Abstract
Abstract A review of short-term (<3 myr: c. 100 kyr to 2.4 myr) Cretaceous sea-level fluctuations of several tens of metres indicates recent fundamental progress in understanding the underlying mechanisms for eustasy, both in timing and in correlation. Cretaceous third- and fourth-order hothouse sea-level changes, the sequence-stratigraphic framework, are linked to Milankovitch-type climate cycles, especially the longer-period sequence-building bands of 405 kyr and 1.2 myr. In the absence of continental ice sheets during Cretaceous hothouse phases (e.g. Cenomanian–Turonian), growing evidence indicates groundwater-related sea-level cycles: (1) the existence of Milankovitch-type humid-arid climate oscillations, proven via intense humid weathering records during times of regression and sea-level lowstands; (2) missing or inverse relationships of sea-level and the marine δ 18 O archives, i.e. the lack of a pronounced positive excursion, cooling signal during sea-level lowstands; and (3) the anti-phase relationship of sea and lake levels, attesting to high groundwater levels and charged continental aquifers during sea-level lowstands. This substantiates the aquifer-eustasy hypothesis. Rates of aquifer-eustatic sea-level change remain hard to decipher; however, reconstructions range from a very conservative minimum estimate of 0.04 mm a −1 (longer time intervals) to 0.7 mm a −1 (shorter, probably asymmetric cycles). Remarkably, aquifer-eustasy is recognized as a significant component for the Anthropocene sea-level budget.
Highlights
This paper focuses on Cretaceous short-term sealevel changes, and provides evidence for mechanisms for eustasy and approaches to their recognition and correlation
Sea-level was generally high compared with today (Conrad 2013), fluctuating within tens of millions of years, we identify shorter superimposed fluctuations with amplitudes of c. 10–40 m
In the greenhouse phase with humid climate, regressions are caused by warming (enhanced hydrological cycle and dominant aquifer-eustasy resulting in falling sea-level through strong continental groundwater storage – see the section ‘Oxygen isotopes’) whereas in icehouse phases, regressions are caused by cooling
Summary
Land-water storage (low-frequency) from the anthropogenic trend (high-frequency) in sea-level fluctuation (e.g. Hamlington et al 2017), both elements of which can either dampen or add to the general trend of continuously rising sea-level in today’s warming climate. In the advancing twenty-first century, right in the middle of the Anthropocene, we have been and are confronted by the consequences of a rapidly warming Earth, quickly waning glaciers and continental ice sheets, continuously rising global sealevel and the prospect that mankind is probably facing an anthropogenic greenhouse world in the very near future (within a few centuries) that will be largely ice-free Against this background, the combination of our insights from greenhouse intervals of Earth’s past with new insights derived from integrated satellite altimetry and gravimetry brings aquifer-eustasy into the scientific focus as an important driver of short-term sea-level changes. (1) The climatic cyclicities controlling glacioeustasy and aquifer-eustasy and the relative scales of their effects on short-term
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