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1Relative sea-level changes and reef development in the northern Coral
2Triangle during the late Quaternary
3
4Kathrine Maxwell1,2, Alessio Rovere3, Hildegard Westphal1,2, Kevin Garas4, Mirasol
5Guinto5, Denovan Chauveau3, Hsun-Ming Hu6,7, Chuan-Chou Shen6,7
6
71Leibniz Centre for Tropical Marine Research (ZMT), Bremen, Germany
82University of Bremen, Department of Geosciences, Bremen, Germany
93Department of Environmental Sciences, Informatics and Statistics, Ca’ Foscari
10 University of Venice, Venice, Italy
11 4Department of Environment and Natural Resources–Mines and Geosciences
12 Bureau (DENR-MGB), Quezon City, Philippines 1100
13 5Forestry Development Center, College of Forestry and Natural Resources,
14 University of the Philippines, Los Baños, Laguna, Philippines
15 6High-Precision Mass Spectrometry and Environment Change Laboratory (HISPEC),
16 Department of Geosciences, National Taiwan University, Taipei, 10617, Taiwan
17 ROC
18 7Research Center for Future Earth, National Taiwan University, Taipei, 10617,
19 Taiwan ROC
20
21 Email: katt.maxwell@gmail.com; kathrine.maxwell@leibniz-zmt.de
22
23 Keywords
24 Relative sea-level change, Coral Triangle, Coral reef terraces, Late Quaternary
25
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26 Abstract
27 The Philippines is located at the apex of the Coral Triangle, and its coasts are fringed
28 by both modern and fossil reefs. Fossil coral reefs provide geologic records of
29 Quaternary relative sea-level changes, giving an insight on past eustatic oscillations
30 and tectonic deformations. In this study, we present a new interpretation of the
31 formation of the Late Pleistocene coral reef terraces at Cape Bolinao, in the northern
32 Coral Triangle. Using high-resolution topographic data coupled with new radiometric
33 ages and reef stratigraphic models, we revise the regional uplift rate and we outline a
34 morpho-chronologic framework for these reef terraces. We determine nine steps of
35 terraces and the highest one (at an elevation of ~155 m) formed during Marine Isotope
36 Stage 5e (MIS 5e, Last Interglacial). The lowest terrace (~4 m) is dated mid-Holocene.
37 Our data provide supporting evidence for the subduction of the Scarborough
38 Seamount Chain beneath Luzon Island.
39
40 1. Introduction
41 Coral reef terraces (CRTs) are formed through repeated bioconstructional and
42 erosional processes, and they can be used as indicators of relative sea-level changes
43 through time (e.g., Anthony, 2008; Rovere et al., 2016). The occurrence and exposure
44 of late Quaternary CRTs above present sea level are proxies of past relative sea-level
45 changes as their formation, growth, and geomorphological development are primarily
46 controlled by changes in eustatic sea level and/or vertical land motions (e.g.,
47 Woodroffe and Webster, 2023). The analysis of sea-level indicators during previous
48 interglacials allows insights into potential drivers of relative sea-level changes in the
49 past (e.g., Rovere et al., 2016). In areas considered tectonically stable (Yucatan
50 peninsula, Mexico, Blanchon et al., 2009) or active (Huon Peninsula, De Gelder et al.,
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51 2022), exposed fossil coral reef terraces provide accessible record of relative sea-
52 level change, such as during the Last Interglacial (Marine Isotope Stage (MIS) 5e,
53 ~130–116 ka) when eustatic sea level was higher than present, by 2 to 9 m (e.g.,
54 Dutton and Lambeck, 2012; Dyer et al., 2021; Dumitru et al., 2023). Meanwhile, in
55 subsiding regions (e.g., Tahiti), sea-level reconstructions are based on
56 sedimentological and palaeontological data from offshore drill cores allowing the
57 investigation of relict reef features related to glacial-interglacial periods (e.g., Camoin
58 et al., 2012).
59
60 In tectonically active regions, exposed sequences of fossil CRT document a combined
61 signal of eustatic sea-level and tectonic uplift (e.g., Chappell, 1974; Lajoie, 1986;
62 Pedoja et al., 2011, 2018; Maxwell et al., 2021; Peñalver et al., 2021). In southwestern
63 Japan, Ota and Omura (1992) examined the CRTs in the Ryukyu Islands and derived
64 different MIS 5e uplift rates for Kikai Island (1.8 mm/yr), and for Hateruma Island (0.3
65 mm/yr) which they suggest reflecting active deformation on the leading edge of an
66 overriding Eurasian Plate. Previous works on the canonical CRTs of Huon Peninsula
67 (Papua New Guinea), situated along the boundary between the Australian and West
68 Pacific plates, identified uplift rates as high as 3.3 to 3.5 mm/yr (e.g., Chappell, 1974;
69 Chappell et al., 1996a). Corrected for tectonics and post-depositional land
70 movements, uplifted CRTs offer a detailed record of relative sea-level change since
71 MIS 5e and has been converted into estimates of ice volume changes over the last
72 climatic cycle (e.g., Chappell, 1974; Bloom et al., 1974; Chappell et al., 1996b;
73 Lambeck and Chappell, 2001; Yokoyama and Esat, 2011). The CRTs in Barbados
74 (e.g., Mesolella, 1967; Broecker et al., 1968; Blanchon and Eisenhauer, 2001;
75 Schellmann and Radtke, 2004) and in Sumba Island in Indonesia (e.g., Pirazzoli et al.,
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76 1991, 1993; Bard et al., 1996) present a relative sea-level record spanning the last 1
77 Ma where Late and Middle Pleistocene terraces are dated.
78
79 Numerical models of reef growth allow modelling the processes that contribute to the
80 creation of CRTs, accounting for changes in sea level, vertical land motion, reef growth
81 rate, wave erosion, and initial substrate slope (e.g., Koelling et al., 2009; Husson et
82 al., 2018; Pastier et al., 2019; De Gelder et al., 2020). These landscape evolution
83 models help understand the geomorphic responses of coral reef sequences to
84 changes in sea level and provide additional constraints on the potential timing and
85 development of the reef sequences, especially when CRT ages are poorly constrained
86 (e.g., Chauveau et al., 2023, 2024; Boyden et al., 2023; De Gelder et al., 2023). For
87 example, in Huon Peninsula, the analysis of high-resolution digital elevation models
88 coupled with numerical models of coral reef terrace formation allowed elucidating the
89 morphogenesis of reef terraces over the last 420 kyr and further suggest that oxygen
90 isotope-based global mean sea-level curves systematically underestimate interstadial
91 sea-level elevations, by up to ∼20 m (De Gelder et al., 2022). In Sumba Island,
92 numerical reef modelling shows that at least two terraces can be created during MIS
93 5e and multiple sea-level peaks during MIS 5e are not required to explain the presence
94 of these terraces (Chauveau et al., 2023).
95
96 In the Philippines, there is little information available about the geomorphological
97 evolution of the emergent CRTs in Cape Bolinao (west Luzon, northern Coral
98 Triangle). We aim to fill this gap by exploring the geomorphic lateral variation and
99 morphological development of the Cape Bolinao CRT using high-resolution digital
100 elevation models and numerical models based on new geomorphic, geochronologic,
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101 and morpho-stratigraphic data . While no comprehensive chronological framework has
102 been proposed for Cape Bolinao so far, we couple our geomorphic analysis with
103 numerical reef modelling to help constrain the chronology, as well as investigate uplift
104 and CRT morphogenesis.
105
106 2. Regional geology and tectonic setting
107 Cape Bolinao (Fig. 1) is located in the western portion of Luzon Island, Philippines, at
108 the northern apex of the Coral Triangle. This is a region known as the epicenter of
109 marine biodiversity (e.g., Hoeksema, 2007; Veron et al., 2009; Veron et al., 2015).
110 From a tectonic standpoint, Cape Bolinao lies at the western portion of the Philippine
111 Mobile Belt (PMB), a 400-km wide deformation zone from Luzon to Mindanao,
112 Philippines, which resulted from the WNW oblique convergence of the Philippine Sea
113 Plate (PSP) and the Eurasian Plate (EU)-Sundaland block (SU) (e.g., Gervasio, 1967;
114 Rangin et al., 1999). It faces the West Philippine Sea and is bounded to the west by
115 the Manila Trench (MT) subduction zone, an active convergent plate margin, along
116 which, the South China Sea oceanic basin is being subducted eastward (e.g., Hayes
117 and Lewis, 1985). Recent estimates based on GPS data reveal a convergence rate of
118 91 mm/yr at the northern end of Luzon and 55 mm/yr in the south and a highly coupled
119 (relatively locked) region between the West Luzon Trough (WLT) and the east of the
