Text S1 Angsa island Our farthest site from the trench is on the west flank of Angsa, an islet that lies only 3 km off the west coast of mainland Sumatra, near the small city of Pariaman (Figure 3). Numerous living Porites microatolls inhabited Angsa’s intertidal reef flat when we visited in mid-1997 (Figure S1). All of these heads exhibited nearly flat upper surfaces indicative of nearly stable HLS through the past few decades. We slabbed two heads, each representative of a different population of the flat-topped microatolls. One microatoll (An97A1) grew on a narrower intertidal reef near the northern edge of the site. The other head (An97A2) was growing in an elevated pool behind an arcuate 40-cm high rampart near the southern edge of the intertidal reef. Both slabs exhibited similar HLS histories, but HLS elevations average about 40 cm higher in An97A2. We do not present the history of this slab, since it provides a record of HLS in an elevated pool, rather than being an indicator of HLS changes on the open reef. The upper surface of slab An97A1 exhibits a pristine micro-topography of ridges and swales (Figures S1b and S2a). These are concentric on the outer parts of the microatoll, and more sinuous on the top of the inner part of the head. The concentric ridges and swales represent minor non-tectonic HLS fluctuations associated with inter-annual fluctuations in HLS ([Natawidjaja, 2003], Chapter 4). The sinuous ridges and swales on the inner part of the slab are overgrowths that prohibited viewing older HLS unconformities in the field but also protected them from erosion. The oldest annual band in the slab grew in the early to mid-1930s (Figure S2a). The pattern of banding of the pre-1955 bands suggests that early in its life, in about 1955, the head was overturned. (The 1947-1955 bands are thinner on the top of the head and thicker on the side. Commonly growth is faster on the top of a submerged head). Free upward growth occurred in the decade after overturning. The first HLS HLS unconformity to give a reliable indication of HLS elevation is on the 1965 band. In the three decades between 1965 and 1996, only 12 annual bands display free upward growth. HLS unconformities limited upward growth in all other years. From the history of HLS unconformities and their relative elevations, we can calculate the average rate of submergence of Angsa islet. The procedure for doing this is laid out in Natawidjaja [2003] and Natawidjaja et al. [2004]. The average rate of submergence of Angsa islet was about 2.2 mm/yr between 1965 and 1997 (Figure S2b). The average rate before the late 1970s appears to have been greater than the average rate in the 1980s and 1990s. Labuan Bajau The Labuan Bajau site is in the southwestern corner of a large bay, on the northern tip of Siberut island (Figure 3). Raised outer perimeters on the modern microatolls indicates moderate submergence in the past few decades. Landward of the large bay is a smaller bay, surrounded by swampy ground. The sample site is at the mouth of this smaller bay, just northeast of its narrow outlet into the larger bay (Figure S3). Currents at the site are swift during tidal flows into and out of the smaller bay. Modern heads populate the outer edge of the intertidal reef. Landward of the modern population is a graveyard of older coral heads, comprising at least three separate populations. The oldest and highest of the heads yielded a mid-Holocene radiocarbon age of 3970 ?100 BP (N. Abram, pers. comm). Many of the older heads are highly eroded; not uncommonly, all that remains is a doughnut-like ring of a former microatoll, resting on the reef substrate. In our experience, this phenomenon is unique to this site. We suspect that strong tidal ebb currents bring concentrations of organic acids out from the murky inner bay and that these dissolve the coral skeletons. Abnormal amounts of erosion are apparent in the modern heads as well. We sampled two heads that appeared to have minimal degradation. As we will demonstrate below, however, chemical erosion has resulted in disparate HLS histories for these two microatolls. The annual banding in Lb99A1 is exceptionally clear, but erosion of the youngest bands gives an uncertainty of a couple years in the absolute ages (Figure S4A). The coral head began to grow in about 1916. The morphology of the oldest bands indicates that the head rolled over in about 1923. Subsequently, the head experienced unrestricted upward and outward growth for at least 7 years. The lack of any aberration in the uppermost part of the 1930 growth band means that its truncation on the upper surface of the microatoll is the result of erosion, not an HLS unconformity. We will see confirmation of this below, in our comparison with the record of a nearby head. The erosional truncation of all bands between 1930 and 1965 allows us to place only lower limits on HLS levels during that period. The microatoll from which this sample was cut has a well-defined shallow outer raised rim. The annual bands exposed in the slab show that this outer raised rim represents a general submergence since about 1965 ?3. The 4 to 5 years of free-upward growth in the mid- to late 1960s suggests a rapid or sudden submergence of about 60 millimeters in about 1965. A sample from a second young microatoll provides additional constraints on recent HLS history. The upper surface of the microatoll from which Lb99A3 was cut has a morphology that differs substantially from the one represented by Lb99A1. Thus we thought, initially, that these two heads grew at different times. Unlike the first microatoll, Lb99A3 has a high central dome and a lower outer rim (Figure S4B). This hat-shaped morphology is commonly indicative of an emergence event. The upper