CL1.8

The East Asian summer monsoon (EASM) is an important component of the climate system and it influences about one-third of the world’s population. Numerous paleoclimate records and climate simulations have been used to study its long-term evolution and response to different forcings. The strong regional dependence of the EASM variation questions the relative role of ice sheets and insolation on the EASM precipitation in different sub-regions in East Asia. A Gaussian emulator, which was generated and calibrated by interpolating the outputs of 61 snapshot simulations performed with the model HadCM3, is used to quantitatively assess how astronomical forcing, CO₂ and northern hemisphere ice sheets affect the variation of the summer precipitation over the last 800 ky. Our results show that in the north of 25°N of the EASM domain, the variation of the summer precipitation is dominated by precession and insolation. This leads to strong 23-ky cycles in the summer precipitation. However, in the southern part (south of 25°N), the impact of ice volume becomes more important, leading to strong 100-ky cycles. Ice sheets influence the summer precipitation in the south mainly through its control on the location of the Intertropical Convergence Zone (ITCZ) which is very sensitive to ice volume. ITCZ is shifted significantly to the south under large ice sheets conditions. Therefore, the region under control of the ITCZ is more sensitive to the influence of ice volume than other regions. Our results also show that obliquity and CO₂ have relatively small effect on the summer precipitation as compared to precession and ice sheets.

How to cite: Lyu, A., Yin, Q., Crucifix, M., and Sun, Y.: Diverse Regional Sensitivity of Summer Precipitation in East Asia to Ice Volume, CO2 and Astronomical Forcing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1051, https://doi.org/10.5194/egusphere-egu21-1051, 2021.

Glacial-to-interglacial variations in East Asian summer and winter monsoon are widely ascribed to orbital and/or global ice-sheet forcing. However, the relative impact of orbital and millennial-scale factors on Pleistocene variations in East Asian monsoon still remain controversial. To better constrain the differential response of seasonal monsoon winds over the last million years we present paired records of siliciclastic silt grain sizes, pollen, minerals, and geochemical tracers obtained from high-sedimentation rate deposits at ODP Sites 1144 and 1146 in the northern South China Sea. The proxy records show that loess-style dust supply of winter monsoon was dominant and fluvial input reduced during peak glacials over the last 900 kyr, moreover, during Heinrich stadials, while fluvial mud marked interglacial regimes as result of enhanced summer monsoon, then completely superimposing the weakened dust input of winter monsoon. A dominating superposition of fluvial mud on top of eolian dust, however, also prevailed during the initial part of most glacial stages during and after the Mid-Pleistocene Transition (MPT), in part possibly modulated by long-term groundwater reserves and/or unknown climate forcings linked to the southern Hemisphere. Prior to the MPT, during glacial stages 24–32, prolonged groundwater reserves and/or a more limited extent of northern-Hemisphere ice sheets, or unknown southern Hemisphere forcing may have controlled an ongoing interglacial-style humid climate in East Asia. In summary, our findings suggest that variations of sediment signals of seasonal East Asian monsoon variability in part may have been more sensitive to secondary factors of groundwater storage, plant cover as well as to the redistribution of insolation energy amongst various climate subsystems than to direct orbital and/or northern ice-sheet forcing.

How to cite: Huang, J. and Sarnthein, M.: One million years of seasonal seesaw in East Asian monsoon winds, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1914, https://doi.org/10.5194/egusphere-egu21-1914, 2021.

How to cite: De Vleeschouwer, D. and Vahlenkamp, M.: Modelling the sedimentary response to orbital variations., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2174, https://doi.org/10.5194/egusphere-egu21-2174, 2021.