120 Scarborough Seamount Chain (SSC) (Hsu et al., 2012).
121
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122
123 Fig. 1. Cape Bolinao and its location along the western Philippines at the apex of the Coral
124 Triangle. a. Map of Southeast Asia and the Coral Triangle (shaded in blue). Circles show the previously
125 reported Last Interglacial (LIG) CRTs in the region as compiled in Maxwell et al. (2021). b. Tectonic
126 elements bordering western Luzon Island and location of Cape Bolinao (in black box). Focal mechanism
127 solutions (>M5) were obtained from the Global CMT Project (Dziewonski et al., 1981; Ekström et al.,
128 2012) and in red lines, the active faults traversing the PMB from the PHIVOLCS (2020). c. Surface
129 classification model (SCM) showing the different paleo-reef surfaces in Cape Bolinao. The highest
130 terrace (TIX, in dark blue) indicates the oldest fossil coral reef terrace mapped and the lowest terrace
131 (in yellow) are the Holocene reefs. Also shown is the location of the present-day reef, the Bolinao-Anda
132 Reef Complex (BARC), from McManus et al. (1992). The survey sites in Patar, Bolinao: (1) Rockview
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133 Point (in Fig. 3a) and (2) Punta Piedra Point (in Fig. 3b) are bounded by boxes. The representative
134 profile, B–BI, (Fig.7) is also shown. Basemaps were created using topography data from Japan
135 Aerospace Exploration Agency (JAXA) ALOS World 3D–30 m (AW3D30)
136 (https://www.eorc.jaxa.jp/ALOS/en/dataset/aw3d30/aw3d30_e.htm) and bathymetry data from the
137 GEBCO Compilation Group (2023) GEBCO 2023 Grid (doi:10.5285/f98b053b-0cbc-6c23-e053-
138 6c86abc0af7b). Maps were created using ESRI ArcGIS Pro 3.1 and the Generic Mapping Tools (GMT)
139 software (Wessel et al., 2019). PSP: Philippine Sea Plate; EU: Eurasian Plate; SU: Sundaland block;
140 MT: Manila Trench; WLT: West Luzon Trough; WPS: West Philippine Sea; NLT: North Luzon Trough;
141 WBF: Western Boundary Fault; SSC: Scarborough Seamount Chain.
142
143 Cape Bolinao is predominantly underlain by the Plio-Pleistocene coralline reefal
144 limestone named the Bolinao Limestone (Mines and Geosciences Bureau, 2010).
145 Lithologic units underlying the Bolinao Limestone are interbedded sandstone,
146 siltstone, and claystone which were assigned Late Pliocene (Piacenzian) based on
147 holoplanktonic gastropods (Janssen, 2007) to Early Pleistocene (1.77 to 0.61 Ma)
148 based on nannofossils and planktonic foraminifers (Wani et al., 2008). These lithologic
149 units were interpreted to indicate an epi- to upper meso-pelagic setting, with depth
150 ranges extending to a maximum of 200-300 m water depth (Janssen, 2007).
151 Underlying the Bolinao Limestone is the Late Miocene to Early Pliocene Santa Cruz
152 Formation (Mines and Geosciences Bureau, 2010). Along the western coast of Cape
153 Bolinao, a sequence of CRTs rising to about ~155 m above mean sea level (amsl) is
154 observed (Fig. 2). This was first described by Maemoku and Paladio (1992) who
155 identified seven terraces from interpretation of aerial photos and altimeter
156 measurements. Their classification was based on elevation and degree of dissection,
157 and they assumed that the highest terrace was MIS 5e in age and the lowest one was
158 Holocene.
159
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160 The extensive modern fringing reef (Fig. 1c) along the northeastern portion of Cape
161 Bolinao is referred to as the Bolinao-Anda Reef Complex (BARC). This reef has an
162 area of ~200 km2, and it is characterized by: (1) a reef flat and lagoon dominated by
163 seagrass with occasional patches of living and dead corals and sandy-muddy
164 substrate; (2) an intertidal reef crest with dead coral rubble and patches of corals; (3)
165 a fore reef characterized by gentle to steep slopes of variable depths (10-30 m) and a
166 talus of sand and coral rubble extending ~22 km into the middle of the Lingayen Gulf
167 (Fig. 2, McManus et al., 1992; Vergara et al., 2010). Branching and massive corals
168 from the families Acroporidae, Pocilloporidae, Poritidae, and Helioporidae are
169 abundant in the BARC (e.g., Shaish, et al., 2010; Quimpo et a., 2020; Torres et al.,
170 2021). The modern reef flat has variable depths from 0.3 to 6 m, with an average of 2
171 m (e.g., Shaish, et al., 2010; Cantarero et al., 2019) and is mostly rocky to sandy and
172 covered with seagrasses and seaweeds with coral boulders and occasional living coral
173 heads (e.g., microatolls) (Fig. 2).
174
175 In our survey sites (that are usually exposed to high-energy wave conditions), the
176 modern reef flat is limited towards the sea by the reef edge, which is in turn
177 characterized by spurs and grooves, with its landward edge marked by discontinuous
178 rocky cliffs and stretches of sandy beaches (Fig. 2). Etched onto rocky cliffs are
179 shoreline angles and sometimes, tidal notches. Shoreline angles (sometimes referred
180 to as inner margins, e.g., Maxwell et al., 2018) are located at the intersection between
181 the reef platform and the sea cliff and are used as a morphological approximation of
182 paleoshorelines (e.g., Lajoie, 1986). Tides in this area are mainly semidiurnal, with a
183 mean tidal range of 0.65 m according to data from the nearest primary station of the
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184 Department of Environment and Natural Resources-National Mapping and Resource
185 Information Authority (DENR-NAMRIA) in San Fernando, La Union.
186
187
188 Fig. 2. Coral reef terraces in Cape Bolinao. a. Schematic diagram of the BARC and the relationship
189 of shoreline angle with present-day sea level. The modern-day reef zonation is based on the
190 descriptions of McManus et al. (1992) and Vergara et al. (2010). b. Aerial photo (view looking SE) of
191 the CRTs as observed in Punta Piedra Point. Lower terraces (TI-TIII) are recognized at elevations <9
192 m and higher terraces are highlighted by dashed lines (TIV and TIX) and black arrows. c-d. Aerial
193 photos of the modern reef flat characterized by a dominance of seagrass with occasional patches of
194 living corals and dead coral boulders. White box shows the location of photo (d), which shows a living
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195 coral (i.e., microatoll, bounded by yellow circle) surrounded by seagrass (bounded by yellow dashed
196 line).
197
198 3. Methods
199 3.1. Coral reef terrace mapping
200 Fossil CRTs were mapped on digital elevation models (DEMs), satellite images, and
201 topographic surveys. We used the 2013 airborne Interferometric Synthetic Aperture
202 Radar-derived-Digital Terrain Model (IFSAR-DTM) with 5-meter posting provided by
203 the National Mapping and Resource Information Authority (NAMRIA) to generate
204 derivative maps (e.g., hillshade, contour, slope, aspect), which allowed us to examine
205 the morphology of the CRT sequences in Cape Bolinao. Using the TerraceM-2
206 program (Jara-Muñoz et al., 2019) and topographic slope and roughness, we
207 generated a surface classification model (SCM) to delineate semi-automatically paleo-
208 reef platforms and steeper areas that may represent paleo-cliffs.
209
210 Systematic topographic surveys, orthogonal to the modern shoreline, were carried out
211 along accessible portions of the coast. We took a particular interest at the section
212 where all the terraces were well preserved and previously described (Maemoku and
213 Paladio, 1992). To measure the location and elevation of points of interest, we used a
214 pair of Emlid REACH RS+ single-band Global Navigation Satellite systems (GNSS)
215 receivers with a Base-Rover configuration and connected with the nearest PURD
216 station of the Philippine Active Geodetic Network (PAGeNet)-NAMRIA. Elevation data
217 was referred to orthometric heights using the Philippine Geoid Model 2018 (PGM2018)
218 of NAMRIA (Gatchalian et al., 2021). We discarded data points with elevation RMS
219 values higher than ±1 m and retained the data point with the lowest elevation RMS
220 whenever several measurements were taken for the same point. Sources of vertical
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221 error include the elevation RMS per data point and the estimated accuracy of the
222 PGM2018 model of ±0.012 m. Taking these into account, the elevation error of our
223 GNSS data range from ±0.011 to ±0.123 m. To supplement and fill the gaps in our
224 survey data (especially in areas where network coverage is poor), we also extracted
225 topographic profiles, orthogonal to the coast, from the IFSAR-DTM (< 1 meter vertical
226 accuracy). Heights of paleo reefs, shoreline angles, and fossil coral samples are
227 reported in meters above mean sea level (m amsl). Data processing and map
228 generation were done using the ArcGIS Pro software by Environmental Systems
229 Research Institute (ESRI).
230
231 3.2. Reef stratigraphy and geochronology
232 Exposed outcrops of CRTs were examined and rock samples were collected and
233 petrographically described. Fossil corals and mollusks found in situ in growth position
234 were collected for radiometric dating. Their elevations were measured and reported in
235 m amsl. Thin sections were prepared and visually examined under the microscope to
236 assess the presence of diagenetic textures. Following the qualitative scheme of
237 McGregor and Abram (2008), we identified the samples as either having “excellent”
238 preservation with no diagenetic textures observed equivalent to calcite below detection
239 levels or having “good” preservation with rare diagenetic textures observed equivalent
240 to <1% calcite. Samples falling outside these categories were discarded. Samples
241 that have preserved aragonitic coral skeletal elements with very little to no evidence
242 of secondary aragonite and calcite and other diagenetic textures were subsampled for
243 radiocarbon and U-Th dating.
