Millennial-scale Climate Cycles Research Articles

Millennial to centennial-scale climate oscillations since 15000 cal yrs BP from Kanwar wetland in the Central Ganga Plain, India

Millennial-scale climate cycles have been meagerly discussed in the terrestrial records from the Indian subcontinent, particularly the wetlands of the Ganga Plain. The mineral magnetic and textural analyses, along with chronology, distinctly record variation in the monsoon intensity since 15000 cal yrs BP in the sediment core collected from the Kanwar Lake in the Central Ganga Plain (CGP) of India. This unique high-resolution data allowed us to analyse monsoon fluctuation in the CGP with respect to changes in the North Atlantic process (internal) and external (solar cycles) forcing. During this period, several palaeoclimatic events, such as the Bølling-Allerød (B/A), Older and Younger Dryas, and the high-frequency fluctuations in the Holocene viz., Greenlandian, Northgrippian, and Meghalayan stages, were observed. The B/A event is correlated with the Indian Summer Monsoon (ISM) intensification period of the CGP. Eleven dry events are recorded at 13600–13200, 13000–11700, 11000, 10600, 9800–9000, 8200, 6100–5300, 4300–4200, 3800, 1800–900 and 300 cal yrs BP. During the Holocene, the sustained episodes of weaker monsoon centered around 10600, 9600, 8200, 6000, 4200, 2800, 1400 and 600 cal yrs BP, where the variability of the proxy data is mostly synchronous with the reduced upwelling intensity in the western Arabian Sea as well as roughly align with ∼1500 years cyclic signals in the drifted ice proxy recorded Bond Events indicating weakened ISM circulation during colder North Atlantic. Two distinct short cycles, spanning 172–344 and 172–688 years, are evident around 12,000 and 14,000 cal yrs BP with an additional occurrence from 86 to 344 years during 2000 and 6000 cal yrs BP, affirming the significance of external forcing, particularly solar activity. For the 1492 years cycle in ISM fluctuation, we postulate it may represent a combined forcing of external (solar) and internal (Atlantic Meridional Overturning Circulation (AMOC)) factors during the Holocene. This is the first high-resolution record available in the CGP, which brings out crucial monsoon reconstruction during the last 15000 years.

The synchrosqueezing transform to evaluate paleoclimate cyclicity

Centennial and millennial-scale climate cycles are observed in paleoclimate proxies distributed globally. For example, periods of ∼2.3, ∼1.5, ∼1, and ≤0.5 thousand years (kyr) are reported in the Holocene, and numerous studies have related cycles with such periods to fluctuations in solar intensity received at the Earth's surface. For the 1.5 kyr cycle, however, there is still much uncertainty surrounding the exact period and presence of the oscillation. This uncertainty is primarily due to difficulty distinguishing cycles from one another; distinguishing this 1.5 kyr cycle from surrounding cycles and determining the onset and cessation of this cycle is crucial to the evaluation of the mechanism driving this cycle. Commonly methods for paleoclimate spectral analysis, such as the Continuous Wavelet Transform and Short-Time Fourier Transform cannot clearly reveal the onset or cessation of sporadic climatic cycles if neighboring spectral peaks are present within an octave due to spectral smearing and leakage. Here, we present a novel application of the Synchrosqueezing Transform in paleoclimate research to obtain improved separation of climate cycles and to more precisely determine the onset and cessation of cycles that are sporadic through the paleoclimate proxy time-series. The SST and its inverse provide a more exact estimate of the onset and cessation of which is useful in comparing to the spectra of other paleoclimate proxies to determine the drivers of specific climate oscillations. We demonstrate that application of the SST to a range of Holocene climate proxies provides consistent identification of the 1.5 kyr cycle.

