Proxy reconstructions from the mid-Holocene (MH: 6,000 years ago) indicate an intensification of the West African Monsoon and a weakening of the South American Monsoon, primarily resulting from ...orbitally-driven insolation changes. However, model studies that account for MH orbital configurations and greenhouse gas concentrations can only partially reproduce these changes. Most model studies do not account for the remarkable vegetation changes that occurred during the MH, in particular over the Sahara, precluding realistic simulations of the period. Here, we study precipitation changes over northern Africa and South America using four fully coupled global climate models by accounting for the Saharan greening. Incorporating the Green Sahara amplifies orbitally-driven changes over both regions, and leads to an improvement in proxy-model agreement. Our work highlights the local and remote impacts of vegetation and the importance of considering vegetation changes in the Sahara when studying and modeling global climate.
In order to examine relative sea-level responses to the postglacial ice-volume change and the glacio-hydro isostatic adjustments (GIA), this study investigated the inner part of the Hangzhou Bay, ...east China, a tectonically relatively stable far-field location, and reconstructed the early to middle Holocene sea-level history. This investigation has established the elevational relationship between modern saltmarsh-mudflat and tidal levels based on diatom analysis for sea-level indicative meaning estimates, produced 17 high-quality sea-level index points, and simulated GIA processes for the study site. These results reveal that the relative sea level rose from −38.3 ± 1.6 m in c. 10,000 cal a BP to the present height by c. 7000 cal. a BP, and the average rate of sea-level rise decreased gradually from 19.6 ± 2.6 mm/a to 2.3 ± 1.5 mm/a during the 3000 years. This period of sea-level history was punctuated by two episodes of accelerated rise around 8200 and 7500 cal a BP. The relative sea level rose to 0.8 ± 1.4 m above msl by c. 6500 cal. a BP, followed by a gradual fall back to the present height at 4500 cal a BP, implying a different response to the potential additional ice melting between 7000 and 4000 cal a BP. A comparison of the sea-level histories between the inner and outer Hangzhou Bay indicates the coastal levering effect due to the marine inundation of the continental shelves. A further comparison between sea-level data from China and Malay Peninsula reveals different GIA effects between the Cathaysia-Yangtze Blocks and the Sundaland Block.
•A modern analogue between diatom zones and tide levels.•RSL rose from −38.3 ± 1.6 m in 10,000 cal a BP to present height by 7000 cal a BP.•Punctuated by two episodes of accelerated rise around 8200 and 7500 cal a BP.•Evidence of coastal levering during the early Holocene.•Different GIA responses of Cathaysia Block and Sundaland Block to IESL.
Human activity has fundamentally altered the global phosphorus (P) cycle. Yet our understanding of when and how humans influenced the P cycle has been limited by the scarcity of long-term P ...sequestration records, particularly outside Europe and North America. Lake sediments provide a unique archive of past P burial rates and allow the human-mediated disruption of the global P cycle to be examined. We compiled the first global-scale and continentally resolved reconstruction of lake-wide Holocene P burial rates using 108 lakes from around the world. In Europe, lake P burial rates started to increase noticeably after ∼4000 calendar years before 1950 CE (cal BP), whereas the increase occurred later in China (∼2000 cal BP) and in North America (∼550 cal BP), which is most likely related to different histories of population growth, land-use and associated soil erosion intensities. Anthropogenic soil erosion explains ∼86% of the observed changes in global lake P burial rates in pre-industrial times. We also provide the first long-term estimates of the global lake P sink over the Holocene (∼2686 Tg P). We estimate that the global mean lake sediment P sequestration since 1850 CE (100 cal BP) is ∼1.54 Tg P yr−1, representing approximately a six-fold increase above the mean pre-industrial value (∼0.24 Tg P yr−1; 11,500 to 100 cal BP) and around a ten-fold increase above the Early-Middle Holocene low-disturbance baseline of 0.16 Tg P yr−1. This study suggests that human activities have been affecting the global P cycle for millennia, with substantial alteration after industrial times (1850 CE).
•We estimated Holocene phosphorus (P) burial rates in sediments of 108 global lakes.•Global lake P burial rates increased notably in the Late Holocene.•Human-induced soil erosion caused early increases in lake sediment P burial rates.•We observed different patterns of lake P burial rates among continents since the Late Holocene.•We provide the first estimates of long-term, global P sequestration in lake sediments throughout the Holocene.