The Pliocene - Pleistocene period (3.6-1.8 Ma) was a significant global cooling time, from very warm, equable climates to high-amplitude glacial-interglacial cycles. The origin of glaciers in the Northern Hemisphere, and the mechanisms by which glacial cycles have expanded since the late Pliocene, remain a subject of ongoing discussion. The studies of the Pliocene orbital scale climate evolution mainly are focused on marine sediments and loess-paleosoil sequences, however, there are few records of continental lacustrine facies during this period. Here we present a 37.6 m high-resolution Sanmen lacustrine sequences during the Pliocene-Pleistocene transition period that indicates the astronomically controlling East Asian climate transition and the Sanmen paloelake evolution. The Rb/Sr series evolution was divided into two parts for astronomical analysis based on the obvious changes observed in curve shape and Evolutionary spectral analysis through the section: 7.4-19 and 19-45 m. Based on evaluation of average accumulation rates from paleomagnetic results, the dominated ~99-cm cycles in the 7.4 to 19 m intervals represent ~41 kyr obliquity cycles. The 19 to 45 m intervals show obvious cycles at ~232-cm, interpreting as ~100 kyr eccentricity. Astronomical tuning combined with paleomagnetic results has been used to establish the 3.83-2.32 Ma high-precision astronomical scale. Rb/Sr series reveals that ~100 kyr eccentricity was the dominant control on lake expansion for Sanmen paleolake evolutionary before 2.75 Ma, after that, dominant obliquity control. Based on re-established the meridional sea surface temperature (SST) gradient between polar Atlantic borehole ODP 982 and the equatorial Atlantic borehole ODP 662, results show that the meridional sea surface temperature gradients increased significantly at 2.75 Ma, with cyclicity changing from the dominant ~140 kyr and ~95 kyr cycles to ~41 kyr at 2.75 Ma, and is coeval with our Rb/Sr record in the Weihe Basin. Crossspectral analysis show that the Rb/Sr and meridional SST gradient are strongly coherent and almost in-phase at these primary orbital periods in the past between 3.83-2.32 Ma. Thus, we conclude that the reorganization of the East Asian climate system at ~2.75 Ma, which coincided with the expansion of Arctic ice sheet, was a response to a dramatic cooling of the global climate and obliquity-driven changes in meridional SST gradients.

How to cite: Zhang, Z., Wang, Z., and Huang, C.: Orbital-scale climatic record in the North China across the Pliocene-Pleistocene transition, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2697, https://doi.org/10.5194/egusphere-egu21-2697, 2021.

Mineral dust is one of the environmental component for forcing the global climatic change, and not only influences the amount of solar radiation incoming the earth surface, but affects atmospheric CO₂ concentrations in the past through wind transport to ocean and subsequent biological pumping. Mineral dust is one of the important driving factors for variations of atmospheric CO₂ content in Quaternary glacial-interglacial cycles. Here, we reconstruct the interaction between the Asian dust flux (as a representative of the global dust flux), the cryosphere system (δ¹⁸O_benthic), and the global carbon cycle since 4 Ma using phase analysis, power decomposition analysis, obliquity sensitivity calculation and evolutionary spectral analysis. The evolutionary spectra show that orbital-scale variability of mineral dust, δ¹⁸O_benthic and δ¹³C_benthic are very similar over the past 4 Ma, except the interval time of 3-2 Ma that shows higher obliquity energy (higher O/T values) of the δ¹⁸O_benthic and δ¹³C_benthic data. Therefore, we suggest that the Asian and/or global dust is acted as a transmitter transporting the periodic signals stored in the Arctic ice sheet to deep-sea δ¹³C_benthic. This is why δ¹³C_benthic data have very similar changes with the Arctic ice sheets on the orbital scale. Sharp increase of global dust flux after 1.6 Ma resulted in a significant weakening of the 405 kyr long eccentricity power of δ¹³C_benthic series because Arctic ice sheet signals strongly inhibit the influences of low-latitude solar insolation variations on deep-sea δ¹³C_benthic system. In addition, we suggest that strengthened global drought and increases of dust fluxes since late Miocene probably forced the anti-phase relationship between δ¹⁸O_benthic and δ¹³C_benthic around 6 Ma, rather than the expansion of Arctic ice sheet. Our results highlight the close coupling between dust fluxes and the global carbon cycle, with deeply influencing marine productivity and land surface processes.