Subglacial precipitates record Antarctic ice sheet response to late Pleistocene millennial climate cycles

Ice cores and offshore sedimentary records demonstrate enhanced ice loss along Antarctic coastal margins during millennial-scale warm intervals within the last glacial termination. However, the distal location and short temporal coverage of these records leads to uncertainty in both the spatial footprint of ice loss, and whether millennial-scale ice response occurs outside of glacial terminations. Here we present a >100kyr archive of periodic transitions in subglacial precipitate mineralogy that are synchronous with Late Pleistocene millennial-scale climate cycles. Geochemical and geochronologic data provide evidence for opal formation during cold periods via cryoconcentration of subglacial brine, and calcite formation during warm periods through the addition of subglacial meltwater originating from the ice sheet interior. These freeze-flush cycles represent cyclic changes in subglacial hydrologic-connectivity driven by ice sheet velocity fluctuations. Our findings imply that oscillating Southern Ocean temperatures drive a dynamic response in the Antarctic ice sheet on millennial timescales, regardless of the background climate state.

Evolution of marine sedimentation in the Bering Sea since the Pliocene

Sediment of the Bering Sea, derived mainly from biogenic, glaciomarine, and, secondarily, riverine sources, reflects the history of oceanographic changes within the basin and climatic changes on the adjacent continents. Integrated Ocean Drilling Program (IODP) Expedition 323 recovered cores that reveal the evolution of sedimentation in the Bering Sea over the past 5 m.y., a period that includes globally significant events such as the early Pliocene warm period, the onset of extensive Northern Hemisphere glaciation, and the Pleistocene glacial-interglacial and millennial-scale climate cycles. To begin to understand the Bering Sea regional response to and role in these global climate change events, we examined the sedimentary constituents of Expedition 323 sites U1339, U1343, and U1344 on the Bering Slope, and U1340 and U1341 on Bowers Ridge. New particle size and petrographic analyses, combined with shipboard lithostratigraphic and physical property data, are used to characterize sediment types and texture and its distribution through space and time. The sediment comprises mainly two components, opaline diatom valves and siliciclastic grains (mainly clay and fine silt size). Approximately 40% of the variance in particle size can be explained by the abundance and preservation of diatom valves, a rough indicator of biogenic opal productivity. Particle size data indicate that productivity was generally higher during interglacials compared to glacials, and higher during the Pliocene warm period, decreasing as Northern Hemisphere glaciation intensified ∼3 m.y. ago. Although the abundance of diatoms in the sediment varied, diatom ooze and diatom mud are the dominant lithologies at Bowers Ridge, indicating that there was a persistent supply of diatoms to the sediment in the open Bering Sea during the past 5 m.y. This study provides a comprehensive view of sediment types and sedimentation processes; future work should be aimed at validating our interpretations of past changes in productivity and siliciclastic sedimentation mechanisms with multiple additional proxies.

Astronomically calibrated 40 Ar/ 39 Ar age for the Toba supereruption and global synchronization of late Quaternary records

The Toba supereruption in Sumatra, ∼74 thousand years (ka) ago, was the largest terrestrial volcanic event of the Quaternary. Ash and sulfate aerosols were deposited in both hemispheres, forming a time-marker horizon that can be used to synchronize late Quaternary records globally. A precise numerical age for this event has proved elusive, with dating uncertainties larger than the millennial-scale climate cycles that characterized this period. We report an astronomically calibrated (40)Ar/(39)Ar age of 73.88 ± 0.32 ka (1σ, full external errors) for sanidine crystals extracted from Toba deposits in the Lenggong Valley, Malaysia, 350 km from the eruption source and 6 km from an archaeological site with stone artifacts buried by ash. If these artifacts were made by Homo sapiens, as has been suggested, then our age indicates that modern humans had reached Southeast Asia by ∼74 ka ago. Our (40)Ar/(39)Ar age is an order-of-magnitude more precise than previous estimates, resolving the timing of the eruption to the middle of the cold interval between Dansgaard-Oeschger events 20 and 19, when a peak in sulfate concentration occurred as registered by Greenland ice cores. This peak is followed by a ∼10 °C drop in the Greenland surface temperature over ∼150 y, revealing the possible climatic impact of the eruption. Our (40)Ar/(39)Ar age also provides a high-precision calibration point for other ice, marine, and terrestrial archives containing Toba sulfates and ash, facilitating their global synchronization at unprecedented resolution for a critical period in Earth and human history beyond the range of (14)C dating.