surface of this head is better preserved than the top of Lb99A1. A small depression on top of the high central dome indicates rapid submergence around 1935. This is consistent with a few centimeters of submergence during the 1935 earthquake, which Natawidjaja et al. [2004] showed was caused by rupture of the subduction interface just to the north, beneath the Batu Islands (Figure 3). Sometime between 1940 and 1950, emergence of about 200 mm occurred. We cannot differentiate between rapid and slow emergence, because erosion has obscured the details of the HLS history. Regardless, an emergence of this magnitude is unusual and may well indicate slip on the subduction interface at depth, perhaps induced by stresses imposed by the 1935 rupture to the north. The gently sloping surface between the annual bands of 1950 and 1965 indicates either a stable HLS or very slow emergence during that period. This is well-constrained by several clear HLS unconformities. The upper surface of the outermost 30 annual bands resembles that of Lb99A1. This suggests that little chemical erosion of the surface has occurred in the past three decades. Most notably, both records show gradual submergence of the heads in the past 30 years and a sudden episode of submergence in 1965?4. Figure S4C displays the HLS histories of the two neighboring slabs. Superimposition of the records shows both the similarity of their records since 1945 and the great discrepancy in their records for the prior decades. Because chemical erosion has obliterated most of the early record of Lb99A1, we rely only on the HLS record of Lb99A3 to infer the pre-1945 history. The principal features of the combined records are these: A small submergence occurred in about 1935, perhaps associated with the 1935 earthquake. A 200-mm emergence event or episode occurred sometime between 1940 and 1950, perhaps a reflection of slip induced on the underlying megathrust by rupture to the north in 1935. Between 1950 and the early 1960s, the site emerged at about 2 mm/yr. In about 1965 a small submergence event occurred. Since the mid-1960s, the site has been submerging at about 2.2 mm/yr. Tabekat Slabs at the Tabekat site have a low height-to-width ratio, which is indicative of low submergence rates. The site is near the mouth of Tabekat bay, along the northeast coast of northern Siberut island (Figure 3). We examined two sites, one on the eastern side of the bay near the southern tip of a long peninsula and one on the western side of the bay. The eastern site is a narrow intertidal flat inhabited by modern Porites and Goniastrea microatolls with the thin, cup-shaped morphology indicative of slow submergence (Figure S5). The western site contains mostly very old (probably mid-Holocene) heads, and is discussed here. We collected two slabs from two modern heads at the eastern site. Both are Porites microatolls, although one has a Goniastrea colony growing on top of it. This enables a comparison of HLS histories of the two different genera. We know from other sites that Goniastrea corallites can survive at elevations a hundred millimeters or so above Porites corallites (for example, Zachariasen et al. [1999], site NP94C). Slab Tbk99D1 contains the longer and simpler record of the two (Figure S6A). It shows an unsteady rise in HLS between about 1950 and its death in 1997 at an average rate of about 3.7 mm/yr. Several drops in HLS interrupt this slow rise. The drop of 1967 is the largest. Most of the rise in HLS occurred between the mid-60s and mid-80s. During the periods 1950-1962 and 1983-1997, the site experienced near-stability. Slab Tbk99D2 gives a more incomplete history, because the initial record is from a Porites head, but the later record is from the Goniastrea colony that surmounted it (Figure S6B). Up to about 1975 the HLS records of microatolls D1 and D2 are roughly the same. But the D2 Porites died in about 1975 and was immediately colonized by a Goniastrea. Because it took the Goniastrea colony about a decade to reach its HLS, we have poor constraints on HLS history from this microatoll during that decade. Furthermore, the Goniastrea’s post-1984 record is too short and irregular to constrain an average submergence rate well. However, its history is consistent with that of the other slab. The plot of both slabs’ HLS histories (Figure S6C) shows these features: Significant HLS die-downs occurred about every 5 years, in 1953, 1962, 1967/1968, 1972, and 1976. The 1962 event, so prominent in records from the Beuasak and Sikici sites and from slabs collected near the Equator [Natawidjaja et al., 2004], exists here, but is not prominent and is smaller than other HLS die-downs. The magnitude of HLS change in 1962 is comparable to non-tectonic fluctuations that have been discussed by Natawidjaja [2003] and Natawidjaja et al. [2004]. Thus, the presence of a tectonic event here in 1962 is not compelling. However, 1962 does seem to mark an abrupt increase in the average rate of submergence. The timing of the HLS unconformities in 1962, 1967, 1972/3, and 1976 are consistent with the occurrence of the moderate to strong ENSO/IOD events in those years [Saji et al., 1999]. Tabekat C Tabekat site C is on the protected side of a 500-m-long islet about 2 km south of the Tabekat cape and only a few hundreds meters south of the principal Tabekat site (Figure 3). The site is a narrow intertidal reef flat, which supports a sparse forest of mangroves (Figure S7). A population of modern heads occurs along the outer edge of the shallow reef platform. Most of these heads are dead, but a few microatolls still had a living band of corallites on their perimeters in 2002. Slab Tbk02C1 was cut from one of these heads, near the edge of the shallow reef. The deep-centered, tea-cup