Keywords: mineral dust; deep sea oxygen isotope (δ¹⁸O_benthic ); deep sea carbon isotope(δ¹³C_benthic); orbital  periods ; inland Asia

How to cite: Cao, M., Wang, Z., Zhang, Z., and Xiao, A.: Mineral dust coupled with climate-carbon cycle on orbital timescales over the past 4 Ma, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4242, https://doi.org/10.5194/egusphere-egu21-4242, 2021.

Projecting future anthropogenic sea-level rise requires a comprehensive understanding of the mechanistic links between climate and short-term sea-level changes under a warming climate. Two different hypotheses, glacioeustasy and groundwater aquifer eustasy, have been proposed to explain short-term, high amplitude sea-level oscillations during past greenhouse intervals. However, the aquifer eustasy hypothesis – supported by subjective evidence of sequence stratigraphy in the Late Triassic greenhouse, has never been rigorously tested. Here we test these competing hypotheses using a recently proposed, objective approach of sedimentary noise modeling for both sea- and lake-level reconstructions for the first time. Sedimentation rate estimates and astronomical calibration of multiple paleoclimate proxies from the lacustrine Newark Basin and the marine Austrian Alps enable the construction of a highly resolved astronomical time scale for the Late Triassic. Using this timescale, sedimentary noise modeling for both lacustrine and marine successions is carried out through the Late Triassic. Lake level fluctuations reconstructed by sedimentary noise modeling and principal component analysis revealed that million-year scale lake-level variations were linked to astronomical forcing with periods of ~3.3 Myr, ~1.8 Myr, and ~1.2 Myr. Our objective water-depth reconstructions demonstrate that lake-level variations in the Newark Basin correlate with sea-level changes in the Austrian Alps, rejecting the aquifer eustasy hypothesis and supporting glacioeustasy as the sea-level driver for the Late Triassic. This study thus emphasizes the importance of high-resolution, objective reconstruction of sea- and lake-levels and supports the hypothesis that fluctuations in continental ice mass drove sea-level changes during the Late Triassic greenhouse.

How to cite: Wang, M., Li, M., Kemp, D. B., and Boulila, S.: Glacial-driven sea-level changes in the Late Triassic, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6117, https://doi.org/10.5194/egusphere-egu21-6117, 2021.

Gravitational interactions among masses in the solar system are recorded in Earth’s paleoclimate history because variations in the geometry of Earth’s orbit and axial orientation modulate solar insolation. However, astronomical models prior to ca. 60 Ma are unreliable due to the unpredictable nature of orbital chaos in the solar system, and therefore such models must be constrained using geological data. Here, we use natural gamma radioactivity and other environmental proxies from paleo-tropical Late Triassic lake deposits of the Newark Rift Basin of eastern North America, previously shown to be paced by variations in axial precession and orbital eccentricity and stratigraphically constrained by U-Pb dating, to explore hitherto undescribed strong variations in orbital inclination in the 201–206 Ma interval (lacustrine, upper Passaic Formation), where lake level variations are particularly muted. We identify the Earth-Saturn 173 kyr orbital inclination cycle and use it to tune the sequence because it exhibits high theoretical stability and metronomic behavior due to the very large mass of Saturn. We tune separately to long-eccentricity as well, with similar effect. Slight, complimentary offsets in the other inclination and eccentricity periods revealed by the Earth-Saturn (s3-s6) and Venus-Jupiter (g2-g5) tunings are apparent that may be due to chaotic variations of the secular fundamental frequencies in the nodal and perihelion orbital precessions of Earth and Venus, respectively. The surprising strength of the inclination cycles in this specific sequence suggest an additional modulating effect of the Earth System on expression of the components of orbital pacing of climate, as well a mechanism to more fully constrain the secular fundamental frequencies of the solar system beyond the ca. 60 Myr limit of predictability that chaos imposes on astronomical solutions.

How to cite: Margulis-Ohnuma, M., Whiteside, J., and Olsen, P.: Strong inclination pacing of climate in Late Triassic low latitudes revealed by the Earth-Saturn tilt cycle, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6638, https://doi.org/10.5194/egusphere-egu21-6638, 2021.

