Low-latitude Summer Insolation Research Articles

Astronomically forced late Paleocene-early Eocene climate variability in the Subei Basin, East China

The latest Paleocene to early Eocene was an interval of globally warm climate. However, the driving mechanisms regulating climate across this interval remain unclear due to the paucity of well-dated and stratigraphically continuous records from terrestrial basins. In this study, we present a continuous ∼430-m lacustrine core record from the QY-1 borehole spanning the late Paleocene-Early Eocene in the Subei Basin, East China. This unique record can be used to better elucidate the response of terrestrial sedimentation to astronomically forced climate change. A 4.54 Myr long astronomical time scale spanning 57.1 to 52.56 Ma is constructed for the second member of the Funing Formation (Ef2) based on magnetostratigraphy and cyclostratigraphy. Low-latitude summer insolation via eccentricity modulation of precession controlled paleolake evolution in the basin during the studied interval. At the same time, we show that the climate system during this greenhouse state was sensitive enough to respond to the small changes in insolation associated with 200-kyr eccentricity cycles. Evidence from gamma ray data suggests that the climate in the Subei Basin changed from warm and humid to arid at ∼54.1 Ma.

Orbitally-forced meridional shifts of the westerlies modulated the hydroclimate of northeast China during the late Eocene

The mid-latitude westerlies are a fundamental component of the global climate system. There has been extensive research about how meridional shifts of the westerlies influenced the wet-dry paleoclimate regimes of mid-latitude Asia during the Quaternary. However, how the westerlies operated in much older geological times, such as the late Eocene, particularly on an orbital time scale, is not fully understood. Here, we conducted detailed cyclostratigraphic and palynological analyses on a late Eocene (41.2–37.8 Ma) mudstone-shale sedimentary sequence from the Fushun Basin in northeast China, which uniquely records orbitally-forced meridional shifts of the westerlies. Cyclostratigraphic analyses reveal that the mudstone-shale rhythms exhibit a prominent short eccentricity cycle, with a modest expression of obliquity cycles and a weak expression of precession cycles. The palynological results show that the vegetation experienced frequent alternations between semi-desert and swamp forest, and the climate exhibited periodic oscillations between warm-dry and cold-wet climates. We propose that a combination of eccentricity-modulated low-latitude summer insolation and obliquity-forced high-latitude incipient ice sheet and meridional temperature gradient variations drove cyclical vegetation and hydroclimate fluctuations in northeast China during the late Eocene by regulating the intensity and displacement of the westerlies and the subtropical high. Our results imply that continued poleward movement of the westerlies and the subtropical high will lead to increased aridification along the southern margin of the westerlies under a continuous global warming scenario.

Tectonic and orbital forcing of the South Asian monsoon in central Tibet during the late Oligocene

The modern pattern of the Asian monsoon is thought to have formed around the Oligocene/Miocene transition and is generally attributed to Himalaya-Tibetan Plateau (H-TP) uplift. However, the timing of the ancient Asian monsoon over the TP and its response to astronomical forcing and TP uplift remains poorly known because of the paucity of well-dated high-resolution geological records from the TP interior. Here, we present a precession-scale cyclostratigraphic sedimentary section of 27.32 to 23.24 million years ago (Ma) during the late Oligocene epoch from the Nima Basin to show that the South Asian monsoon (SAM) had already advanced to the central TP (32°N) at least by 27.3 Ma, which is indicated by cyclic arid-humid fluctuations based on environmental magnetism proxies. A shift of lithology and astronomically orbital periods and amplified amplitude of proxy measurements as well as a hydroclimate transition around 25.8 Ma suggest that the SAM intensified at ~25.8 Ma and that the TP reached a paleoelevation threshold for enhancing the coupling between the uplifted plateau and the SAM. Orbital short eccentricity-paced precipitation variability is argued to be mainly driven by orbital eccentricity-modulated low-latitude summer insolation rather than glacial-interglacial Antarctic ice sheet fluctuations. The monsoon data from the TP interior provide key evidence to link the greatly enhanced tropical SAM at 25.8 Ma with TP uplift rather than global climate change and suggest that SAM's northward expansion to the boreal subtropics was dominated by a combination of tectonic and astronomical forcing at multiple timescales in the late Oligocene epoch.