244
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245 After screening, we sent eight coral subsamples to the High-Precision Mass
246 Spectrometry and Environment Change Laboratory (HISPEC) at the National Taiwan
247 University to be analyzed for U-Th isotopic compositions using a high-resolution multi-
248 collector inductively coupled plasma mass spectrometer (MC-ICP-MS), Thermo Fisher
249 Neptune. Chemistry followed the procedure described in Shen et al. (2003) and
250 instrumental analysis used the protocols given in Shen et al. (2013). Analytical errors
251 are reported at the two-sigma (2) of the mean. 230Th ages with 2-sigma uncertainty
252 ranges are reported in years before present (year BP) relative to 1950 AD.
253 Radiocarbon dating of three coral subsamples and two mollusk subsamples was
254 conducted in the Beta Analytic Radiocarbon Dating Laboratory in Miami, Florida,
255 U.S.A by Accelerator Mass Spectrometry (AMS) method. The OxCal version 4.4
256 (Bronk Ramsey, 2009) was used to calibrate conventional radiocarbon ages using the
257 MARINE20 Marine Radiocarbon Age Calibration Curve (Heaton et al., 2020). We
258 calculated the local DeltaR correction of -262 ± 63 for Cape Bolinao based on coupled
259 14C and 230Th ages of three coral samples using an online application for DeltaR
260 calculation (Reimer and Reimer, 2017). Radiocarbon ages presented in this study are
261 reported in calibrated years before present (cal BP) relative to 1950 AD.
262
263 3.3. Terrace morphology and deformation
264 To determine the morphology and lateral extent of the CRTs in Cape Bolinao, we
265 generated stacked swath profiles using the TopoToolbox 2, a MATLAB-based
266 software for topographic analysis by Schwanghart and Scherler (2014). We calculated
267 parallel topographic swath profiles and stacked them together (e.g., Armijo et al., 2015;
268 Fernández-Blanco et al., 2020; De Gelder et al., 2022). This analysis allows extracting
269 the average elevation (and corresponding minimum and maximum values) along
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270 parallel profile lines avoiding the biases introduced by discrete topographic profiles
271 (e.g., De Gelder et al., 2022). Using the 5-meter posting (<1 m vertical accuracy)
272 IFSAR-DTM, we generated stacked swath profiles calculated from 100-300 parallel
273 lines (50-m spacing between each line) with swath width of 50 and 100 m. In general,
274 the CRTs on the stacked swath profiles are shown as areas with clusters of overprinted
275 topographic profiles and better visualization of the terraces depend on the viewing
276 angle with respect to the dip of the terraces (De Gelder et al., 2022; Chauveau et al.,
277 2023). We find that the east-southeastward viewing angle (which corresponds to the
278 dip of the highest terrace towards the ESE), with swaths roughly parallel to the present-
279 day coastline, provides a better image of the overall morphology and elevation of the
280 CRTs in Cape Bolinao.
281
282 We also performed shoreline angle analysis on the IFSAR-DTM using TerraceM-2
283 (Jara-Muñoz et al., 2019) to help evaluate the overall deformation in the study area.
284 We mapped shoreline angles by systematically placing 100 swath profiles orthogonal
285 to the trace of the paleocliff or the terrace inner edge. From these 200-m wide swath
286 profiles, we identified the elevations, distributions, and number of terraces per site.
287 This analysis gives us additional information not only of the deformation but also of the
288 preservation of the CRTs in Cape Bolinao.
289
290 3.4. Reef modelling
291 To explore the morphology, geometry, and possible chronology of the CRTs in Cape
292 Bolinao, we used a kinematic profile evolution model that incorporates model
293 parameters such as potential reef growth rate, marine erosion, initial slope, vertical
294 land motion rate, and a chosen sea-level curve (see details in Husson et al., 2018;
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295 Pastier et al., 2019). Using the REEF code (Husson et al., 2018; Pastier et al., 2019),
296 we produced reef simulations with parameters constrained by local observations and
297 previous works to help us better examine and understand the reef development in
298 Cape Bolinao since the Late Pleistocene.
299
300 We chose a representative cross-section profile that shows the best preservation of
301 the reef terraces in Cape Bolinao. We used a potential reef growth rate (RG) of 10
302 mm/yr based on reported accretion rate of 10-13 mm/yr derived from Holocene reef
303 cores in Currimao, northwest Luzon (Shen et al., 2010). The maximum reef growth
304 depth (MRGD) was set to 30 m following Flores et al. (2023), who identified coral reefs
305 at this depth northeast of Cape Bolinao, and the optimal reef growth depth (ORGD)
306 and wave erosion maximum depth (WEMD) were respectively set to 2 m and 3 m
307 (Chauveau et al., 2023, 2024). We used 5 degrees as the initial slope (IS) value based
308 on the reported inclination of the gently-sloping Sta. Cruz Formation, which serves as
309 the basement rock of the CRTs in Cape Bolinao (Bureau of Mines and Geosciences,
310 1985a; 1985b). We used an erosional potential (E) of 60 mm3/yr (Chauveau et al.,
311 2023). For the vertical land motion value (U), we used different scenarios, with uplift
312 rates changing between 0 mm/yr (no uplift) to 1.3 mm/yr (that is, the maximum uplift
313 rate considering the age of the highest terrace MIS 5e, as reported in literature).
314 Finally, we model eustatic sea-level changes using five different sea-level curves:
315 Waelbroeck et al. (2002), Bintanja et al. (2005), Grant et al. (2014), Rohling et al.
316 (2009), and Spratt and Lisiecki (2016).
317
318 4. Results
319 4.1. Geomorphology and stratigraphy of Cape Bolinao reef terraces
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320 The analysis of high-resolution topographic data allowed us to delineate nine paleo-
321 reef platforms and associated paleo-cliffs in Cape Bolinao, herein named Terrace I
322 (TI) to Terrace IX (TIX), from lowest to highest (Fig. 1c). We identified two generations
323 of reef terraces based on morphology and heights of terrace cliffs: (1) the lower
324 terraces characterized by narrow (from several meters wide to <500 m wide) terraces
325 rising to 9 m with 2-to-3-m-high terrace cliffs (Fig. 3) and (2) the higher terraces
326 characterized by terrace widths of >100 m and rising from ~20 to 155 m with 10-to-30-
327 m-high terrace cliffs (Fig. 2b). The lower terraces (TI to TIII) are measured and
328 identified using high-resolution GNSS topographic surveys, while the higher terraces
329 (TIV to TIX), that are best preserved along the western coast of Cape Bolinao, are
330 delineated both in the field and from analysis of high-resolution DEMs.
331
332 The Cape Bolinao terraces display typical reef facies composed of framework
333 components such as in situ corals (from the families of Poritidae, Merulinidae, and
334 Acroporidae) and skeletal fragments of corals, coralline red algae, echinoid spines,
335 serpulids, Halimeda grains, lithoclasts, mollusk shells, and benthic (and encrusting)
336 foraminifera, that can be observed in thin sections (Fig. 4, Supplementary Fig. 1). For
337 corals and other aragonitic components observed above TI, the original skeleton and
338 intra-skeletal porosity are replaced and filled by blocky calcite spar and sometimes
339 dissolved leaving molds (Supplementary Fig. 2).
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340
341
342 Fig. 3. Surveyed lower terraces in Cape Bolinao. We identified three steps of terraces from
343 topographic surveys in Patar, Bolinao (location shown in Fig. 1c). a. Outcrop photo (white dashed line
344 outlines survey transect) and topographic profile surveyed from Rockview Point show two steps with TI
345 measured at 3.87 ± 0.01 m and TIII measured at 8.02 ± 0.01 m. Most of the mid-Holocene coral samples
346 were collected from TI at this site. b. Field photo (white dashed line outlines survey transect) and
347 topographic profile surveyed from Punta Piedra Point revealing two steps with TII measured at 5.72 ±
348 0.02 m and TIII measured at 8.57 ± 0.02 m. c-d. Aerial photographs illustrating the general morphology
349 of the lower CRTs in Patar, Bolinao and their position several meters above sea level. Arrows point to
350 the paleo-reef surfaces of identified terraces.
351
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352
353 Fig. 4. Corals and composition of Cape Bolinao reef terraces. a. Outcrop photo of TI showing in
354 situ corals (e.g., Porites) (bounded by white dashed contours). b-d. In situ corals from (b) TII, (c) TIII,
355 and (d) TIX from the families of Acroporidae and Merulinidae that are diagenetically altered. e-f. Outcrop
356 photos of (e) terrace TVII where we collected a Halimeda-rich floatstone and (f) terrace TIV, which is
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357 characterized by cobbles and boulders composed mainly by diagenetically altered corals (bounded by
358 white dashed contours).
359
360 The lowest terrace, TI, is best observed in Rockview Point at a measured elevation of
361 3.87 ± 0.01 m and is characterized by a width of 5-10 m (Fig. 3a). It is limited seaward
362 by gently sloping sand-covered beach deposits and it transitions gradually into the
363 sand-covered terrace TII/TIII. The paleo-reef surface of TI is characterized by in situ
364 massive and branching corals from the families of Poritidae, Merulinidae, and
365 Acroporidae. Coral samples, composed of aragonite skeleton with little to no evidence
366 of diagenesis, yielded 230Th ages of 6.38-5.71 kyr BP and radiocarbon ages of 6.05-
367 5.64 kyr cal BP (Tables 1 and 2, Supplementary Fig. 2). The Porites and Acropora
368 corals yielded relatively older ages compared to Goniastrea corals with δ234U initial
369 values (Table 1) within the acceptable limits (see Chutcharavan et al., 2018). In Punta
370 Piedra, TI is composed of beachrock with abundant large benthic foraminifera (e.g.,
371 Calcarinidae), gastropods, and reworked pristine Porites (PAT-012822-3A) and
372 Acropora (PAT-012822-3E) corals dated 5.73 and 5.25 kyr BP, respectively.
373
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374 Table 1. Uranium and thorium isotopic compositions and 230Th ages for coral samples by MC-ICPMS, Thermo Electron
375 Neptune at NTU
Sample
Lat.