Millennial-scale climate cycles in Permian–Carboniferous rhythmites: Permanent feature throughout geologic time?

Two late Paleozoic glacial rhythmite successions from the Itarare Group (Parana Basin, Brazil) were examined for paleoclimate variations. Paleomagnetic (characteristic remanent magnetization, ChRM) and magnetic susceptibility (K z ) measurements taken from the rhythmites are interpreted as paleoclimatic proxies. Ratios of low-frequency components in the K z variations suggest Milankovitch periodicities; this leads to recognition of other, millennial-scale variations reminiscent of abrupt climate changes during late Quaternary time, and are suggestive of Bond cycles and the 2.4 k.y. solar cycle. We infer from these patterns that millennial-scale climate change is not restricted to the Quaternary Period, and that millennial forcing mechanisms may have been prevalent throughout geologic time.

Orbital and millennial-scale environmental and sedimentological changes in the Okhotsk Sea during the last 350 kyr

The chlorine content (CC), magnetic susceptibility, water content, density and coarse sediment fraction (CF/IRD) of an Okhotsk Sea sediment core PC-7R have been measured for the last 350 kyr. The PC-7R age model was constructed by matching the dated series of relative paleointensity lows recognized in geomagnetic intensity records, marine isotope stages (MIS) determined in the lithophysical proxies, CC and color reflectance b* variability and tephrochronology. The orbital changes of the OS lithophysical proxy D, WC and CF (IRD) lag behind Northern Hemispheric summer radiation around 6–12 kyr consistently with M. Milankovich's view of climate control by solar radiation through the northern ice sheets volume. On the basis of inphasing of the millennial-scale climate cycles in the North Hemisphere for last 75 kyr, we suggest that some 48 pronounced cold events observed in the OS records during MIS 2–10 are Heinrich-equivalent event anomalies, which have counterparts in climate records of the North Atlantic, Greenland and East Asian.

Atmospheric CO 2 and Climate on Millennial Time Scales During the Last Glacial Period

Reconstructions of ancient atmospheric carbon dioxide (CO2) variations help us better understand how the global carbon cycle and climate are linked. We compared CO2 variations on millennial time scales between 20,000 and 90,000 years ago with an Antarctic temperature proxy and records of abrupt climate change in the Northern Hemisphere. CO2 concentration and Antarctic temperature were positively correlated over millennial-scale climate cycles, implying a strong connection to Southern Ocean processes. Evidence from marine sediment proxies indicates that CO2 concentration rose most rapidly when North Atlantic Deep Water shoaled and stratification in the Southern Ocean was reduced. These increases in CO2 concentration occurred during stadial (cold) periods in the Northern Hemisphere, several thousand years before abrupt warming events in Greenland.

Geographic coherence of millennial-scale climate cycles during the Holocene

Varve thickness measurements and variation in sediment texture have been compiled for an intermittently varved sequence of Holocene sediments at ODP Site 893 in the Santa Barbara Basin (SBB), off California. A varve-based age model is generated from the varve counts, using interpolation across homogeneous intervals. With this age model, the varve thickness and sediment texture time series are compared to published GISP2 stable oxygen isotope and aerosol data as well as to the atmospheric Δ 14C record. Variation in sediment texture at Site 893 shows the presence of a ∼ 1000 year cycle, which is attributed to fluctuations in the degree of ventilation of the basin. Cross spectral analysis shows that this cycle is coherent with the GISP2 δ 18O record and with atmospheric Δ 14C throughout the Holocene. The correlation with the Δ 14C record indicates that this ∼ 1000 year cycle is related to solar cycles, which appear to have a noticeable impact especially on circum-North Atlantic climate. The presence of the 1000 year cycle in the SBB is attributed to a tele-connection with the Northern high latitudes, most likely via intermediate water formation in the North Pacific. In contrast, the GISP2 aerosol record and variation in varve thickness at ODP Site 893 show a coherent ∼ 2750 year cycle, which does not correlate with the ∼ 1000 year cycle. Varve thickness in SBB has been shown to correlate with annual rainfall, which occurs mainly during winter when prevailing winds slacken. Both the GISP2 aerosol and SBB varve thickness records are therefore interpreted as a measure for a combination of wind stress and aridity. The correlation is negative, in that thick varves/wet climate in SBB occur during times that the aerosol load is high at GISP2 (dry). The presence of two distinct patterns of millennial scale variation points to regional trends in climate change during the Holocene. However, the fact that both patterns occur at two distant localities suggest that they represent differential behaviour of supra-regional climate systems of which the effects can be traced over large parts of the globe.