Orbital controls on eastern African hydroclimate in the Pleistocene

Understanding eastern African paleoclimate is critical for contextualizing early human evolution, adaptation, and dispersal, yet Pleistocene climate of this region and its governing mechanisms remain poorly understood due to the lack of long, orbitally-resolved, terrestrial paleoclimate records. Here we present leaf wax hydrogen isotope records of rainfall from paleolake sediment cores from key time windows that resolve long-term trends, variations, and high-latitude effects on tropical African precipitation. Eastern African rainfall was dominantly controlled by variations in low-latitude summer insolation during most of the early and middle Pleistocene, with little evidence that glacial–interglacial cycles impacted rainfall until the late Pleistocene. We observe the influence of high-latitude-driven climate processes emerging from the last interglacial (Marine Isotope Stage 5) to the present, an interval when glacial–interglacial cycles were strong and insolation forcing was weak. Our results demonstrate a variable response of eastern African rainfall to low-latitude insolation forcing and high-latitude-driven climate change, likely related to the relative strengths of these forcings through time and a threshold in monsoon sensitivity. We observe little difference in mean rainfall between the early, middle, and late Pleistocene, which suggests that orbitally-driven climate variations likely played a more significant role than gradual change in the relationship between early humans and their environment.

Eccentricity-paced monsoon variability on the northeastern Tibetan Plateau in the Late Oligocene high CO2 world.

Constraining monsoon variability and dynamics in the warm unipolar icehouse world of the Late Oligocene can provide important clues to future climate responses to global warming. Here, we present a ~4-thousand year (ka) resolution rubidium-to-strontium ratio and magnetic susceptibility records between 28.1 and 24.1 million years ago from a distal alluvial sedimentary sequence in the Lanzhou Basin (China) on the northeastern Tibetan Plateau margin. These Asian monsoon precipitation records exhibit prominent short (~110-ka) and long (405-ka) eccentricity cycles throughout the Late Oligocene, with a weak expression of obliquity (41-ka) and precession (19-ka and 23-ka) cycles. We conclude that a combination of eccentricity-modulated low-latitude summer insolation and glacial-interglacial Antarctic Ice Sheet fluctuations drove the eccentricity-paced precipitation variability on the northeastern Tibetan Plateau in the Late Oligocene high CO2 world by governing regional temperatures, water vapor loading in the western Pacific and Indian Oceans, and the Asian monsoon intensity and displacement.

A study of Holocene Asian summer and winter monsoon change by an analog of climate factors between millennial and modern interannual scales

The East Asian monsoon exerts a profound influence on environmental change in the East Asian region. Various factors have been hypothesized as the dominant Asian monsoon forcings, however, the forcings can change from interannual to millennial timescales. The linkages between monsoon forcings at different timescales remain unclear. To better understand the connection of the variabilities and mechanisms of the East Asian monsoon at various timescales, we present a modern analog. Various climatic data, monsoon indices, and circulation factor calculations were used to identify the variabilities and controlling factors of the modern East Asian summer and winter monsoons. Paleo-climatic proxies from a region sensitive to both summer and winter monsoons were used in concert with monsoon simulation data to reconstruct and analyze paleo-monsoon variations and mechanisms. Results showed that the weakening of the Holocene Asian summer and winter monsoons is closely linked to low-latitude summer insolation and mid-latitude winter insolation, while modern summer and winter monsoons are related to global circulation, sea surface temperature, and sea ice change. We confirm that the driving mechanism of the monsoon was dependent on timescale.

Asynchronous Holocene Asian monsoon vapor transport and precipitation

Millennial-scale variations in the Asian monsoon vapor transport are always associated with long-term changes in monsoon rainfall according to the conventional understanding. However, recent advances in monsoon studies suggested that regional monsoon rainfall has different responses to monsoon vapor transport. This paper presents a comparison between monsoonal vapor and precipitation records along the path of the Asian summer monsoon vapor transport. Pedogenic carbonate δ18O values in both surface sediment and Holocene eolian and lacustrine sediments from the Shiyang River drainage basin were used to investigate the variations of the Asian monsoon vapor transport. Meanwhile, Holocene lake-level changes of the terminal lake were reconstructed to reflect the monsoon precipitation variability. The results show that millennial-scale monsoon vapor transport is in accordance with low-latitude summer insolation, while the increasing summer monsoon rainfall during the early Holocene lags the monsoon water vapor transport. In addition, the relatively cold oceans left over from glacial periods and slow melting of ice sheets may temper the onset of the rainfall. These findings can provide insights into the interpretation of the asynchronous Holocene records from lakes and caves in East Asia.