Climatic and environmental changes in the northeastern Tibetan Plateau are controlled by the Asian summer monsoon (ASM) and the westerlies, two key circulation components of the global climate system ...which directly affect a large human population and associated ecosystems in eastern Asia. During the past few decades, a series of Holocene palaeoclimatic records have been obtained from sediment cores from Lake Qinghai and from various other geological archives in the surrounding area of the northeastern Tibetan Plateau. However, because of uncertainties regarding the sediment chronologies and the climatic significance of the proxies used, the nature of Holocene climatic changes in the region remains unclear and even controversial. Here we review all major classes of the published data from drilled cores from Lake Qinghai, as well as other evidence from lakes and aeolian deposits from surrounding areas, in order to reconstruct changes in moisture patterns and possible summer monsoon evolution in the area during the Holocene. Combining the results of moisture and precipitation proxies such as vegetation history, pollen-based precipitation reconstruction, aeolian activity, lake water depth/lake level changes, salinity and sediment redness, we conclude that moisture and precipitation began to increase in the early Holocene, reached their maximum during the middle Holocene, and decreased during the late Holocene - similar to the pattern of the East Asian summer monsoon (EASM) in northern China. It is clear that the region experienced a relatively dry climate and weak EASM during the early Holocene, as indicated by relatively low tree pollen percentages and fluctuating pollen concentrations; generally low lake levels of Lake Qinghai and the adjacent Lake Hurleg and Lake Toson in the Qaidam Basin; and widely distributed aeolian sand deposition in the Lake Qinghai Basin and the nearby Gonghe Basin to the south, and in the eastern Qaidam Basin to the west. We argue that the ostracod δ18O record, which is widely used as a proxy of effective moisture and summer monsoon intensity in lake sediments, at least in Lake Qinghai, and which exhibits light values in the early Holocene and heavier values thereafter, cannot be used to reflect the strength of the EASM or the intensity of monsoon precipitation - as is also the case for leaf wax δ2H records. Rather, we argue that as is the case of the Chinese speleothem δ18O record, which also is often interpreted as an EASM proxy, it reflects variation in the δ18O of precipitation. Overall, we suggest that the EASM significantly affected precipitation in the northeastern Tibetan Plateau during the Holocene; and that it increased in strength during the early Holocene, reached a maximum during the middle Holocene and decreased during the late Holocene.
•Lake Qinghai and the adjacent lakes were generally low and aeolian sands were widely distributed in the northeastern Tibetan Plateau during the early Holocene.•Ostracod δ18O records from Lake Qinghai cannot be used to reflect the strength of the East Asian summer monsoon or the intensity of monsoon precipitation.•Moisture and precipitation at Lake Qinghai and its environs began to increase in the early Holocene, reached their maximum during the middle Holocene, and decreased during the late Holocene.
This study presents modeling evidence that the Indian Ocean Dipole (IOD) is enhanced in its frequency and amplitude under mid‐Holocene (8‐ka) conditions. This enhancement is identified in a global ...climate model simulation driven by the 8‐ka insolation, in which negative sea surface temperature anomalies grow more than the present‐day simulation in the eastern Indian Ocean. The mechanism of IOD amplification is explained in terms of a positive feedback among sea surface temperature, precipitation, and wind speeds over the eastern Indian Ocean. A series of idealized experiments in which the present‐day insolation seasonal cycle over the tropics is systematically perturbed in the model demonstrates that the IOD frequency and amplitude are proportional to the seasonal cycle strength of the atmospheric circulation off Sumatra‐Java, consistent with proxy records in the mid‐Holocene. Greater surface warming over Eurasia due to increased insolation in boreal summer is not critical to the amplification of the IOD.
Plain Language Summary
Indian Ocean Dipole (IOD) mode is a prominent coupled air‐sea variability in the Indian Ocean, which affects weather over the surrounding regions. In order to clarify the sensitivity of the IOD activity to changing climate condition from the past to present, we compared two long climate simulations: one with the present‐day forcing and the other with the mid‐Holocene (8,000 years ago) forcing that is characterized by a strong seasonal cycle of insolation. The mid‐Holocene climate simulation clearly showed an enhancement of the IOD activity in its amplitude and frequency. The insolation changes during the mid‐Holocene are complex, including intensified seasonal cycle and annual mean over the globe, so that for further exploring mechanisms for the enhanced IOD, we conducted a series of idealized experiments in which perturbations in insolation were confined to the seasonal cycle over the tropics. The intensification of IOD is similarly seen in the experiment, and the degree of the enhancement is found proportional to the rate of seasonal cycle change over the northern Indian Ocean. The process that gives rise to the IOD modulation is mainly controlled by the strength of mean surface winds off Sumatra during the monsoon onset season.