Linking continental-slope failures and climate change: Testing the clathrate gun hypothesis

It has been suggested that the release of clathrates rather than expansion of wetlands is the primary cause of the rapid increases observed in the ice-core atmospheric methane record during the Pleistocene. Because submarine sediment failures can involve as much as 5000 Gt of sediment and have the capacity to release vast quantities of methane hydrates, one of the major tests of the clathrate gun hypothesis is determining whether the periods of enhanced continental-slope failure and atmospheric methane correlate. To test the clathrate gun hypothesis, we have collated published dates for submarine sediment failures in the North Atlantic sector and correlated them with climatic change for the past 45 k.y. More than 70% by volume of continental-slope failures during the past 45 k.y. was displaced in two periods, between 15 and 13 ka and between 11 and 8 ka. Both these intervals correlate with rising sea level and peaks in the methane record during the Bolling-Allerod and Preboreal periods. These data support the clathrate gun hypothesis for glacial-interglacial transitions. The data do not, however, support the clathrate gun hypothesis for glacial millennial-scale climate cycles, because the occurrence of sediment failures correlates with Heinrich events, i.e., lows in sea level and atmospheric methane. A secondary use of this data set is the insight into the possible cause of continental-slope failures. Glacial-period slope failures occur mainly in the low latitudes and are associated with lowering sea level. This finding suggests that reduced hydrostatic pressure and the associated destabilization of gas hydrates may be the primary cause. The Bolling-Allerod sediment failures were predominantly low latitude, suggesting an early tropical response to deglaciation, e.g., enhanced precipitation and sediment load to the continental shelf or warming of intermediate waters. In contrast, sediment failures during the Preboreal period and the majority of the Holocene occurred in the high latitudes, suggesting either isostatic rebound–related earthquake activity or reduced hydrostatic pressure caused by isostatic rebound, causing destabilization of gas hydrates.

Ice sheet-thermohaline circulation interactions in a climate model of intermediate complexity

A vertically integrated dynamic ice sheet model is coupled to the atmosphere-ocean-sea ice-land surface climate model recently developed by Wang and Mysak (2000). The background lateral (east-west) ice sheet discharge rate used by Gallee et al. (1992) is reduced and the planetary emissivity is increased (to parameterize the cooling effect of a decrease of the atmospheric CO2 concentration), in order to build up substantial ice sheets during a glacial period and hence set the stage for ice sheet-thermohaline circulation (THC) interactions. The following iceberg calving scheme is then introduced: when the maximum model height of the North American ice sheet reaches a critical value (2400 m), a prescribed lateral discharged rate is imposed on top of the background discharge rate for a finite time. Per a small prescribed discharge rate, repeated small iceberg calving events occur, which lead to millennial-scale climate cycles with small amplitudes. These are a crude representation of Dansgaard-Oeschger oscillations. Over one such cycle, the zonally averaged January surface air temperature (SAT) drops about 1.5°C at 72.5°N. However, a large prescribed lateral discharge rate leads to the shut down of the THC. In this case, the January SAT drops about 5°C at 72.5°N, the sea ice extent advances equatorward from 57.5° to 47.5°N and the net ice accumulation rate at the grid of maximum ice sheet height is reduced from 0.24 to 0.15 m/y. Since data strongly suggest that a collapsed THC was not a steady state during the last glacial, we restore the THC by increasing the vertical diffusivity in the North Atlantic Ocean for a finite time. The resulting climate cycles associated with conveyor-on and conveyor-off phases have much larger amplitudes; furthermore, the strong iceberg calving events lead to a larger loss of ice sheet mass and hence the period of the oscillations is longer (several thousand years).