The lower Cambrian successions of Southern Morocco (West Gondwana) feature some of the oldest trilobites and archaeocyaths fossil remains in the world, as well as some of the largest carbon isotope excursions of the Phanerozoic. Combined with multiple state-of-the-art U-Pb radio-isotopic constraints, these sections are key references for lower Cambrian stratigraphy. We suggest that some of the regularly alternating lithological sequences, which show various orders of nested cyclicity, carry a signal of astronomical climate forcing. In agreement with a U-Pb Bayesian age model, the primary lithological alternation corresponds with the precession astronomical cycle while clear amplitude modulation patterns reflect a short-term eccentricity imprint. Both small- and large-scale features are laterally continuous at great distances. Changes in lithology may have been primary controlled by changes in terrigenous input. This integrated astrochronological age model has the potential to result in unprecedented early Cambrian timescales for major paleoenvironmental changes like the appearance of key fossils and large carbon isotope excursions. Often, lower Cambrian and Precambrian strata lack classical stratigraphical tools like good index fossils or magnetostratigraphy. In comparison with younger strata, the combined use of high-quality radio-isotopic dating and high-precision astrochronology might be even more crucial to disentangle important events in the early evolution of life and climate on our planet.

How to cite: Sinnesael, M., Millard, A. R., and Smith, M. R.: Astrochronology for the oldest Cambrian trilobites in Moroccan West Gondwana, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9782, https://doi.org/10.5194/egusphere-egu21-9782, 2021.

The late Miocene to early Pliocene was a time of global cooling, albeit in a warmer-than present climate state. Increased marine primary productivity characterizes this interval, often referred to as the late Miocene-early Pliocene biogenic bloom (~9-3.5 Ma). To explain its manifestation, paleoceanographers often involve ocean gateway or monsoon-related mechanisms, formulating hypotheses of increased or redistributed nutrients in the ocean. However, the exact cause-and-effect chains remain obscure, since important diachronicity is observed across ocean basins for the main phase and the termination of this event. Here, we compile proxy data for late Miocene to Pliocene paleoproductivity from all major ocean basins, including calcareous and siliceous plankton groups. By systematically evaluating the age-depth model accuracies of previously published records we demonstrate that a globally synchronous and long-sustained reduction in primary productivity was initiated with a sharp decline between 4.6 and 4.4 Ma. Our compilation supports that relatively rapid processes (~200 kyr) influenced nutrient availability towards the end of the biogenic bloom. By evaluating different mechanisms influencing the ocean nutrient budget on such time scales, we propose orbital forcing as an important candidate to have tipped the balance towards a less productive ocean. We show that this decline in productivity coincided with a prolonged period of low orbital eccentricity and a shift towards lower-amplitude obliquity. This specific astronomical configuration prevents the development of extreme seasonal contrasts which could lead to reduced nutrient supply to the ocean due to decreased riverine influx.

How to cite: Karatsolis, B. T., Lougheed, B., De Vleeschouwer, D., and Henderiks, J.: Long-term, sustained reduction in ocean productivity initiated at 4.6-4.4 Ma, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10071, https://doi.org/10.5194/egusphere-egu21-10071, 2021.