Long.
Elev.
Coral
W
238U
232Th
234U
[230Th/238U]
230Th/232Th
Age (kyr ago)
Age (kyr BP)
234Uinitial
ID
(m)e
Samp
(g)
10-6g/g a
10-12g/g
measureda
activityc
atomic (× 10-6)
uncorrected
correctedc,d
correctedb
PAT-
012822-2A
119.783
16.317
3.72 ± 0.01
P
0.22
2.4043
± 0.0019
1501.7
±
2.6
147.1
±
1.4
0.06193
±
0.00015
1634.7
± 4.8
6.045
±
0.017
5.957
± 0.019
149.6
± 1.4
PAT-
012822-2B
119.783
16.317
3.56 ± 0.01
A
0.22
2.9485
± 0.0022
355.3
±
2.1
144.8
±
1.3
0.059375
±
0.000075
8124
± 48
5.802
±
0.010
5.725
± 0.010
147.2
± 1.3
PAT-
012822-2D
119.783
16.317
3.37 ± 0.01
P
0.23
2.4486
± 0.0020
3280.5
±
4.6
145.1
±
1.6
0.06620
±
0.00021
815
± 2.8
6.486
±
0.023
6.382
± 0.028
147.8
± 1.6
PAT-
012822-2E
119.783
16.317
3.52 ± 0.02
G
0.22
2.2894
± 0.0019
74.1
±
2.1
142.9
±
1.4
0.059105
±
0.000070
30090
± 846
5.784
±
0.010
5.710
± 0.010
145.3
± 1.5
PAT-
012922-2F
119.783
16.317
3.87 ± 0.01
G
0.24
2.1940
± 0.0021
116.7
±
1.9
142.9
±
1.4
0.060198
±
0.000076
18665
± 300
5.894
±
0.011
5.819
± 0.011
145.3
± 1.5
PAT-
012922-2G
119.783
16.317
3.87 ± 0.01
G
0.22
2.1671
± 0.0019
2835.9
±
4.0
143.2
±
1.4
0.06255
±
0.00020
788.1
± 2.7
6.129
±
0.022
6.026
± 0.027
145.6
± 1.4
PAT-
012822-3A
119.781
16.311
2.46 ± 1
Po
0.24
2.3209
± 0.0016
2429.1
±
3.4
145.5
±
1.3
0.05893
±
0.00017
928.4
± 2.8
5.753
±
0.018
5.656
± 0.022
147.9
± 1.3
PAT-
012822-3E
119.781
16.311
2.46 ± 1
Ar
0.26
2.8897
± 0.0025
109.4
±
1.8
146.8
±
1.4
0.053937
±
0.000066
23492
± 387
5.2483
±
0.0094
5.174
± 0.009
148.9
± 1.4
376 Analytical errors are 2s of the mean.
377 a[238U] = [235U] x 137.77 (±0.11‰) (Hiess et al., 2012); 234U = ([234U/238U]activity - 1) x 1000.
378 b234Uinitial corrected was calculated based on 230Th age (T), i.e., 234Uinitial = 234Umeasured × e234×T, and T is corrected age.
379 c[230Th/238U]activity = 1 - e–230T + (234Umeasured/1000)[230/(230 - 234)](1 - e–(230 - 234) T), where T is the age. Decay constants are 9.1705 × 10-6 yr-1 for 230Th, 2.8221
380 x 10-6 yr-1 for 234U (Cheng et al., 2013), and 1.55125 x 10-10 yr-1 for 238U (Jaffey et al., 1971).
381 dAge corrections, relative to 1950 AD, were calculated using an estimated atomic 230Th/232Th ratio of 4 (± 2) x 10-6 (Shen et al., 2008).
382 eMeasured elevations and uncertainties of coral samples are reported in m amsl. Elevations of reworked samples were derived from IFSAR-DTM with vertical
383 accuracy of <1m.
384 Coral Sample: P = Porites sp., A = Acropora sp., G = Goniastrea sp., Po = Porites sp. (overturned), Ar = Acropora sp. (reworked). W: Weight (g)
385
386 Table 2. Radiocarbon ages for carbonate samples by NEC accelerator mass spectrometers and 4 Thermo IRMSs at Beta
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Sample ID
Latitude
Longitude
Elevation
(m)
Coral Sample
Measured
Radiocarbon
Age (BP)
IRMS δ13C (‰) b
Conventional
Radiocarbon
Age (BP) a
Calendar Calibration
(95.4 % probability) c, d,
e
PAT-012822-2A
119.783
16.317
3.72 ± 0.01
Porites sp.
5240 ± 30
-5.0
5570 ± 30
6.050 ± 0.103
PAT-012822-2B
119.783
16.317
3.56 ± 0.01
Acropora sp.
4920 ± 30
-1.0
5310 ± 30
5.760 ± 0.102
PAT-012822-2E
119.783
16.317
3.52 ± 0.02
Goniastrea sp.
4800 ± 30
-1.3
5190 ± 30
5.638 ± 0.104
PAT-012822-2C
119.783
16.317
3.37 ± 0.01
Tridacna sp.
41350 ± 730
+2.2
41790 ± 730
44.094 ± 0.616
PAT-012822-1A
119.782
16.317
6.82 ± 0.01
Tridacna sp.
41720 ± 780
+2.2
42160 ± 780
44.431 ± 0.684
387
388 aThe "Conventional Radiocarbon Age" was calculated using the Libby half -life (5568 years), is corrected for total isotopic fraction and was used for calendar
389 calibration where applicable. The Age is rounded to the nearest 10 years and is reported as radiocarbon years before present (BP), “present" = AD 1950.
390 bThe reported IRMS 13C (‰) values were measured separately in an IRMS (isotope ratio mass spectrometer).
391 c OxCal version 4.4 (Bronk Ramsey, 2009) was used to calibrate conventional radiocarbon ages using the MARINE20 Marine Radiocarbon Age Calibration
392 Curve (Heaton et al., 2020).
393 d We used local DeltaR = –262 ± 30 based on coupled 230Th and 14C ages of three coral samples from Cape Bolinao calculated using an online application for
394 DeltaR calculation (Reimer and Reimer, 2017).
395 e Calculated mean and 1 standard deviation (in kyr cal BP)
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396 The second terrace, TII, is best recognized discontinuously along rocky exposures on
397 the coast and is regularly subjected to sea sprays especially during high-energy wave
398 conditions. Along the profile in Punta Piedra Point (Fig. 3b), TII is characterized by a
399 rocky jagged surface with a measured elevation of 5.72 ± 0.02 m and a narrow width
400 of <20 m. It is limited to the sea by rocky almost vertical cliffs and it is limited landward
401 by meter-high terrace risers of TIII. Branching corals, already influenced by
402 diagenesis, are observed on the terrace surface (Fig. 4b). Where observed, tidal
403 notches are etched on the vertical cliffs of TII / TIII exposed to the sea. In our survey
404 sites, the retreat point (2-3 m depth) of tidal notches were observed to coincide with
405 elevation of the identified TI surface (Fig. 3).
406
407 We identified TIII terrace at a measured elevation of 8.57 ± 0.02 m (rising to about 14
408 m based on DEM-derived shoreline angle analysis). This is the widest Holocene
409 terrace, with a width of 200 to 500 m and follows the coastline for more than 10 km.
410 TIII is better preserved along the northwest-facing side of the coast. The terrace
411 surface is highly vegetated with occasional outcropping rocks and is highly modified,
412 as the area is transformed by coastal developments. Along the profile in Punta Piedra
413 Point (Fig. 3b), the terrace is characterized by a rocky surface dominated by in situ
414 massive and branching corals from the families of Poritidae, Merulinidae, and
415 Acroporidae that are diagenetically altered (Fig. 4c, Supplementary Fig. 2). Because
416 of the high degree of diagenesis observed on the corals from this terrace, we found
417 no coral material suitable for radiometric dating.
418
419 The fourth terrace, TIV, has a mean elevation of 31 ± 14 m and is characterized by
420 terrace width of 0.1 to 1 km, which increases from west to north. The surface of TIV is
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421 dissected by several drainage systems. Outcrop exposures reveal that this terrace is
422 composed of abundant sub-rounded to sub-angular diagenetically altered coral rubble
423 and boulders within a sandy matrix (Fig. 4f, Supplementary Fig. 1d). Petrographic
424 description of an outcrop sample reveals abundant lithoclasts (e.g., Halimeda
425 floatstone, bored coral fragments, and skeletal packstone) and broken fragments of
426 benthic foraminifera, coralline red algae, and echinoid plates are also present
427 (Supplementary Fig. 1d).
