On the generation and degradation of emerged coral reef terrace sequences: First cosmogenic 36Cl analysis at Cape Laundi, Sumba Island (Indonesia)
Introduction
Sumba is an actively rising island in Indonesia where an emerged coral reef terrace sequence records the progressive emergence of the island. The sequence at Cape Laundi, on the north coast of the island, reaches ∼470 m in elevation and includes at least 18 successive coral reef terraces (CRTs). This sequence has a well-preserved and potentially valuable record of Quaternary sea level, paleoclimate and tectonics, for which dating of the CRTs is crucial. The previous studies of this CRT sequence (Pirazzoli et al., 1991; 1993; Bard et al., 1996) have identified significant temporal discrepancies within the CRTs, i.e., different ages of corals within the same CRT and similar ages of corals on several CRTs. Pirazzoli et al. (1991; 1993) and Bard et al. (1996) proposed that the CRTs have a polycyclic nature in order to explain age diachronism (Fig. 1). Pirazzoli et al. (1993) suggested that marine erosion can reshape the CRT surface and promote the bioconstruction of a new coral-colony on an older one during sea level highstands (Fig. 1A). Bard et al. (1996) indicated that a decrease in the rate of uplift to a low rate would in recurrent similar relative sea levels, causing several phases of reef development on a pre-existing surface (Fig. 1B). The role of marine erosion on the morphogenesis of CRTs has been discussed since a long time (e.g., Chappell, 1974; Hearty el al., 2008). Despite the persistence in recent publications of a simplistic definition of CRTs as constructive marine terraces; it is now clearly accepted in many syntheses that a CRT surface results from the combination of bioconstruction, erosion at sea level and accumulation of the eroded sediments (Pirazzoli, 2005; Cabioch, 2011; Murray-Wallace and Woodroffe, 2014; Pedoja et al., 2018; Pastier et al., 2019).
Apart from the role of marine erosion and bioconstruction reoccupation, what is the role of continental denudation in age diachronism on the same CRT? On a polycyclic CRT, continental denudation could partially strip the thin superficial layer of a young fossil coral reef and exhume older corals in several places (Fig. 1C). Since the stratigraphy of the CRTs on Sumba is not described and is difficult to observe in the canyons that incise them, and since the preservation of paleo-soils is unlikely in the polycyclic CRTs by their subsequent marine abrasion during a new transgression, we have chosen to combine the cosmogenic 36Cl method (e.g., Lal, 1988; 1991; Bierman, 1994), new 230Th/U dating and previous dating to highlight the processes of marine erosion, reoccupation and continental denudation affecting the CRTs of Cape Laundi. The 36Cl method has already been carried out on CRTs in Barbados, resulting in quantification of the continental denudation rate (Lal et al., 2005).
In this study, we measured the cosmogenic 36Cl isotopes concentration of 34 in situ surface samples collected from the oldest CRT to the current reef shelf and took several samples on each CRT from the inner edge to the lower cliff in order to detect variation of continental denudation on them (Fig. 1C; 2). Moreover, we analyzed the 36Cl concentration in a 2.5 ± 0.1 m deep core of the lowermost CRT to attempt to (1) constrain its exposure time to cosmic rays (i.e., the age at which it emerged) if the concentration of 36Cl decreases exponentially at depth (e.g., Braucher et al., 2011), or (2) to detect several exposure phases (i.e., reoccupation stages) by 36Cl concentration peaks at depth (Fig. 1A; 1B). We conducted 230Th/U dating of two coral colonies in growth position collected on the Holocene landform and used these 230Th/U ages to calculate a coastal denudation rate from the 36Cl concentrations of samples taken from the top of the active Holocene sea cliff. We discuss our results in terms of 1) the influence of marine erosion and constructive reoccupation components on CRTs morphogenesis, 2) continental denudation rates of carbonates and their heterogeneity, as well as 3)their comparison with global trends.
Section snippets
Emerged coral reef terrace sequences
Morphologically, a CRT is an expanse of reefal limestone with a surface that is flat or slightly sloping seawards, limited by a change in slope seaward and landward. Seaward, the change in slope (i.e., a distal edge associated with a more or less steep cliff; Fig. 2), is usually described as the paleo-reef crest (e.g., Pirazzoli et al., 1991; Rovere et al., 2016). Landward, at the inner edge, a CRT is characterized by a break in slope, sometimes interpreted as a shoreline angle suggesting the
Onshore and offshore data
We mapped the inner edges of the CRTs at Cape Laundi using a high resolution (2 m) Digital Elevation Model (DEM) produced from stereoscopic satellite images (Pleaides, CNES) with MicMac freeware (e.g., Rupnik et al., 2016). We acquired topographic and bathymetric profiles, using a real kinematic differential global positioning system (RTK DGPS) onshore, and a Humminbird 700 series sonar offshore (Fig. 4). Onshore, our profiles were carried out perpendicular to the main inner edges of the
Cape Laundi: offshore and onshore landforms
Improved DEM resolution, new bathymetric data and field observations allowed us to improve the mapping of Cape Laundi. The precise description of the coastal morphology is essential to better understand the processes of its formation and destruction. Offshore, two submerged ∼200 m wide surfaces (named –I1 and –I2; Fig. 6), were newly identified. Their morphology is consistent with paleo-lagoons: flat in their central part (at −38 ± 1 and −53 ± 1 m for –I1 and –I2, respectively; Fig. 6) and
Discussion
Significant denudation rates obtained at the sea level and on the CRT sequence point to marine erosion and continental denudation as the cause of the age diachronism on a single CRT (Fig. 1A; 1B). This hypothesis also requires reef reoccupation over several highstands (R2 over R1 on Fig. 1). Our results provide the opportunity to discuss the dynamics of these processes at Cape Laundi. We first highlight the role of reef reoccupation processes and marine erosion in shaping CRTs. Then, we discuss
Conclusions
At Cape Laundi, previous studies (Pirazzoli et al., 1991; 1993; Bard et al., 1996) have identified age discrepancies on CRTs. We disentangled the roles of continental denudation, coastal erosion, and marine reoccupation in the promiscuity of diachron coral colonies on the same CRT surface. The 36Cl concentrations of 34 surface limestone samples taken from different morphological zones of this CRT sequence allowed us to calculate continental denudation rates, ranging from 2.5 ± 0.3 to
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by public funds received of the program "Investissements d'Avenir" managed by the French National Research Agency (ANR-10-EQPX-20 and ANR-10-LABX-19-01, Labex Mer, CLIMORESO, C. Authemayou) , the INSU Tellus Syter program (SECOMAS, C. Authemayou), and the CNES TOSCA program (CETTROPICO, C. Authemayou). We thank the German Aerospace Center for providing us the TanDEM-X data for our study zone. We thank the State Ministry of Research and Technology of Indonesia “RISTEK”
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