Genesis and Architecture of Sequences of Quaternary Coral Reef Terraces: Insights From Numerical Models

Abstract

The variety of coral reefs morphologies highlights their sensitivities to several forcings; fossil reefs stack in sequences that are accordingly diverse. In order to understand their genesis and architectures, we devised a numerical approach, accounting for Quaternary sea level oscillations, vertical land motion, initial slope, wave erosion, and reef growth. We first test our model on the subsiding sequence of Hawaii and on the uplifting sequence of Wangi‐Wangi (Sulawesi) that bears active barriers. We then construct a parametric study that we analyze based on a comprehensive yet compact description of sequences as barcodes, that depict the vertical distribution of a few geometrical characteristics (number, width, and height of the terraces and barriers). We find that geological factors suffice to explain the variety of architectures of reefal sequences at first order, regardless of additional ecosystemic processes. Vertical land motion and foundation slopes are the prime players, while reef growth rates only play a minor role. Barriers may develop both in uplift and subsidence mode, and their preservation attests for the erosional power. Last, we reappraise the genesis of sequences and find that sequences do not fingerprint discrete events of sea level oscillations but a continuous process harrowed by stochastic events: Major sea level fluctuations can be overrepresented by several terraces or conversely absent; reoccupations may yield composite terraces representing multiple events. Overall, sequences shall not be regarded as stacks of reef bodies forming during sea level highstands, which implies that the commonly assumed bijective relationship between sea level highstands and terraces shall be abandoned.

How do sea-level curves influence modeled marine terrace sequences?

“…The author also modeled the positive correlation between uplift rates and the number of terraces preserved within a sequence, which had been previously recognized in nature (Merritts and Bull, 1989;Armijo et al, 1996). Along the same lines, LEMs of Melnick (2016) and Pastier et al (2019) emphasized that for low uplift rates, the continuous reoccupation of abrasion platforms, coupled to paleo-cliff diffusion, hinders the direct conversion of terrace elevations to uplift rates.…”

High interstadial sea levels over the past 420ka from the Huon Peninsula, Papua New Guinea

“…We chose three representative crosssections (Fig. 4) and used the REEF code (updated from refs 30,31 ). We test the impact of our RSL estimates by considering 3 different sea-level curves (Fig.…”

“…6). The second potential limitation is that a terrace surface might form several meters below the peak in sea-level (SL-Terrace DIF ), depending on the morphology formed during preceding sea-level oscillations or erosion of a barrier reef 31 . We quantified these effects (Supplementary Fig.…”

The influence of rock uplift rate on the formation and preservation of individual marine terraces during multiple sea level stands

“…Later, Lajoie (1986) merged this work with seminal studies on constructional coral reef terraces (e.g., Broecker et al 1968, Chappell, 1974 and declared that "a general consensus has developed" linking strandlines and high stands on rising coastlines. This morphostratigraphic approach relies on a bijective assumption that requires each individual terrace to have a unique age linked to a unique high stand (Pastier et al, 2019). It is commonly employed at sites where independent dating of multiple terraces is unavailable or limited to a small subset.…”

“…The observation of composite ages on individual coral reef terraces (e.g., Bard et al, 1996) and the occasional absence of specific MIS high stands in extensive coral terrace series (e.g., Pedoja et al, 2018) also calls into question the bijective rationale. Pastier et al (2019) highlight that a sea level curve cannot be straightforwardly related to a coral reef terrace record since some terrace sequences may lack certain high stands and/or preserve steps formed at lower sea level stands.…”