Climate‐environmental Deteriorations in a Greenhouse Earth System: Causes and Consequences of Short‐Term Cretaceous Sea‐Level Changes (a Report on IGCP 609)

response to enhanced atmospheric greenhouse gases and melting of the Earth’s continental ice shields have become issues of continuously growing interest for the scientific community as well as the public, pointing to threads of societies in a future greenhouse Earth System. As the sea level constitutes a fundamental boundary for (human) life on our planet and sealevel changes drive not only major shifts in the landscape but endanger also the greater part of big cities of the world, a global sea-level rise even on the scale of a few meters has major impact on mankind. Main drivers of recent short-term (on a geological time frame) sea-level rise initiated by greenhouse global warming are accelerated discharge of melt water from continental ice shields and mountain glaciers into the oceans and thermal expansion of seawater and potentially the oceanic forcing of ice sheet retreat on ice shelves as well. The processes and coherences behind are highly complex, and coupled feedback mechanisms of global warming are considered to affect global climate and, thus, the whole continents as well. The Cretaceous greenhouse period provides a deep-time view on greenhouse phase Earth System processes and planetary boundaries (e.g. Hu et al., 2012; Hay, 2017), and provides invaluable data for a better understanding of the causes and consequences of global (eustatic) short-term sea-level changes over a very long-time interval with different, intermittently ‘extreme’ climates. In that, the Cretaceous greenhouse, especially the mid-Cretaceous (Aptian to Turonian, ca. 126–66 Ma) hothouse periodserves as a natural laboratory to learn for a future greenhouse Earth System out of the glacial-interglacial cyclicity of the Pleistocene. IGCP 609 (Li et al., 2016) addressed correlation, causes and consequences of significant short-term (cycles of 3 and 4 order, i.e. about 0.5–3.0 Ma and a few tens of thousands to 0.5 Ma, respectively) sea-level changes which are recorded in Cretaceous sedimentary sequences worldwide. Such cyclic sealevel changes and corresponding sequences are usually explained by the waxing and waning of continental (polar) ice sheets. However, though Cretaceous eustasy involves processes like brief glacial episodes for which evidence has been given (and resulting glacio-eustasy models, e.g. Miller et al., 2005) the presence of continental ice sheets during the Cretaceous is still disputed, and remains particularly enigmatic for the midCretaceous hothouse episodes and global average temperature maxima during the Cenomanian to Turonian (Hu et al., 2012). IGCP 609 placed emphasis on the causes and mechanisms of short-term eustatic sea-level changes in the mid-Cretaceous hothouse during which the presence of continental ice sheets was highly improbable and, thus, other mechanisms have to be taken into consideration to explain significant short-term eustatic changes. Major progress within the last six years 2013–2018 concern: (1) Cretaceous sequence stratigraphy put into a numerical time frame (e.g., Haq, 2014; Haq and Huber, 2017). Major mechanisms for global and regional sea-level changes have been quantified, and regional, tectonically induced, versus global mechanisms for sea-level change were discussed and quantified (Wagreich et al., 2016; Sames et al., 2016; see Fig. 1). (2) Various proxy correlations, such as oxygen and carbon isotopes, and interpretations for sea-level reconstructions in the Cretaceous were challenged and feedback mechanisms were evaluated in detail (e.g. Wendler and Wendler, 2016). 3) Orbital forcing in the (long) Milankovitch band was identified as the main driver of sea-level cycles also during greenhouse times (e.g. Wendler et al., 2014, 2016a) pointing to climate control of continental water storage. (4) Processes and triggering mechanisms of short-term sea-level fluctuations during greenhouse periods remain controversial, but evidence is growing for the revival of the ‘aquifer-eustasy’ (Fig. 1) and‘limno-eustasy’ hypotheses (Hay and Leslie, 1990; Föllmi, 2012).(5) Against the background that short-term cyclic sea-level changes are climate driven and, thus, ultimately orbitally controlled – and that, besides the characteristic remnant magnetization, this is the only signal potentially recorded in contemporaneous non-marine deposits as well – the stratigraphic application of short-term climate cycles more and more comes under scrutiny as promising tool for non-marine to marine correlations (e.g. Sames, 2017). (6) Case studies within the project came from various regions of the world; published studies include, e.g., Austria (Neuhuber et al., 2016; Wolfgring et al., 2018b), China (Xi et al., 2016; Wu et al., 2017; Li et al., 2018), Egypt (Fathy et al., 2018), Tanzania (Wendler et al., 2016a), Jordan (Wendler et al., 2016b), Pakistan (Iqbal et al., 2019), Spain (Socorro et al., 2017), Turkey (Yilmaz et al., 2018; Wolfgring et al., 2018a), NW Europe (Hart et al., 2016), Russia Climate-environmental Deteriorations in a Greenhouse Earth System: Causes and Consequences of Short-Term Cretaceous Sea-Level Changes (a Report on IGCP 609)