428
429 Terrace TV is at elevation of 55 ± 15 m is characterized by terrace width of 0.5 to 1.8
430 km which increases from west to north. Situated at an elevation of 81 ± 13 m, terrace
431 TVI has a width of ~0.1 to 0.3 km increasing towards the northeast. Petrographically,
432 gastropods and random sections of large benthic foraminifera (e.g., Calcarina) are
433 abundant in an outcrop sample from this terrace (Supplementary Fig. 1c). Coral
434 fragments are encrusted by coralline red algae and acervulinid foraminifera and
435 aragonite skeleton is replaced by calcite spar.
436
437 The seventh terrace, TVII, is at an elevation of 102 ± 10 m and has a terrace width of
438 0.2 to 3 km increasing from southwest to northeast. It is highly dissected along the
439 northern portion of the cape where it is the widest and is better preserved along the
440 northwest- to north-facing sides of the coast (Fig. 1c). Along the western coast, its
441 outer edge is marked by alternating ridges and channels, that are reminiscent of spurs
442 and grooves presently observed along the modern reef edge. Petrographic description
443 of a sample collected from along the profile reveals Halimeda-rich floatstone with
444 random sections of benthic foraminifera and other skeletal components in micritic
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445 matrix (Fig. 4e, Supplementary Fig. 1b). Halimeda fragments are enveloped by thin
446 micrite rim then filled by calcite spar.
447
448 Terrace TVIII has a narrow width (~0.1 to 0.3 km) and a mean elevation of 120 ± 14
449 m. The highest reef platform, TIX, is a broad, generally planar paleo-reef surface with
450 terrace width of ~5 km and ranges in elevation from 130 to 153 m (with a mean
451 elevation of 143 ± 2 m). The DEM shows its surface to be highly dissected, with circular
452 depressions, possibly sinkholes (Fig. 1c). Diagenetically altered corals from the
453 families of Poritidae and Merulinidae were recognized on this terrace (Fig. 4d).
454 Because of the high degree of diagenesis of the samples, we found no material
455 suitable for radiometric dating (Supplementary Fig. 2). Petrographically, outcrop
456 samples identified as coral rudstone to packstone reveal corals as major components
457 while gastropods, benthic foraminifera (Amphistegina, Calcarina), encrusting
458 foraminifera, coralline red algae, echinoid spines, and serpulids were minor
459 (Supplementary Fig. 1a). Terraces TVIII and TIX are generally preserved along the
460 west-facing side of the coast from the central to the southern portion of the cape.
461
462 4.2. Terrace deformation in Cape Bolinao
463 To examine the lateral extent and large-scale morphology of the CRTs in Cape
464 Bolinao, we generated stacked swath profiles (Fig. 5). Generally, we observe slight
465 variability in terrace elevations in which the CRTs are gently sloping down towards the
466 northeast to where the present-day modern reefs are observed to be widest (Fig. 5e).
467 We also recognize the lateral variability of terrace preservation from northeast to
468 southwest as higher terraces (TVIII-TIX) are preserved along the southwest while the
469 lower terraces (TV to TVII) are better preserved along the northeast. TIV is observed
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470 to have a poorly developed terrace topography (terrace surfaces are irregular) which
471 might be due to its composition of mostly coral boulders and cobbles. Also, the number
472 of terraces varies as fewer terraces are observed along the southwest and most
473 terraces are preserved on the central portion of the cape, along the west-northwest-
474 facing side of the coast. By comparing different viewing angles, we determined an
475 ESE-ward directed tilt for the higher terraces (TVIII-TIX) which is generally in
476 agreement with an eastward tilt direction previously suggested by Maemoku and
477 Paladio (1992).
478
479
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480 Fig. 5. Morphology and deformation pattern of Cape Bolinao CRTs. a. Slope map of Cape Bolinao
481 CRTs with the location of the 100 swaths we used for the shoreline angle analysis (in boxes b, c, d)
482 and the ~230 parallel lines (50-m spacing between each line) we used to calculate the representative
483 topographic swath profiles with widths of 50 and 100 m (in box e). b-d. Shoreline angle analysis for the
484 (b) northern portion of the cape with cross section from S-N, (c) central portion (northwest-facing side),
485 (d) southern portion with cross section from E-W. Gray lines represent the topographic swaths
486 (horizontal distance in km) calculated using TerraceM while the gray circles represent the elevation and
487 position of delineated shoreline angles with vertical uncertainties. A representative profile is shown
488 highlighted by blue and red (shoreline angles) circles. e. Representative stacked swath profiles
489 generated in Cape Bolinao. The highest terrace, TIX, is generally observed along the central and
490 southern portion of the cape and is characterized by a broad, generally planar paleo-reef surface. Also
491 prominent is the general flat morphology of the reef terraces which are gently sloping down towards the
492 NE to where the modern reefs are presently observed to be widest. Colored lines highlight the
493 approximate delineation of the mapped terraces (TIII to TIX).
494
495 We also systematically tracked the elevation of the shoreline angles to better quantify
496 the overall deformation in the area. Based on ~390 calculated shoreline angles, we
497 confirm the observations from the stacked swath profile analysis pointing to a lateral
498 variability in the preservation and number of terraces along strike and general dip
499 direction towards the ESE (Fig. 5b, 5c, 5d; Supplementary Fig. 3). From these coupled
500 analyses, we observe the Holocene terraces fringing the entire coast from northeast
501 to southwest although we recognize variabilities in terms of elevations. The elevations
502 of CRTs derived from the combined stacked swath profile analysis and shoreline angle
503 analysis are within the values we measured in the field.
504
505 4.3. Reef sequence models
506 By taking into consideration different parameters (i.e., sea-level curve, vertical uplift,
507 reef growth rate, erosion rate, initial substrate slope), we can infer the conditions which
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508 supported the development of the current morphology of the terraces. We used the
509 elevation and width of the CRTs derived from our high-resolution geomorphic analysis
510 and directly compared it with the reef simulations produced. We chose a
511 representative cross-section profile (BI-B) that shows the best preservation of the reef
512 terraces in Cape Bolinao and superimposed it with stacked swath profiles calculated
513 parallel to this. For our reef models, we explored the parametric fields of sea level and
514 uplift and used constant values (based on local observations and previous works) for
515 the rest of the parameters. By focusing our attention on these parameters, we can
516 examine the effects of late Quaternary sea-level variations and changes in vertical
517 uplift in the development of the CRTs in the region.
518
519 To reproduce the maximum terrace elevation at 155 m amsl, uplift rates should be
520 more than 0.4 mm/yr irrespective of the sea-level curves used. However, given the
521 lateral extent of the Cape Bolinao CRTs, low uplift rates are not enough to reproduce
522 the terrace width of ~5 km for the highest paleo-reef surface, TIX. We then use higher
523 uplift rates (>1.10 mm/yr) to increase the reef terrace widths. Higher uplift rates also
524 produce terraces above 155 m. In Cape Bolinao, the highest terrace is situated at 155
525 m and no terraces above it are presently observed. With these restrictions, uplift rates
526 of 1.15-1.20 mm/yr were used to achieve TIX terrace elevation at 155 m amsl with
527 minimum width of 1 km. Using the best fit uplift rate of 1.17 mm/yr, regardless of the
528 sea-level curves used, the terrace at ~155 m elevation is designated MIS 5e (Fig. 6).
529 Based on the simulations, the lowest terrace is MIS 1 with a modelled terrace width of
530 0.5 to 1 km and elevations close to present sea level.
531
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532
533 Fig. 6. Reef modelling results for the Cape Bolinao CRTs using different sea-level curves and
534 constant parameters. a. Sea-level curves used to reproduce the CRTs in Cape Bolinao. The color
535 codes below indicate the Marine Isotope Stages (MIS) from MIS 1 to 11. b. Reef simulations reproduced
536 using different sea-level reconstructions. The paleo-reef terraces for TIX and TIV are highlighted by
537 colored lines and model parameters are also shown. c. Representative stacked swath profile along the
538 central portion of the coast where a complete suite of CRTs was observed. For comparison, shown
539 here is the general topography showing the elevations of the CRTs along the best preserved area in
540 Cape Bolinao. SL16: Spratt and Lisiecki (2016), W02: Waelbroeck et al. (2002), B05: Bintanja et al.
541 (2005), R09: Rohling et al. (2009), G14: Grant et al. (2014), U: Uplift (Vertical land motion rate), RG:
542 Potential reef growth rate, E: Erosion rate, IS: Initial slope, MRGD: Maximum reef growth depth, ORGD:
543 Optimal reef growth depth, WEMD: Wave erosion maximum depth.
544
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545 5. Discussion
546 5.1. Cape Bolinao CRT development during the Late Pleistocene
547 As highlighted by previous works (e.g., De Gelder et al., 2020; Chauveau et al., 2024),
548 different sea-level curves shape different CRT morphologies; hence, the choice of the
549 sea-level curve will most likely determine the overall fossil reef morphology. Because
550 of this, we ran our simulations and tested five different sea-level curves: Waelbroeck
551 et al. (2002), Bintanja et al. (2005), Grant et al. (2014), Rohling et al. (2009), and Spratt
552 and Lisiecki (2016) to reproduce the coral reef sequences in Cape Bolinao. Comparing
553 the simulations produced, we find the sea-level curve of Waelbroeck et al. (2002) to
554 closely reproduce the elevations of the reef terraces we observe today. This curve is
555 based on North Atlantic and Equatorial Pacific Ocean benthic foraminifera oxygen
556 isotopic ratios calibrated with relative sea-level data based on corals over the last
557 climatic cycle. The other sea-level reconstructions were meanwhile based on a global
558 compilation of benthic oxygen isotope data (Bintanja et al., 2005), on oxygen isotopic
559 ratios of planktonic foraminifera and bulk sediment from the Red Sea (Rohling et al.,
560 2009), on U/Th-dated speleothem oxygen isotopic ratio record synchronized with an
561 Asian monsoon signal with dust and SL records (Grant et al., 2014), and on the
562 principal component analysis of earlier compilations (Spratt and Lisiecki, 2016).
