Astronomically forced changes in groundwater reservoirs and sea level during the greenhouse world

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<p indent="0mm">Sea level rise caused by global warming is one of the biggest challenges facing humankind. Deepening the understanding of the mechanism of global sea level change during the greenhouse period is the key to meeting this challenge. Large-scale, rapid sea-level changes frequently occurred during the greenhouse period lacking ice sheets. However, conventional sea level change models cannot explain this phenomenon, exposing their shortcomings. Estimation of global pore space in sediments and big data studies have indicated that global sea level changes may also be related to climate-related continental groundwater volume changes. We review the mechanism of sea level change caused by astronomically driven continental groundwater activities and propose a new Sponge Continent hypothesis: astronomical forcing drives climate change, causing filling and discharge of continental aquifers (by analogy, a sponge), which may in turn impact large-scale changes in global sea levels and inland lake level during the greenhouse period. We summarize recent technological breakthroughs related to the reconstruction of deep time sea- and lake-level changes. First, lake level change has been proposed as an indicator for the fluctuation of continental groundwater volume. Second, the sedimentary noise model and SediRate-Fischer method were proposed for the reconstruction of sea- and lake-level changes from sedimentary successions. Among these, the sedimentary noise model is developed based on the principle of the Milankovitch theory of climate change and time series analysis techniques. The noise model contains two complementary methods: dynamic noise after orbital tuning (DYNOT) and lag-1 autocorrelation coefficient (<italic>ρ</italic>₁). The DYNOT method measures the noise intensity in paleoclimate time series, i.e., the ratio of non-Milankovitch variation to total variation. By calculating the lag-1 autocorrelation coefficient of the target paleoclimate time series, the <italic>ρ</italic>₁ model can independently test the noise changes related to sea level variations. The <italic>ρ</italic>₁ value decreases with the increase of noise intensity, and vice versa. The relative intensity of the sedimentary noise is anti-phased with the sea level change. When the sea level increases, the sedimentary noise decreases, and vice versa. This model can also be applied to the studies of lake level changes. Third, cyclo-magnetostratigraphy can provide a global synchronous stratigraphic correlation framework, which is vital for the evaluation of the aquifer eustasy and Sponge Continent hypothesis. The hypothesis of aquifer eustasy and Sponge Continent can be supported by geological evidence from the greenhouse period. This paper reviews recent progress related to astronomically forced sea level changes and/or lake level changes in chronological order. Some of them support the predicted antiphase relationship between the sea level curve and the contemptuous lake level curve as predicted by the Sponge Continent hypothesis. However, many unsolved issues about the nature and mechanisms of sea level change are to be explored. First, estimates of groundwater volume remain in dispute. Second, there is a lack of quantitative indicators for deep time groundwater variations. Third, the timing and mechanism of groundwater changes in continental aquifers are still unconstrained. Potential research directions are also discussed, including groundwater storage estimate, research methods, storage-drainage mechanism of groundwater reservoirs, etc.