Astronomical Forcing And Sedimentary Noise Modeling Of Lake-level Changes

Challenges in interpreting continental sequence stratigraphy and typically uncertain geochronology hinder the understanding of paleolake evolution and hydrocarbon exploration in terrestrial basins.Lake-level changes directly control the type and distribution of lacustrine deposits and are closely related to the formation and distribution of petroleum and mineral resources.It is important to reveal the process and mechanism of terrestrial stratigraphic sequence and lake-level changes.Long-period high-precision lake level reconstruction and the mechanism of lake-level variation remains unclear.The sedimentary noise model is a new approach to sea-level reconstruction based on the study of cyclotomic stratigraphy,which has been proposed in recent years.This model quantifies the signal-noise profile of climate proxies and shows that the signal-to-noise ratio of the sedimentary record is related to the relative water depth of the paleo-ocean.When the water depth is deeper,the sedimentary noise recorded by paleoclimate proxies will be lower than when the sea level is shallower,and vice versa.Therefore,the sea-level changes can be reconstructed by analyzing the sediment noise.This study extends this approach of sea-level reconstruction to lake-level change for the first time and test the validity of sedimentary noise model for lake-level reconstruction with abundant evidence.It finds a new way of studying the long-term and high-resolution lake level changes.We recalibrated astronomical age scales using multiple paleoclimate proxies for the Triassic Newark Basin and the Paleogene Dongpu and Qianjiang Depressions.Based on cyclostrarigraphyic results,the sedimentary noise model is used to quantitatively reconstruct high-resolution lake-level changes and discuss the astronomical forcing in the lake-level changes.We also explore the mechanisms of response to global paleoclimatic events,land-sea water exchange.Long-period lacustrine records and numerous chronostratigraphic studies in the Triassic Newark Basin provide a perfect opportunity to reconstruct lake-level changes and assess the robustness of the sedimentary noise model in terrestrial basins.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 reveal that million-year scale lake-level variations were linked to astronomical forcing with periods of ~3.3Myr,~1.8Myr,and ~1.2Myr.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.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 lakelevels and supports the hypothesis that fluctuations in continental ice mass drove sealevel changes during the Late Triassic greenhouse.The Dongpu and Qianjiang Depressions are lacustrine basins with abundant hydrocarbon resources.An accurate chronology for the Paleogene stratigraphy is lacking,and the mechanism of lake-level variation remains unclear.In this study,we utilize highresolution gamma ray logs to conduct a cyclostratigraphic analysis of the Shahejie,Jinghezhen and Qianjiang Formations,which are the successions characterized by sandstones and dark mudstones interbedded with thin salt rocks.Time series analysis reveals evidence for 405kyr eccentricity cycles in the gamma ray series,which is supported by statistical modeling of optimal sedimentation rates.Tuning of the gamma ray data to this 405kyr eccentricity cyclicity enables a long astronomical time scale to be constructed.This astrochronology is anchored to the astronomical age of the Dongying Formation/Shahejie Formation boundary(28.86Ma)in the Bohai Bay Basin and Paleogene/Eocene boundary(23.03Ma),thus providing absolute timescales for the studied interval.Using this anchored astrochronology,we show that a recently established sedimentary noise model for inferring sea level change in marginal marine settings can be used to similarly infer lake level fluctuations in terrestrial basins.In addition,the lake level changes in the reconstructed Paleogene Dongpu and Qianjiang Depressions are in response to the global paleoclimate.During the Eocene and Oligocene transitions around 33.9Ma,both Dongpu and Qianjiang Depressions experienced large-scale lake level decline,which is consistent with the global Oi-1cooling event,while the global hot event period also corresponds to the lake level rise.During the Eocene,the lake level change curve shows a significant 1.2Myr cycle that is in anti-phase with the 1.2Myr amplitude modulation cycle predicted by astronomical solutions,while during the Oligocene,the lake-level record is in-phase with the 1.2Myr cycle.This phase shift is likely related to the occurrence of the Antarctic ice sheet during the Eocene-Oligocene transition.It also reveals that the lake level variation is controlled by both the astronomical forcing of 2.4Myr and 4.6Myr,and the lake level record has an anti-phase variation relationship with these two cycles predicted by astronomical theoretical models.It is suggested that relatively strong rainfall during the period of maximum eccentricity may lead to enrichment of continental groundwater and decrease in the sea level,and vice versa.Previous studies have suggested that the depression may have experienced sea erosion during the Paleogene,and therefore the change of lake level is likely to be directly controlled by sea level,which provides a unique opportunity to test the aquifer eustasy hypothesis.In particular,sedimentary noise modeling of the tuned gamma ray series reveals high-resolution changes in sedimentary noise that are indicative of lake-level variations linked to million-year scale astronomical forcing,such as 2.4Myr and 1.2Myr.These inferred changes in lake level are supported by previously published sequence stratigraphic interpretations.Moreover,an evolutionary correlation coefficient(COCO)analysis of the gamma ray series also indicates recurrent distortions in sedimentation that may be linked to lake level changes.This study provides new methods for the assessment of paleolake level variations,as well as insights into the connection between astronomical forcing and lake evolution across long timescales in terrestrial basins.