Timing, cause and consequences of mid-Holocene climate transition in the Arabian Sea

We reconstruct centennial scale quantitative changes in surface seawater temperature (SST), evaporation-precipitation (from Mg/Ca and δ18O of surface dwelling planktic foraminifera), productivity (from relative abundance of Globigerina bulloides), carbon burial (from %CaCO3 and organic carbon [%Corg]) and dissolved oxygen at sediment–water interface, covering the entire Holocene, from a core collected from the eastern Arabian Sea. From the multi-proxy record, we define the timing, consequences and possible causes of the mid-Holocene climate transition (MHCT). A distinct shift in evaporation-precipitation (E-P) is observed at 6.4 ka, accompanied by a net cooling of SST. The shift in SST and E-P is synchronous with a change in surface productivity. A concurrent decrease is also noted in both the planktic foraminiferal abundance and coarse sediment fraction. A shift in carbon burial, as inferred from both the %CaCO3 and %Corg, coincides with a change in surface productivity. A simultaneous decrease in dissolved oxygen at the sediment–water interface, suggests that changes affected both the surface and subsurface water. A similar concomitant change is also observed in other cores from the Arabian Sea as well as terrestrial records, suggesting a widespread regional MHCT. The MHCT coincides with decreasing low-latitude summer insolation, perturbations in total solar intensity and an increase in atmospheric CO2.

Holocene Dune Mobility and Forcing Mechanisms at the Northern Margin of the East Asian Monsoon

Abstract:Dune fields at the northern margin of the East Asian monsoon (EAM), are mosaics of mobile and vegetation‐stabilized aeolian dunes. These sand dunes are highly sensitive to environmental change, thus the distribution and the timing of their development may provide important clues to past environmental dynamics. Due to the strong wind erosion and dune migration, long and continuous stratigraphic records are seldom preserved. Synthesizing a large body of events, ultimately producing a relatively complete and high‐resolution record, may be a proper method to investigate the dune development history and climate change. In this study, we synthesized a large body of luminescence ages for aeolian deposits from the Mu Us, Otindag, Horqin dune fields at the northern margin of the EAM. The results show that these dune fields, as a whole experienced a most extensive mobility during the early Holocene, followed by a widespread shift toward limited mobility and soil development in the mid‐Holocene, and widespread reactivation occurred during late Holocene. The dune developments are directly linked to the effective moisture change controlled by the EAM changes, which respond to the low latitude summer insolation variation. The increased subsidence at the margin contrary to the core EAM, the delay from the feedback of the soil‐vegetation‐air coupled system, the increased evaporation due to the high temperature all play partial role in the lag of the margin EAM effective moisture change to the low latitude summer insolation. The asynchronous end of the wetter mid‐Holocene mainly responds to the southeastwardly shift of the precipitation belt, while the regional sensitivity, response speed and internal feedback also contributed. The correspondence between dune records and North Atlantic drift‐ice records of the rapid climate changes implies a close relationship between North Atlantic climate and the frequent dune activity at the northern margin of EAM.

High- and low-latitude forcing of the Nile River regime during the Holocene inferred from laminated sediments of the Nile deep-sea fan

Sediments deposited on deep-sea fans are an excellent geological archive to reconstruct past changes in fluvial discharge. Here we present a reconstruction of changes in the regime of the Nile River during the Holocene obtained using bulk elemental composition, grain-size analyses and radiogenic strontium (Sr) and neodymium (Nd) isotopes from a sediment core collected on the Nile deep-sea fan. This 6-m long core was retrieved at ∼700m water-depth and is characterized by the presence of a 5-m thick section of finely laminated sediments, which were deposited between 9.5 and 7.3ka BP and correspond to the African Humid Period (AHP). The data show distinct changes in eolian dust inputs as well as variations in discharge of the Blue Nile and White Nile. Sedimentation was mainly controlled by changes in fluvial discharge during the Holocene, which was predominantly forced by low-latitude summer insolation and by the location of the eastern African Rain Belt. The changes in relative contribution from the Blue Nile and White Nile followed changes in low-latitude spring/autumn insolation, which highlights the role of changes in seasonality of the precipitation on the Nile River regime. The relative intensity of the Blue Nile discharge was enhanced during the early and late Holocene at times of higher spring insolation (with massive erosion and runoff during the AHP at times of high summer insolation), while it was reduced between 8 and 4ka at times of high autumn insolation. The gradual insolation-paced changes in fluvial regime were interrupted by a short-term arid event at 8.5–7.3ka BP (also associated with rejuvenation of bottom-water ventilation above the Nile fan), which was likely related to northern hemisphere cooling events. Another arid event at 4.5–3.7ka BP occurred as the apex of a gradually drier phase in NE Africa and marks the end of the AHP.