563
564 The best-fitting simulation is obtained using the sea-level curve of Waelbroeck et al.
565 (2002) and an uplift rate of 1.17 mm/yr (Fig. 7). In general, the reef simulation clearly
566 replicated the TIX terrace at an elevation of ~155 m and the elevations of the fringing
567 terraces (TVII to TIV). In terms of terrace widths, it showed relatively wider (>0.5 km)
568 planar surfaces for TIX, TVII, and TIV and narrow (<300 m) terrace surfaces for TVIII
569 and TVI, which is in agreement with our morphological analysis. The modelled
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570 morphology and width of terrace TV, however, is at odds with our morphological
571 analysis. It is worth noting that, while the reef simulation also produced terraces at
572 elevations higher than 160 m, we did not observe any terraces higher than 155 m in
573 Cape Bolinao. Also, it did not fully reproduce the width of TIX (~5 km), that we observe
574 today. To explain this, we refer to the underlying lithologic units (interpreted to indicate
575 an epi- to upper meso-pelagic setting) beneath the reefal terraces of Cape Bolinao.
576 We postulate that a relative sea-level change (~200-300 m) is needed to reduce paleo-
577 water depths and subsequently provide a suitable substrate for later shallow-marine
578 reef development. The gently-dipping beds provided more accommodation space for
579 the development of an extensive Late Pleistocene reef platform or table reef.
580 Continuous uplift superimposed on a changing sea level led to the emergence of this
581 table reef followed by formation of fringing reefs (corresponding to lower reef terraces)
582 along its slope. Husson et al. (2018) pointed out that relative sea-level change (uplift
583 or subsidence) exposes pristine domains of the shore to reef growth and expands the
584 accommodation space. Vertical land motion therefore fosters reef carbonate
585 productivity and, in addition, productivity is higher for shallow slopes than for steep
586 ones as the former provide wider accommodation spaces, which can provide favorable
587 foundations for reef growth during subsequent reoccupations (Husson et al., 2018;
588 Pastier et al., 2019).
589
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590
591
592 Fig. 7. Best-fit reef simulation. a. Reef simulation using the sea-level reconstruction of Waelbroeck
593 et al. (2002) and uplift rate of 1.17 mm/yr. b. Representative topographic profile (B-BI, Fig. 1c) measured
594 along the central portion of the cape where a complete suite of terraces is observed. Dashed lines allow
595 comparison of the field observations with the corresponding modelled terraces and arrows point to the
596 shoreline angles calculated along this profile. RG: Potential reef growth rate, U: Uplift (Vertical land
597 motion rate), E: Erosion rate, IS: Initial slope, MRGD: Maximum reef growth depth, ORGD: Optimal reef
598 growth depth, WEMD: Wave erosion maximum depth, SL: Sea-level curve used.
599
600 With our best-fitting simulation, we can propose a morpho-chronological framework
601 for the Cape Bolinao CRTs, with TIX formed at peak MIS 5e, and the lowest terrace
602 (corresponding to TI to TIII) formed during MIS 1. The reef simulations reproduce two
603 terraces (a well-defined planar surface and an indistinct lower terrace) for MIS 5e and
604 MIS 5a. Sea-level reconstructions for the MIS 5e show either a single peak
605 (Waelbroeck et al., 2002; Bintanja et al., 2005; Grant et al., 2014) or multiple peaks
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606 (Rohling et al., 2009). Our numerical reef simulations reveal that at least two terraces
607 can be created during MIS 5e using the curves of Spratt and Lisiecki (2016),
608 Waelbroeck et al. (2002), and Bintanja et al. (2005) while three terraces can be
609 reproduced using the curves of Grant et al. (2014) and Rohling et al. (2009). The
610 simulations suggest that more than one terrace can be created during MIS 5e and
611 multiple sea-level peaks are not required to create such terraces within one isotopic
612 stage. Our results are comparable with the findings of Chauveau et al. (2023) and De
613 Gelder et al. (2023) from their works in Sumba Island in which their simulations show
614 that a single peak during MIS 5e can form multiple terraces associated with MIS 5e.
615 We also propose that the antecedent topography (the terraces which formed during
616 MIS 6) influenced the pattern and morphology of the overlying MIS 5e terraces as
617 former terraces were reoccupied during the MIS 5e transgression. For Sumba Island
618 CRTs, Chauveau et al. (2023) showed that antecedent CRTs influence new reef
619 constructions and more likely explain the presence of multiple CRTs associated with
620 MIS 5e. The results of our reef modelling support the proposition of Pastier et al. (2019)
621 challenging the commonly assumed bijective relationship between sea-level
622 highstands and terraces (that is, one-to-one correspondence between a reef terrace
623 and a sea-level highstand).
624
625 5.2. Tectonic deformation along the Manila Subduction Zone
626 With our detailed morphological analysis coupled with reef modelling, we determine
627 that the most likely long-term uplift rate is 1.17 mm/yr for Cape Bolinao. This value is
628 comparatively lower than previously estimated by Maemoku and Paladio (1992) in
629 Cape Bolinao (1.3 mm/yr) and higher than MIS 5e uplift rates estimated by Maxwell et
630 al. (2018) from CRTs in northwest Luzon (0.13 to 0.3 mm/yr). We observed the highest
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631 reef platform (MIS 5e reef) to be titled towards the ESE and the paleo-reef surfaces
632 are striking towards the NNE. The deformation patterns observed as well as the high
633 uplift rates estimated are more likely attributed to subduction processes along the
634 Manila Trench, in particular, the subduction of the Scarborough Seamount Chain
635 (SSC), a NE-striking South China Sea extinct spreading ridge, beneath the west Luzon
636 forearc region (e.g., Pautot and Rangin, 1989; Armada et al., 2020).
637
638 Based on analyses of seismic reflection data and bathymetric data, Armada et al.
639 (2020) provided new evidence of bathymetric highs (i.e., seafloor relief related to
640 seamounts and ridges) being subducted eastward beneath the Luzon Island with
641 vertical deformation in the forearc region being concentrated at 17°N to 15.5°N
642 latitudes. Our work provides a supporting evidence of SSC subduction beneath the
643 west Luzon forearc during the late Quaternary causing high uplift in the region and
644 tilting of the reef terrace towards the ESE. Consequently, this might also explain the
645 difference in long-term uplift rates estimated from the CRTs in Cape Bolinao and in
646 northwest Luzon.
647
648 The Luzon forearc basin can be separated into the North Luzon Trough (NLT) and the
649 West Luzon Trough (WLT) with boundary at the 17°N latitude (Fig. 1b, Armada et al.,
650 2020) and Hsu et al. (2012) suggests a partially locked fault zone near 15–16.5°N
651 beneath the WLT adjacent to the SSC. While the CRTs of Cape Bolinao are situated
652 in this highly coupled region, the CRTs of northwest Luzon are adjacent the NLT,
653 characterized by a relatively smooth seafloor. By examining the relationship between
654 observed coastal late Pleistocene uplift rates and various geodynamic parameters,
655 Henry et al. (2014) suggests that the first order parameter explaining coastal uplift
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656 along subduction zones is the small-scale heterogeneities of the subducting plate (i.e.,
657 subducting aseismic ridges). With this, we suggest that roughness of the seafloor (i.e.,
658 the impinging SSC) might likely be the reason for the difference in long-term uplift rates
659 between the two regions.
660
661 The subduction of SSC might also be the likely explanation on the variation of mid-
662 Holocene coral ages collected from the lower terraces from both localities. The
663 younger mid-Holocene corals in Cape Bolinao might have experienced a more recent
664 uplift compared to the relatively older mid-Holocene corals in northwest Luzon (Fig. 8,
665 Table 3). In addition, the older corals in northwest Luzon (i.e., Currimao, Badoc) are
666 better preserved than those in Cape Bolinao as we did not find pristine corals in our
667 sites older than 6.4 kyr. It may be the case that the corals in Cape Bolinao have been
668 more affected by meteoric diagenesis than the ones in northwest Luzon. While high
669 uplift rates explain much to this as the reefs may be subaerially exposed earlier and
670 longer, the presence of submarine groundwater discharge in Cape Bolinao (Cantarero
671 et al, 2019) might also contribute.
672
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673
674
675 Fig. 8. Elevation of dated fossil Holocene corals in west Luzon and northwest Luzon. Plotted are
676 the elevation and ages of dated Holocene corals (plotted as symbols) collected along west Luzon Island.