Paleoceanographic studies reconstructing surface paleoproductivity and benthic conditions allow us to measure the effectiveness of the biological pump, an important mechanism in the global climate system. In order to assess surface productivity changes and their effect on the sea-floor environment, a multiproxy paleoceanographic analysis was conducted on the core SAT-048A (1542 m.b.s.l.), recovered from the continental slope of the southernmost Brazilian continental margin, western South Atlantic. We assessed sea surface productivity using different planktonic foraminiferal proxies: (1) the relative abundances of the species Globigerina bulloides and Globigerinita glutinata and (2) the δ¹³C signal of shells of the species Globigerinoides ruber ruber. To assess the organic matter (OM) flux to the seafloor, the foraminiferal planktonic:benthic ratio and the δ¹³C signal of shells of the benthic foraminifer Uvigerina spp. were used. To study dissolution effects occurring at the sea-floor, the Fragmentation Intensity (i.e., the proportion of fragments and broken foraminiferal shells), the number of planktonic foraminiferal tests per gram of dry sediment, and the CaCO₃ and Sand contents of the sediment were measured. Superimposed on the climate-induced changes related to the last glacial-interglacial transition, the reconstruction indicates paleoproductivity changes synchronized with the precessional cycle. From the reconstructed data, it was possible to identify the glacial and postglacial stages: surface productivity, flux to the seafloor, and dissolution rates of planktonic foraminiferal tests where high during the glacial and low during the postglacial. Furthermore, within the glacial, enhanced productivity was associated with higher insolation values, which can be explained by increased NE summer winds that strengthened the Brazil Current transport and, in turn, promoted meandering and upwelling of the nutrient rich South Atlantic Central Water. Changes in the Atlantic Meridional Overturning Circulation and the reorganization of bottom water masses may change the CO₃^2- saturation levels and, consequently, influence carbonate preservation. However, the δ¹³C values from shells of Uvigerina spp. are different from present-day δ¹³C values from dissolved inorganic carbon for the Upper Circumpolar Deep Water and the North Atlantic Deep Water, which is likely linked to varying OM fluxes. Future studies (e.g., εNd in benthic Foraminifera) must quantify the effect of the reorganization of the bottom water masses on the dissolution of the planktonic foraminiferal tests, to better understand the effect of the biological pump removing carbon from the seawater and its subsequent sequestration in the seafloor sediments.

How to cite: Suarez Ibarra, J. Y., Fraga Frozza, C., Monticelli Petró, S., de Lara Palhano, P., and Gómez Pivel, M. A.: Orbital cycle-related benthic-pelagic fluctuations in Foraminifera during the last glacial-interglacial interval in the western South Atlantic, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10245, https://doi.org/10.5194/egusphere-egu21-10245, 2021.

How to cite: Leloup, G. and Paillard, D.: An attempt to understand δ13C cycles with a simple conceptual model., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11364, https://doi.org/10.5194/egusphere-egu21-11364, 2021.

Fire regimes are changing due to anthropogenic climatic drivers and fuel management challenges in all regions of Earth. However, the planet is also subject to natural background variability due to Earth’s orbital parameters (Milkankovitch cycles). To date no studies have assessed a sedimentary record that is sufficiently long or has a resolution that is high enough to assess both long eccentricity and precessional forcings on fire.  Here we present a ~350,000 yr record of wildfire activity, using fossil charcoal from Jurassic sediments.

The studied interval is part of the astronomically constrained Upper Pliensbachian of the Mochras borehole, Cardigan Bay Basin. The site was located within the Laurasian Seaway, south of the Viking Corridor that linked the north-western Tethys Ocean to the Boreal Sea, at a palaeolatitude of ~35°N. Clear lithological couplets of carbonate-rich and TOC-enhanced beds are observed, which show an orbital control on deposition. High resolution macrocharcoal (>125 um) and microcharcoal (10-125 um) abundance data have been obtained at a ~2 ky resolution over the studied interval. Charcoal data are coupled to estimates of variations in the hydrological cycle using clay mineral analyses, along with palynofacies and elemental analyses, and lithological and biogeochemical signatures.

We show that fire activity was strongly increased during (1) a period of maximum eccentricity (405,000 yr cycle) and (2) inferred maximum in seasonal contrast due to precession (20,000 yr cycles). In these periods with a strong seasonality, charcoal abundance indicates enhanced wildfire activity. This is coupled to a more seasonal pattern of rainfall as indicated by the relative abundance of smectite versus kaolinite. We argue that the shift to a more seasonal climate would have led to the increase in dry-adapted conifer forests. Conifers have biochemical and morphological traits that make them particularly flammable whether dry or live.This climate induced change in vegetation contributed to increased wildfire activity in the seasonal dry periods.

Increase in wildfire activity on an orbital time scale indicates that currently wildfires should be suppressed as Earth is close to an eccentricity minimum, such that man may have counteracted a relatively fire limited period.

How to cite: Hollaar, T. P., Baker, S. B., Hesselbo, S. P., Deconinck, J.-F., Mander, L., Ruhl, M., and Belcher, C. M.: Long and short orbital forcing of Jurassic wildfires., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15594, https://doi.org/10.5194/egusphere-egu21-15594, 2021.