Different orbital rhythms in the Asian summer monsoon records from North and South China during the Pleistocene

Here we construct a Pleistocene astronomical timescale for the Nihewan fluvio–lacustrine sediments (North China), via tuning a stacked summer monsoon index generated from grain size and low-field magnetic susceptibility records to orbital obliquity and precession. Combining the summer monsoon records retrieved from the Nihewan and the Chinese loess deposits in North China on the one hand, and those from the stalagmites and the marine deposits in South China on the other, the Asian summer monsoon records from North and South China appear to show different orbital rhythms during the Pleistocene. The monsoon records from both the Nihewan Basin and Chinese Loess Plateau are equally characterized by dominant obliquity (41 kyr) before ~ 0.9 Ma and dominant eccentricity (100 kyr) after this time, closely following the marine δ 18O record. In contrast, the δ 18O record of stalagmites from South China (Wang et al., 2001, 2008b; Cheng et al., 2009) and the iron oxide proxy record from the South China Sea (Zhang et al., 2007, 2009; Ao et al., 2011), which are considered as a proxy indicator of the Asian summer monsoon intensity in South China, reveal a dominant cyclicity of precession (23 kyr) over the past 1.8 Myr, closely following the solar insolation curve instead. We further present a possible interpretation of the different orbital rhythms in the Asian summer monsoon records from North and South China. The orbital rhythm in the summer monsoon records from North China is mainly modulated by the migration of the Asian summer monsoon due to changes in sea level and Northern Hemisphere ice volume during glacial–interglacial cycles. Strong summer monsoon may have reached North China mainly during interglacial periods. During glacial periods when the sea level was low and Northern Hemisphere ice volume was large, the southeasterly migration of summer monsoon would make North China beyond the reach of the summer monsoon. Thus the summer monsoon records from North China primarily show a cyclic oscillation similar to the glacial–interglacial climate cycles as indicated by cycle-by-cycle correlation between monsoon and marine oxygen isotope records. However, the summer monsoon always prevailed over South China during both glacial and interglacial periods. Therefore, the orbital-scale variability of the summer monsoon in South China shows a direct response to the orbital variations in the low-latitude summer insolation, without significant influence from the migration of monsoon during glacial–interglacial cycles.

Glacial-interglacial water cycle, global monsoon and atmospheric methane changes

The causes of atmospheric methane (CH4) changes are still a major contention, in particular with regards to the relative contributions of glacial-interglacial cycles, monsoons in both hemispheres and the late Holocene human intervention. Here, we explore the CH4 signals in the Antarctic EPICA Dome C and Vostok ice records using the methods of timeseries analyses and correlate them with insolation and geological records to address these issues. The results parse out three distinct groups of CH4 signals attributable to different drivers. The first group (~80% variance), well tracking the marine δ18O record, is attributable to glacial-interglacial modulation on the global water cycle with the effects shared by wetlands at all latitudes, from monsoonal and non-monsoonal regions in both hemispheres. The second group (~15% variance), centered at the ~10-kyr semi-precession frequency, is linkable with insolation-driven tropical monsoon changes in both hemispheres. The third group (~5% variance), marked by millennial frequencies, is seemingly related with the combined effect of ice-volume and bi-hemispheric insolation changes at the precession bands. These results indicate that bi-hemispheric monsoon changes have been a constant driver of atmospheric CH4. This mechanism also partially explains the Holocene CH4 reversal since ~5 kyr BP besides the human intervention. In the light of these results, we propose that global monsoon can be regarded as a system consisting of two main integrated components, one primarily driven by the oscillations of Inter-Tropical Convergence Zone (ITCZ) in response to the low-latitude summer insolation changes, anti-phase between the two hemispheres (i.e. the ITCZ monsoon component); and another modulated by the glacial-interglacial cycles, mostly synchronous at the global scale (i.e. the glacial-interglacial monsoon component). Although atmospheric CH4 record integrates all wetland processes, including significant non-monsoonal contributions, it is the only and probably the best proxy available to reflect the past changes of global monsoon. However, the utility of CH4 as a proxy of monsoon changes at any specific location is compromised by its bi-hemispheric nature.