677 The symbols represent the coral ages reported by previous works in Pangasinan (Ramos and Tsutsumi,
678 2010, in inverted triangle), in Currimao (Shen et al., 2010, in diamond symbols), in Currimao and Badoc
679 (Maxwell et al., 2018, in triangle symbols), and in Currimao, Badoc, and Palawan (Maeda et al., 2004,
680 in circles). Shen et al. (2010) examined reef cores thereby providing an older, longer record for the
681 Holocene reef development in northwest Luzon. The color codes for the symbols represent the dating
682 method used: (1) light blue for 230Th ages and (2) dark blue for ages derived using radiocarbon. Also
683 shown here are the calculated paleo-relative sea level (shown as blue rectangles when IMCalc
684 (Lorscheid and Rovere, 2019) is used and dashed lines if an average reef depth of 5 m is considered).
685 Most of the dated corals are collected from the surfaces of the lowest terrace (TI). We observe that
686 corals from Cape Bolinao and Pangasinan are relatively younger than the corals from northwest Luzon.
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687 Table 3. Summary of sea-level indicators in west Luzon and Palawan Islands, Philippines and nomenclature used.
688
Cape Bolinao CRTs (This Study)
Bolinao Reef Terraces
(Maemoku and
Paladio, 1992)
Currimao CRTs
(Maxwell et al., 2018)
Palawan Notches
Terrace
Level
Elevation
(m)
Elevation
uncertainty
(m)
Coral
Ages (kyr)
Terrace
Width
(km)
Elevation
(m)
Terrace
Level
Elevation
(m)
Coral Ages
(kyr)
Terrace
Level
Elevation
(m)
Coral Ages
(kyr)
Indicator
TI*
3.87
± 0.01
6.382-5.71
<10 m
3.4 - 3.7
7.373 - 6.163
TI
1.5 ± 0.15
6.7 - 5.69
LTN
TII*
5.72
± 0.02
<20 m
6.1 - 6.6
6.774 - 6.533
TII
TIII*
8.57
± 0.02
0.02 - 0.5
6.8 - 8.6
BVII
8.4 - 9.2
TIII
TIV
31.42
± 13.75
0.1 - 1.0
21 - 23.1
BVI
TV
55.15
± 14.66
0.5 - 1.8
30.2 - 59.2
BV
TVI
80.55
± 12.52
0.1 - 0.3
43.6 - 74.7
BIV
TVII
102.15
± 9.77
0.2 - 3
58.6 - 107.1
BIII
TVIII
119.99
± 14.14
0.1 - 0.3
100-136
BII
TIX
143.34
± 11.50
~ 5.0
120-155
BI
24 - 35
119.7 - 108.4
LPT
6.8 ± 0.15
126.5 ± 20
UTN
689
690 * measured elevation from discrete topographic survey transects
691 LPT: Late Pleistocene Terrace; LTN: Lower tidal notch (from Maeda et al. (2004)); UTN: Upper tidal notch (from Omura et al. (2004)).
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692 6. Conclusions
693 This work presents a new interpretation of the formation of CRTs in the northern Coral
694 Triangle as a function of relative sea-level changes and tectonic uplift along an active
695 margin during the Late Pleistocene. With new stratigraphic and geochronologic data
696 coupled with high-resolution morphological analysis and reef modelling, we are able
697 to examine in detail the Cape Bolinao CRTs in western Luzon, Philippines and provide
698 a record of sea-level changes in the West Philippine Sea during the last glacial cycle.
699 We are also able to estimate a new uplift rate for Cape Bolinao and examine terrace
700 deformation patterns providing additional evidence for the subduction of the
701 Scarborough Seamount Chain beneath the Luzon Island. While our analysis is limited
702 by lack of robust age constraints and glacial isostatic adjustment (GIA) corrections for
703 the region, our study serves as the first attempt to provide a morpho-chronological
704 framework for the development of late Quaternary CRTs in the northern Coral Triangle
705 thereby presenting a key dataset especially in a region that is poorly studied. However,
706 the knowledge on the vertical distribution of the carbonate sequences in Cape Bolinao
707 (and other sites in the region) is still incomplete but required to fully constrain the reef
708 response to sea-level changes. More importantly, future efforts should be done to
709 constrain the timing of the older reef terraces using different techniques.
710
711 Acknowledgements
712 The authors acknowledge the doctoral program Marie Skłodowska-Curie Innovative
713 Training Network (ITN) 4D-REEF, coordinated by Willem Renema. The authors also
714 thank the following institutions: the National Mapping and Resource Information
715 Authority (NAMRIA) for generously providing the 2013 airborne IFSAR-DTM
716 topographic data; the Hydrography Branch of NAMRIA for their provision of actual tide
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717 gauge and bathymetric data; the Philippine Active Geodetic Network (PAGeNet) -
718 Geodesy Division/Mapping and Geodesy Branch of NAMRIA for granting access to
719 real time corrections during RTK-GNSS surveys and for efficient technical support;
720 and the Mines and Geosciences Bureau for their provision of necessary permits for
721 the samples. The authors are grateful to the local government officials of the
722 municipalities of Bolinao, Burgos, and Dasol and the province of Pangasinan for their
723 support and permission to conduct field activities. We also thank Mari Shylla Joaquin
724 for the logistical support. 230Th dates using solution MC-ICPMS protocol for
725 stalagmites were determined at the High-Precision Mass Spectrometry and
726 Environment Change Laboratory (HISPEC), Department of Geosciences, National
727 Taiwan University, supported by grants from Taiwan ROC MOST (111-2116-M-002-
728 022-MY3 to C.-C.S.) and National Taiwan University (112L894202 to C.-C.S.). This
729 manuscript is part of a project that has received funding from the European Research
730 Council (ERC) under the European Union’s Horizon 2020 research and innovation
731 programme (Grant agreement No. 802414). K.M. and H.W. were supported by funding
732 from the European Union’s Horizon 2020 research and innovation programme under
733 the Marie Sklodowska-Curie Actions (Grant agreement No 813360). This publication
734 is part of Kathrine Maxwell’s doctoral thesis.
735
736 CRediT authorship contribution statement
737 K. Maxwell: Conceptualization; Formal Analysis; Investigation; Methodology; Data
738 curation; Writing – original draft; Writing – review & editing. A. Rovere:
739 Conceptualization; Writing – review & editing; Supervision; Funding acquisition. H.
740 Westphal: Conceptualization; Writing – review & editing; Supervision; Funding
741 acquisition; Resources. K. Garas: Investigation; Writing – review & editing. M. Guinto:
This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=4888948
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742 Investigation; Writing – review & editing. D. Chauveau: Formal analysis; Writing –
743 review & editing. H.-M. Hu: Formal analysis; Writing – review & editing. C.-C. Shen:
744 Formal analysis; Writing – review & editing; Funding acquisition; Resources
745
746 Declaration of competing interest
747 The authors declare that they have no known competing financial interests or personal
748 relationships that could have appeared to influence the work reported in this paper.
749
750 Funding sources
751 This research was supported by the program 4D-REEF, funded by the European
752 Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-
753 Curie Actions (grant no. 813360) to Hildegard Westphal. Alessio Rovere received
754 financial support from the ERC starting grant “WARMCOASTS” (grant no. ERC-StG-
755 802414). 230Th dates using solution MC-ICPMS protocol for stalagmites were
756 determined at the High-Precision Mass Spectrometry and Environment Change
757 Laboratory (HISPEC), Department of Geosciences, National Taiwan University,
758 supported by grants from Taiwan ROC MOST (111-2116-M-002-022-MY3 to Chuan-
759 Chou Shen) and National Taiwan University (112L894202 to Chuan-Chou Shen).
760
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1032 Yokoyama, Y., Esat, T., 2011. Global Climate and Sea Level: Enduring Variability and Rapid
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1035
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1036 Supplementary Figures
1037
1038
1039 Supplementary Fig. 1. Representative thin sections from the coral reef terraces in Cape Bolinao.
1040 a. A sample, which comes from TIX (MIS 5e) terrace, shows a coral (white arrow, probably Porites) that
1041 is diagenetically altered. b. A sample from TVII (MIS 5c) shows Halimeda (white arrow) as the dominant
1042 skeletal component. c. A sample from TVI (MIS 5a) terrace shows abundant mollusks and large benthic
1043 foraminifera (Calcarina, white arrow) with a coral fragment enveloped by micritic rim and encrusting
1044 foraminifera. d. A sample collected from TIV (MIS 3) terrace showing dominant lithoclasts probably
1045 coming from the higher terraces (white arrow, Halimeda floatstone lithoclast).
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1046
1047
1048
1049 Supplementary Fig. 2. Field photos and thin section images of the representative fossil corals in Cape Bolinao. Thin section images show different diagenetic
1050 textures observed on the fossil corals. a-b. Fossil corals from TI show little to no diagenetic textures. c-d. Fossil corals from TII already show diagenetic features
1051 although coral skeletal features are still distinguishable. e-f. Fossil corals from TIII show high-degree of diagenesis and original aragonite is completely replaced
1052 by low-Mg calcite. g-h. Fossil corals from TIX show high-degree of diagenesis and evidence of weathering/soil formation.