Key Points
Both IOD frequency and amplitude increase under a stronger seasonal cycle in insolation during the mid‐Holocene
IOD activity varies coherently with the strength of the mean wind circulation off Sumatra during boreal summer
Change in insolation over the extratropics is not crucial in modulating the IOD amplitude
Extreme hydrologic events such as storms and floods have the potential to severely impact modern human society. However, the frequency of storms and their underlying mechanisms are limited by a ...paucity of suitable proxies, especially in inland areas. Here we present a record of speleothem magnetic minerals to reconstruct paleoprecipitation, including storms, in the eastern Asian monsoon area over the last 8.6 ky. The geophysical parameter IRMsoft-flux represents the flux of soil-derived magnetic minerals preserved in stalagmite HS4, which we correlate with rainfall amount and intensity. IRMsoft-flux exhibits relatively higher values before 6.7 ky and after 3.4 ky and lower values in the intervening period, consistent with regional hydrological changes observed in independent records. Abrupt enhancements in the flux of pedogenic magnetite in the stalagmite agree well with the timing of known regional paleofloods and with equatorial El Niño–Southern Oscillation (ENSO) patterns, documenting the occurrence of ENSO-related storms in the Holocene. Spectral power analyses reveal that the storms occur on a significant 500-y cycle, coincident with periodic solar activity and ENSO variance, showing that reinforced (subdued) storms in central China correspond to reduced (increased) solar activity and amplified (damped) ENSO. Thus, the magnetic minerals in speleothem HS4 preserve a record of the cyclic storms controlled by the coupled atmosphere–oceanic circulation driven by solar activity.
Tropical mountain environments, such as the Rwenzori Mountains in equatorial Africa, are thought to be particularly sensitive to climate change. Ongoing warming in the Rwenzori is impacting local ...environments and communities through glacial retreat, fires, and flooding. Paleoclimate reconstructions from elsewhere in Africa suggest considerable warming accompanied glacier retreat during the last glacial termination, from ∼21 thousand years before present (ka) through the early to mid-Holocene. Quantifying these changes has been difficult but could help to assess future impacts in the Rwenzori. Here, we present a ∼21 thousand-year (kyr) temperature reconstruction based on the relative abundance of branched glycerol dialkyl glycerol tetraethers (brGDGTs) from Lake Mahoma (2,990 m above sea level; m asl) in the Rwenzori Mountains, Uganda. Our record, paired with existing Rwenzori glacial moraine 10Be exposure ages, suggests that deglacial warming and glacial retreat began by ∼20 ka and accelerated at ∼18–18.5 ka. The timing of the onset of rapid warming matches the timing of the post-glacial rise in radiative forcing from atmospheric greenhouse gases (GHGs) from Antarctic ice cores (Brook et al., 1996; Marcott et al., 2014; Monnin et al., 2004; Schilt et al., 2010). Our temperature reconstruction registers ∼4.9 °C warming from the Last Glacial Maximum (LGM) to the late Holocene. This increase is larger than the average ∼2-4 °C warming observed in records from lower elevation sites in tropical East Africa, but similar to that observed at other high-elevation sites in this region. The increased warming at higher elevations thus confirms that the temperature lapse rate steepened during the LGM over this region. Our results also indicate ∼3 °C of warming during the mid-Holocene relative to the late Holocene. This suggests that the freezing-level height rose above Rwenzori summit elevations at that time, likely causing complete deglaciation of the Rwenzori Mountains from ∼5 to 7 ka. The mid-Holocene is thus a potential analog for the glacial and environmental changes that these mountains are likely to experience in the coming decades. Overall, the timing and magnitude of temperature change observed in our record has important implications for climate model projections of future warming in tropical Africa.
•We produce a 21 kyr temperature record from alpine Lake Mahoma, Uganda.•Rwenzori glaciers and vegetation are very sensitive to past temperature change.•∼3 °C warming during middle Holocene caused complete deglaciation in Rwenzori.•Regional temperature records indicate a steeper tropical lapse rate during the LGM.•Tropical lapse rate shoaled and stabilized during the middle and late Holocene.
Based on the latest database, we analyze the characteristics of the changing trends of sunspot number (SSN) and temperatures of the global mean surface (GMST) as well as six latitude bands during the ...Holocene, aiming to explore the long-term responses of the Holocene temperatures to solar activity. We adopt two methods, i.e., the 300-year moving average and the singular spectrum analysis to obtain the long-term trends of signals. We find that the average changes in the amplitude of temperatures in the Northern Hemisphere (NH) (3.65 °C) was greater than that in the Southern Hemisphere (SH) (1.28 °C) during the entire Holocene. There was one peak of temperatures (at 6500 BP) and two peaks of solar activity (at 4500 BP and 2000 BP) during our study interval. Spatially, the temperatures in the NH were more sensitive to solar forcing than those in the SH, especially in the latitude bands of 0°–30°N and 60°N-90°N. Moreover, the latitude band 0°-30°N had the strongest correlation with solar activity (C.C. = 0.38 for the 300-year moving average and C.C. = 0.55 for the singular spectrum analysis, p < 0.01), while the latitude band 60°S-90°S had the weakest correlation with solar activity (not significant in the statistical sense). Regarding the time response, solar forcing nearly did not affect the terrestrial temperatures in the early Holocene (8700 BP-6500 BP). While positive correlations started to be strong since 4500 BP and became stronger during 2000 BP-100 BP. All in all, the terrestrial temperatures were consistent with solar activity in the long-term trend during the Holocene, especially in the NH. Although many mysteries in historical climate reconstructions remain un-resolved, evaluating the impact of solar force on terrestrial temperature is crucial to reconstruct and predict terrestrial climate. Much deeper specific research on solar-terrestrial mechanisms deserves to be put on the agenda.