The tropical rainbelt and productivity changes off northwest Africa: A 31,000-year high-resolution record

Abstract Benthic foraminiferal assemblage compositions and sedimentary geochemical parameters were analyzed in two radiocarbon dated sediment cores from the upwelling area off NW Africa at 12°N, to reconstruct productivity changes during the last 31 kyr. High-latitude cold events and variations in low-latitude summer insolation influenced humidity, wind systems, and the position of the tropical rain belt over this time period. This in turn caused changes in intensity and seasonality of primary productivity off the southern Northwest African continental margin. High accumulation rates of benthic foraminifera, carbonate, and organic carbon during times of north Atlantic melt water events Heinrich 2 (25.4 to 24.3 kyr BP) and 1 (16.8 to 15.8 kyr BP) indicate high productivity. Dominance of infaunal benthic foraminiferal species and high numbers of deep infaunal specimens during that time indicate a strong and sustained supply of refractory organic matter reworked from the upper slope and shelf. A more southerly position of the tropical rainbelt and the Northeast trade wind belt during Heinrich 2 and 1 may have enhanced wind intensity and almost permanent upwelling, driving this scenario. A phytodetritus-related benthic fauna indicates seasonally pulsed input of labile organic matter but generally low year-round productivity during the Last Glacial Maximum (23 to 18 kyr BP). The tropical rainbelt is more expanded to the North than during Heinrich events, and relatively weak NE trade winds resulted in seasonal and weak upwelling, thus lower productivity. High productivity characterized by a seasonally high input of labile organic matter, is indicated for times of orbital forced warming, such as the African Humid Period (9.8 to 7 kyr BP). An intensified African monsoon during boreal summer and the northernmost position of the tropical rainbelt within the last 31 kyr resulted in enhanced river discharge from the northward-extended drainage area initiating intense phytoplankton blooms. In the late Holocene (4 to 0 kyr BP) strong carbonate dissolution may have been caused by even more enhanced organic matter fluxes to the sea floor. Increasing aridity on the continent and stronger NE trade winds induced intensive, seasonal coastal upwelling.

Persistent summer expansion of the Atlantic Warm Pool during glacial abrupt cold events

Palaeoclimate records and numerical model simulations indicate that changes in tropical and subtropical sea surface temperatures and in the annual average position of the intertropical convergence zone are linked to high-latitude climate changes on millennial to glacial–interglacial timescales. It has recently been suggested that cooling in the high latitudes associated with abrupt climate-change events is evident primarily during the northern hemisphere winter, implying increased seasonality at these times8. However, it is unclear whether such a seasonal bias also exists for the low latitudes. Here we analyse the Mg/Ca ratios of surface-dwelling foraminifera to reconstruct sea surface temperatures in the northeastern Gulf of Mexico for the past 300,000 years. We suggest that sea surface temperatures are controlled by the migration of the northern boundary of the Atlantic Warm Pool, and hence the position of the intertropical convergence zone during boreal summer, and are relatively insensitive to winter conditions. Our results suggest that summer Atlantic Warm Pool expansion is primarily affected by glacial–interglacial variability and low-latitude summer insolation. Because a clear signature of rapid climate-change events, such as the Younger Dryas cold event, is lacking in our record, we conclude that high-latitude events seem to influence only the winter Caribbean climate conditions, consistent with the hypothesis of extreme northern-hemisphere seasonality during abrupt cooling events.

