prior to the deposition of the Kawakawa/Oruanui tephra. Milder interstadial conditions prevailed between 25,470 (26,090–25,270) and 24,400 (24,840–24,120) cal. yr BP and between 22,630 (22,930–22,340) and 21,980 (22,210–21,580) cal. yr BP, separated by a return to cold stadial conditions between 24,400 and 22,630 cal. yr BP. A final episode of grass-dominated vegetation, indicating cold stadial conditions, occurred from 21,980 (22,210–21,580) to 18,490 (18,670–17,950) cal. yr BP. The decline in grass pollen, indicating progressive climate amelioration, was well advanced by 17,370 (17,730–17,110) cal. yr BP, indicating that the onset of the termination in south Westland occurred sometime between ca 18,490 and ca 17,370 cal. yr BP. A similar general pattern of stadials and interstadials is seen, to varying degrees of resolution but generally with lesser chronological control, in many other paleoclimate proxy records from the New Zealand region. This highly resolved chronology of vegetation changes from southwestern New Zealand contributes to the examination of past climate variations in the southwest Pacific region. The stadial and interstadial episodes defined by south Westland pollen records represent notable climate variability during the latter part of the Last Glaciation. Similar climatic patterns recorded farther afield, for example from Antarctica and the Southern Ocean, imply that climate variations during the latter part of the Last Glaciation and the transition to the Holocene interglacial were inter-regionally extensive in the Southern Hemisphere and thus important to understand in detail and to place into a global context. [Copyright &y& Elsevier]

The scarcity of research in NE China on lake ecosystem responses to large-scale climate oscillations since the last deglaciation limits our abilities for informing conservation practices and policies in the context of recent global warming. Here, a high-resolution, sedimentary diatom record covering the past ∼17,500 years was retrieved from Lake Moon, a small, hydrologically closed crater lake in the remote central part of the Great Khingan Mountain Range in NE China. We compare diatom changes with geochemical data from the same core, and with regional palynological and dust deposition records, to better understand the influences of long-term environmental and climatic variability on aquatic ecosystems in this climatically sensitive region. Several abrupt and pronounced shifts in dominance among diatom taxa corresponded to marked fluctuations in the climate regime since the last deglaciation. During the close of the Last Glacial Period, a series of short-lived diatom shifts that were indicative of an increase in nutrients, signaled the transition from the cold period of the Heinrich event 1 (∼17,500–∼14,700 cal yr BP) to the warmer Bølling - Allerød interstadial period (∼14,700–∼12,900 cal yr BP). The onset of the cold Younger Dryas period was marked by a brief rise to dominance of benthic taxa (∼12,900–∼11,800 cal yr BP), followed by a pronounced lake ecosystem shift to a new trophic state at the Pleistocene-Holocene transition (∼11,800–∼9900 cal yr BP), evidenced by an abrupt rise to dominance of several eutrophic diatom indicators (e.g. Cyclostephanos dubius , Aulacoseira ambigua and Stephanodiscus parvus). This nearly complete compositional turnover was indicative of an increase in phosphorus supply to the lake with the onset of a warmer and moister climate and an increased frequency of strong dust storms. During the mid-Holocene (∼6000 cal yr BP), a striking increase in the relative abundance of Discostella pseudostelligera and Asterionella formosa , at the expense of previously dominant eutrophic indicators, signified another lake ecosystem change from well-mixed, turbid and phosphorus-rich conditions to a lower nutrient state with longer open-water periods and increased thermal stability. The turnover to oligotrophic diatom taxa was likely in response to variations in seasonal temperature, precipitation and dust deposition. The diatom shifts of Lake Moon during the past ∼17,500 years were directly or indirectly mediated by climate change that affected thermal stratification, productivity, lake level and trophic state. Our results indicate that climate change had an overarching control on aquatic ecosystem changes in the mountain regions of NE China since the last deglaciation. • A continuous diatom record tracking aquatic ecosystem dynamics in NE China since the last deglaciation. • Significant ecological reorganizations corresponded to the Heinrich 1 event, the Bølling - Allerød interstadial and the Younger Dryas. • Aquatic ecosystem during the Holocene responded to seasonal variations in temperature, precipitation and dust deposition. • Climate change and its mediating effects on landscape processes had an overarching control on aquatic ecosystem changes. [ABSTRACT FROM AUTHOR]

In this study, the high-resolution palynological data derived from two cores taken from Mogan Lake in the Gölbaşı Basin of the Ankara region in central Anatolia (Turkey) are presented. The results provide the palaeobotanical, palaeoclimatological and palaeoenvironmental records of the last ca. 3100 years for the region which is characterised by rich natural resources and a long human occupation history. The major climatic events and the archaeological-historical periods, which are well established in previous works, along with textual sources, are related to vegetation history inferred from the palynological investigation in the study area. Five main vegetation phases are distinguished and interpreted. Our findings demonstrate that the phases of limited woody vegetation cover are chiefly coupled to several dry climate periods, with some exceptions. Steppe vegetation in an open landscape accompanied by more cultural indicators and lesser arboreal elements is inferred during the Near-East Aridification Phase at the LBA–Iron Age transition (c. 1200–850 BCE), Roman Warm Period before 700 CE, Medieval Warm Period at about 1020 CE, the 1500s and twentieth century. Only during the three dry episodes in the 1600s, 1800s and at the start of Modern Warming Period (in the twentieth century) partial arboreal recovery is deduced, and land-cover patterns are related to both natural and human-induced factors. The most extensive pine-dominated forest cover coincides mainly with the three wet climatic episodes, in Galatians times of the Hellenistic Period at c. 2080 BP, the 700s CE of the middle Byzantine era, the Seljuqs Dynasty and the earlier Ottoman Periods (c. 1165–1432/1435 CE). The former episode is represented by remarkable signs of human interference, while the others have marks of sporadically less cultivated landscape. It is concluded that a combination of various factors can be taken into account as the drivers of vegetation changes in the Ankara region, but climate appears to have been the primary agent in shaping the land-cover. [ABSTRACT FROM AUTHOR]