As the dominant large herbivore in midcontinent North America since the terminal Pleistocene, bison (Bison spp.) have been a fundamental component of ecosystems and economies. Despite the importance ...of bison in late Quaternary North America, large-scale (regional to continental) patterns of bison biogeography are not well understood. Here we integrate archaeological and paleontological bison occurrence data with simulated climate data to better understand long-term drivers of bison distribution and abundance in North America. We used these records to model bison distribution and abundance over the past 20 thousand years at 1-thousand-year intervals. Our results show that late Quaternary changes in the distribution and abundance of bison were influenced by large-scale trends in temperature and precipitation. The distribution of bison since the Bølling–Allerød Interstadial (ca. 14 ka) is primarily explained by seasonal temperature patterns (mean temperature of the coldest quarter is the most important variable for 12 of the 14 1-thousand-year intervals). The modeled climate of bison distributions progressively narrowed since the Last Glacial Maximum (ca. 20 ka) as bison populations retracted from disjunct Pleistocene refugia and congregated in midcontinent rangelands. Through the Pleistocene-Holocene transition, bison experienced rapidly warming summer temperatures that increased faster in midcontinent North America than other regions and the continent as a whole. Model results suggest that Holocene bison abundance was influenced by hydroclimatic shifts that affected the quality and availability of forage. Bison abundances decreased through the dry early and mid-Holocene and increased when moisture availability improved in the late Holocene. We infer that bison have thrived under a broad range of environmental conditions since the Last Glacial Maximum and that the climatic and biogeographic space occupied by bison narrowed in recent millennia.
•Regional extirpations and expansions of bison linked to changing climate conditions.•Bison abundance positively associated with moisture availability in open ecosystems.•Mean temperature of the coldest quarter is a strong predictor of post-LGM bison distribution.•Bison climate niche gradually narrowed from the LGM to present.
The Indo-Gangetic floodplains have many abandoned meander cut-offs and oxbow lakes that can serve as potential archives for high-resolution paleoclimatic reconstruction. Previously, a few of these ...lakes have been investigated for understanding the regional Holocene climatic history, but these studies are generally limited by a lack of high chrono-stratigraphic resolution. To develop an understanding of the role of local and/or regional climate forcings, an oxbow lake (Baraila Tal) in the Central Ganga Plains has been studied using multiple biotic proxies such as pollen, phytoliths, diatoms, and sponge spicules. Millennial-scale climatic fluctuations and vegetation history for the Early to Mid-Holocene period have been reconstructed using 17 AMS radiocarbon dates from two trenches, 3–4 m deep. Five major biotic zones have been delineated between 13.2 and 4.8 kyr BP. Zone I (∼13–12 kyr BP) shows small patches of dry tropical woody flora along with tall C4 grass indicating a moderately dry climate. The presence of low-temperature herbs indicates an active NE monsoon but the paucity of arboreals indicates a weakened Indian Summer Monsoon (ISM). The high percentages of Phacotus lenticularis cysts in Zone II (12–9 kyr BP) indicate the presence of stagnant, lime-rich inland waters under relatively colder climatic conditions. The appearance of evergreen forest groves around 11.5 kyr BP indicates an increase in humidity, and lake deepening was inferred between 10.9 and 10.3 kyr BP. Sediment accumulation rate shows a sudden drop at ∼10.3 kyr BP indicating a complete cut-off of the oxbow lake from the fluvial channel. Zone III (9–8 kyr BP) records millennial-scale drying with a complete absence of aquatic pollen between 8.9 and 7.7 kyr BP. The expansion of short C4 grass in Zone IV (8–7 kyr BP) further indicates drier conditions. Zone V (7 –∼5 kyr BP) shows very high percentages of sponge spicules that flourish in clear waters, and represents a strengthened ISM and a subsequent lake expansion.
•High resolution paleoclimatic reconstruction from an oxbow lake in the central Ganga plains.•A multi-proxy approach using pollen, phytoliths, diatoms, and sponge spicules.•Millennial-scale climatic fluctuations and vegetation history for Early to Mid-Holocene period using 17 AMS dates.•Records the onset of the Holocene epoch at ∼11.8 kyr and a warm and humid mid-Holocene.
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