Holocene moisture evolution in arid central Asia and its out-of-phase relationship with Asian monsoon history

We synthesize palaeoclimate records from the mid-latitude arid Asian region dominated today by the Westerlies (“arid central Asia” (ACA)) to evaluate spatial and temporal patterns of moisture changes during the Holocene. Sediment records from 11 lakes with reliable chronologies and robust proxies were selected to reconstruct moisture histories based on a five-class ordinal wetness index with assigned scores from the driest to wettest periods at individual sites for 200-year time slices. The proxies used in these records include pollen and diatom assemblages, sediment lithology, lake levels, and geochemistry (mainly isotope) data. The results of our synthesis show that ACA as a whole experienced synchronous and coherent moisture changes during the Holocene, namely a dry early Holocene, a wetter (less dry) early to mid-Holocene, and a moderately wet late Holocene. During the early Holocene most of the lakes experienced very low water levels and even dried out before ca 8 ka (1 ka=1000 cal a BP). Hence the effective-moisture history in ACA is out-of-phase with that in monsoonal Asia as documented by numerous palaeoclimate records. In monsoonal Asia, a strong summer monsoon and humid climate characterized the early Holocene, and a weakened summer monsoon and drier climate prevailed during the late Holocene, which were mainly controlled by changes in low-latitude summer insolation. In contrast, we propose that the pattern of Holocene effective-moisture evolution in the westerly dominated ACA was mainly determined by North Atlantic sea-surface temperatures (SSTs) and high-latitude air temperatures that affect the availability, amount and transport of water vapor. Also, topography of the Tibetan Plateau and adjacent Asian highlands could have contributed to the intensification of dry climate in ACA during the early Holocene, as a result of strengthening the subsidence of dry air masses, associated with stronger uplift motion on the plateau by intense heating under a stronger summer insolation. Summer insolation might have played a key role in directly controlling moisture conditions in ACA but only after the northern hemisphere ice-sheets had disappeared in the mid- and late Holocene.

A methane-based time scale for Vostok ice

Tuning the Vostok methane signal to mid-July 30°N insolation yields a new ice-core gas time scale. This exercise has two rationales: (1) evidence supporting Kutzbach's theory that low-latitude summer insolation in the northern hemisphere controls the strength of tropical monsoons, and (2) interhemispheric CH 4 gradients showing that the main control of orbital-scale CH 4 variations is tropical (monsoonal) sources. The immediate basis for tuning CH 4 to mid-July insolation is the coincident timing of the most recent (pre-anthropogenic) CH 4 maximum at 11,000–10,500 calendar years ago and the most recent July 30°N insolation maximum (all ages in this paper are in calendar years unless specified as 14C years). The resulting CH 4 gas time scale diverges by as much as 15,000 years from the GT4 gas time scale (Petit et al., Nature 399 (1999) 429) prior to 250,000 years ago, but it matches fairly closely a time scale derived by tuning ice-core δ 18O atm to a lagged insolation signal (Shackleton, Science 289 (2000) 1897). Most offsets between the CH 4 and δ 18O atm time scales can be explained by assuming that tropical monsoons and ice sheets alternate in controlling the phase of the δ 18O atm signal. The CH 4 time scale provides an estimate of the timing of the Vostok CO 2 signal against SPECMAP marine δ 18O, often used as an index of global ice volume. On the CH 4 time scale, all CO 2 responses are highly coherent with SPECMAP δ 18O at the orbital periods. CO 2 leads δ 18O by 5000 years at 100,000 years (eccentricity), but the two signals are nearly in-phase at 41,000 years (obliquity) and 23,000 years (precession). The actual phasing between CO 2 and ice volume is difficult to infer because of likely SST overprints on the SPECMAP δ 18O signal. CO 2 could lead, or be in phase with, ice volume, but is unlikely to lag behind the ice response.

Initiation of the Australian summer monsoon 14,000 years ago

Definitive evidence for the onset of monsoon moisture in northern Australia following the arid phase of the Last Glacial Maximum has been elusive, in large part due to the lack of long, continuous sediment records. Here we report new evidence from two separate drainage basins in the Kimberley region of monsoonal northwestern Australia. At both sites, radiocarbon ages on (basal) biogenic materials in flood deposits indicate stream flow, as high or higher than maximum historic levels, began about 14 cal ka BP. These results suggest that the Australian summer monsoon was fully active several millennia earlier than previously expected. The early onset of the monsoon and the widespread evidence for diminished monsoon activity in the late Holocene cannot be reconciled with primary forcing of the Australian monsoon by changes in low-latitude summer insolation over Australia.