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1053
1054 Supplementary Fig. 3. Stacked Swath Profiles (of different viewing angles) calculated for Cape
1055 Bolinao: along (a) NNE-SSW, (b) N-S, (c) E-W, and (d) SE-NW.
1056
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1057 Figure captions
1058
1059 Fig. 1. Cape Bolinao and its location along the western Philippines at the apex
1060 of the Coral Triangle. a. Map of Southeast Asia and the Coral Triangle (shaded in
1061 blue). Circles show the previously reported Last Interglacial (LIG) CRTs in the region
1062 as compiled in Maxwell et al. (2021). b. Tectonic elements bordering western Luzon
1063 Island and location of Cape Bolinao (in black box). Focal mechanism solutions (>M5)
1064 were obtained from the Global CMT Project (Dziewonski et al., 1981; Ekström et al.,
1065 2012) and in red lines, the active faults traversing the PMB from the PHIVOLCS
1066 (2020). c. Surface classification model (SCM) showing the different paleo-reef
1067 surfaces in Cape Bolinao. The highest terrace (TIX, in dark blue) indicates the oldest
1068 fossil coral reef terrace mapped and the lowest terrace (in yellow) are the Holocene
1069 reefs. Also shown is the location of the present-day reef, the Bolinao-Anda Reef
1070 Complex (BARC), from McManus et al. (1992). The survey sites in Patar, Bolinao: (1)
1071 Rockview Point (in Fig. 3a) and (2) Punta Piedra Point (in Fig. 3b) are bounded by
1072 boxes. The representative profile, B–BI, (Fig.7) is also shown. Basemaps were created
1073 using topography data from Japan Aerospace Exploration Agency (JAXA) ALOS
1074 World 3D–30 m (AW3D30)
1075 (https://www.eorc.jaxa.jp/ALOS/en/dataset/aw3d30/aw3d30_e.htm) and bathymetry
1076 data from the GEBCO Compilation Group (2023) GEBCO 2023 Grid
1077 (doi:10.5285/f98b053b-0cbc-6c23-e053-6c86abc0af7b). Maps were created using
1078 ESRI ArcGIS Pro 3.1 and the Generic Mapping Tools (GMT) software (Wessel et al.,
1079 2019). PSP: Philippine Sea Plate; EU: Eurasian Plate; SU: Sundaland block; MT:
1080 Manila Trench; WLT: West Luzon Trough; WPS: West Philippine Sea; NLT: North
1081 Luzon Trough; WBF: Western Boundary Fault; SSC: Scarborough Seamount Chain.
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1082 Fig. 2. Coral reef terraces in Cape Bolinao. a. Schematic diagram of the BARC and
1083 the relationship of shoreline angle with present-day sea level. The modern-day reef
1084 zonation is based on the descriptions of McManus et al. (1992) and Vergara et al.
1085 (2010). b. Aerial photo (view looking SE) of the CRTs as observed in Punta Piedra
1086 Point. Lower terraces (TI-TIII) are recognized at elevations <9 m and higher terraces
1087 are highlighted by dashed lines (TIV and TIX) and black arrows. c-d. Aerial photos of
1088 the modern reef flat characterized by a dominance of seagrass with occasional
1089 patches of living corals and dead coral boulders. White box shows the location of photo
1090 (d), which shows a living coral (i.e., microatoll, bounded by yellow circle) surrounded
1091 by seagrass (bounded by yellow dashed line).
1092
1093 Fig. 3. Surveyed lower terraces in Cape Bolinao. We identified three steps of
1094 terraces from topographic surveys in Patar, Bolinao (location shown in Fig. 1c). a.
1095 Outcrop photo (white dashed line outlines survey transect) and topographic profile
1096 surveyed from Rockview Point show two steps with TI measured at 3.87 ± 0.01 m and
1097 TIII measured at 8.02 ± 0.01 m. Most of the mid-Holocene coral samples were
1098 collected from TI at this site. b. Field photo (white dashed line outlines survey transect)
1099 and topographic profile surveyed from Punta Piedra Point revealing two steps with TII
1100 measured at 5.72 ± 0.02 m and TIII measured at 8.57 ± 0.02 m. c-d. Aerial
1101 photographs illustrating the general morphology of the lower CRTs in Patar, Bolinao
1102 and their position several meters above sea level. Arrows point to the paleo-reef
1103 surfaces of identified terraces.
1104
1105 Fig. 4. Corals and composition of Cape Bolinao reef terraces. a. Outcrop photo
1106 of TI showing in situ corals (e.g., Porites) (bounded by white dashed contours). b-d.
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1107 In situ corals from (b) TII, (c) TIII, and (d) TIX from the families of Acroporidae and
1108 Merulinidae that are diagenetically altered. e-f. Outcrop photos of (e) terrace TVII
1109 where we collected a Halimeda-rich floatstone and (f) terrace TIV, which is
1110 characterized by cobbles and boulders composed mainly by diagenetically altered
1111 corals (bounded by white dashed contours).
1112
1113 Fig. 5. Morphology and deformation pattern of Cape Bolinao CRTs. a. Slope map
1114 of Cape Bolinao CRTs with the location of the 100 swaths we used for the shoreline
1115 angle analysis (in boxes b, c, d) and the ~230 parallel lines (50-m spacing between
1116 each line) we used to calculate the representative topographic swath profiles with
1117 widths of 50 and 100 m (in box e). b-d. Shoreline angle analysis for the (b) northern
1118 portion of the cape with cross section from S-N, (c) central portion (northwest-facing
1119 side), (d) southern portion with cross section from E-W. Gray lines represent the
1120 topographic swaths (horizontal distance in km) calculated using TerraceM while the
1121 gray circles represent the elevation and position of delineated shoreline angles with
1122 vertical uncertainties. A representative profile is shown highlighted by blue and red
1123 (shoreline angles) circles. e. Representative stacked swath profiles generated in Cape
1124 Bolinao. The highest terrace, TIX, is generally observed along the central and southern
1125 portion of the cape and is characterized by a broad, generally planar paleo-reef
1126 surface. Also prominent is the general flat morphology of the reef terraces which are
1127 gently sloping down towards the NE to where the modern reefs are presently observed
1128 to be widest. Colored lines highlight the approximate delineation of the mapped
1129 terraces (TIII to TIX).
1130
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1131 Fig. 6. Reef modelling results for the Cape Bolinao CRTs using different sea-
1132 level curves and constant parameters. a. Sea-level curves used to reproduce the
1133 CRTs in Cape Bolinao. The color codes below indicate the Marine Isotope Stages
1134 (MIS) from MIS 1 to 11. b. Reef simulations reproduced using different sea-level
1135 reconstructions. The paleo-reef terraces for TIX and TIV are highlighted by colored
1136 lines and model parameters are also shown. c. Representative stacked swath profile
1137 along the central portion of the coast where a complete suite of CRTs was observed.
1138 For comparison, shown here is the general topography showing the elevations of the
1139 CRTs along the best preserved area in Cape Bolinao. SL16: Spratt and Lisiecki
1140 (2016), W02: Waelbroeck et al. (2002), B05: Bintanja et al. (2005), R09: Rohling et al.
1141 (2009), G14: Grant et al. (2014), U: Uplift (Vertical land motion rate), RG: Potential
1142 reef growth rate, E: Erosion rate, IS: Initial slope, MRGD: Maximum reef growth depth,
1143 ORGD: Optimal reef growth depth, WEMD: Wave erosion maximum depth.
1144
1145 Fig. 7. Best-fit reef simulation. a. Reef simulation using the sea-level reconstruction
1146 of Waelbroeck et al. (2002) and uplift rate of 1.17 mm/yr. b. Representative
1147 topographic profile (B-BI, Fig. 1c) measured along the central portion of the cape
1148 where a complete suite of terraces is observed. Dashed lines allow comparison of the
1149 field observations with the corresponding modelled terraces and arrows point to the
1150 shoreline angles calculated along this profile. RG: Potential reef growth rate, U: Uplift
1151 (Vertical land motion rate), E: Erosion rate, IS: Initial slope, MRGD: Maximum reef
1152 growth depth, ORGD: Optimal reef growth depth, WEMD: Wave erosion maximum
1153 depth, SL: Sea-level curve used.
1154
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1155 Fig. 8. Elevation of dated fossil Holocene corals in west Luzon and northwest
1156 Luzon. Plotted are the elevation and ages of dated Holocene corals (plotted as
1157 symbols) collected along west Luzon Island. The symbols represent the coral ages
1158 reported by previous works in Pangasinan (Ramos and Tsutsumi, 2010, in inverted
1159 triangle), in Currimao (Shen et al., 2010, in diamond symbols), in Currimao and Badoc
1160 (Maxwell et al., 2018, in triangle symbols), and in Currimao, Badoc, and Palawan
1161 (Maeda et al., 2004, in circles). Shen et al. (2010) examined reef cores thereby
1162 providing an older, longer record for the Holocene reef development in northwest
1163 Luzon. The color codes for the symbols represent the dating method used: (1) light
1164 blue for 230Th ages and (2) dark blue for ages derived using radiocarbon. Also shown
1165 here are the calculated paleo-relative sea level (shown as blue rectangles when
1166 IMCalc (Lorscheid and Rovere, 2019) is used and dashed lines if an average reef
1167 depth of 5 m is considered). Most of the dated corals are collected from the surfaces
1168 of the lowest terrace (TI). We observe that corals from Cape Bolinao and Pangasinan
1169 are relatively younger than the corals from northwest Luzon.
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