shape of the microatoll, with its several treads and risers ascending outward from the low, central flat, indicates a history of fast submergence. The presence of small arcuate lumps on the surface of the outer rim indicates that erosion can not have been much more than about 50 mm [Natawidjaja et al., 2004]. The Tbk02C1 slab reveals that HLS has been rising rapidly at the site for the past six decades (Figure S8). A least-squares fit to HLS unconformities over the entire coral life yields an average submergence rate of about 7.5 mm/yr. The rate has not been constant throughout that period. From about the mid-1940s to 1980, the microatoll grew upward nearly with the formation of only 2 clear HLS unconformities (in 1955 and 1971). A few unconformities may also have formed in the years just prior to 1971, but erosion of the upper surfaces of the bands obscures the HLS history. Numerous annual HLS unconformities between 1980 and 2002 suggests that the rate of subsidence has been only about 3 mm/yr in the past quarter century. Teluk Saibi Saibi Bay (Teluk Saibi) is about half-way down Siberut’s northeastern coast (Figure 3). The site lies on a mangrove-covered islet on the northern side of the bay mouth. Numerous modern microatolls inhabit the intertidal reef (Figure S9). However, none of them have living tissue – the entire population appears to have died during the 1997 ecological disaster. Tiny live hemispherical colonies of Porites had begun to re-colonize the dead tops of the heads by the time of our visit in 2002. About 250 m west of the slabbed modern microatoll, near the edge of the mangrove forest, are a few very old, severely eroded heads. A U-Th analysis yields a mid-Holocene age for these heads (5920 ± 60 years BP). We recovered slab Tsa02B1 from a radial cut into a 2-m wide modern microatoll. The microatoll has a cup-shaped morphology with a 500-mm-wide raised outer rim (Figure S10). Although it did not appear so in the field, the cross section reveals that the outer raised rim has been tilted – the HLS unconformities on the left (interior) side of the rim are higher than those on the right (exterior) side. Since the rim is attached to the older, central flat at about the 1971 band, this tilt implies that the entire microatoll tilted. This contradiction between the apparent lack of tilt noted in the field and the clear evidence for tilt in the cross-section make the analysis of the cross-section questionable. Nonetheless, we analyze the cross-section, below, assuming no tilt. Then we discuss the implications tilt would have for the results. The central flat has two distinct surfaces. The surface that formed prior to about 1933 is several centimeters lower than the surface that formed between about 1941 and 1966. Based upon our experience with the erosion of microatoll surfaces, we judge the older surface to have been eroded more than 50 millimeters, the younger surface to have experienced 30 to 50 millimeters of erosion, and the top of the outer raised rim to have experienced between 10 and 30 millimeters of erosion [Natawidjaja et al., 2004]. The slab records sea-level history back to about 1928 (Figure S10). The cup-shaped morphology attests to a general history of submergence. The average rate of submergence has been low, only about 4.1 mm/yr. However, the microatoll shows clearly that the rate of submergence has varied over the past seven decades. Most of the rise in HLS occurred at the beginning of or during the two periods between 1935 and 1942 and between 1971 and 1986. About 80 mm of uninhibited upward growth occurred in the latter half of the 1930s, perhaps in association with the Mw 7.7 earthquake that was generated by rupture of the megathrust beneath the Batu islands, less than 100 km to the northwest [Natawidjaja et al., 2004]. The coral bands show that HLS was nearly stable from 1943 to 1965. A decline in elevation of the tops of the annual bands between 1966 and 1971 suggests emergence of 80 mm during those years. An alternative explanation is that these bands were severely eroded after an 80-mm emergence event in about1 1971. This is plausible, given that a Mb 5.9 earthquake occurred in the vicinity in that year (its ISC epicenter is about 50 km to the WSW). The rapid, unrestricted upward growth that formed the outer raised rim began in about 1971. A period of HLS stability began in the mid-1980s. The symmetry of the outer raised rim shows that it tilted after growth had ceased in about 1998. The tilt makes determination of the absolute elevation of HLS difficult for the period of the last 20 years. The double row of HLS values in Figure S10 graph indicates the difference in elevation of interior and exterior unconformities due to this tilt. The tilt calls into question the validity of the analysis for the interior flat, which is firmly attached to the outer rim. If the entire head has tilted, all the rates determined above would be higher. Nonetheless, the discrete changes of 1935, 1971 and 1987 would still have occurred. Saibi The Saibi site is on the northeastern coast of central Siberut island, east of the village of Sibuda Oinan (Figure 3). The village after which the site is named, Muarasaibi, is a few kilometers up the coast to the northwest (Figure 2E). The Saibi site is a 30- to 60-m-wide shallow reef on the southern tip of an islet, backed by a thick mangrove swamp (Figure S11). The presence of big, dead snags rooted on the shallow reef tens of meters out from the living mangrove swamp is spectacular evidence for recent fast submergence. High height-to-width ratios and deep cup-shaped upper surfaces also indicate that recent subsidence has been fast. We cut slab Sa99A1 from the southwestern radius of a modern head near the outer edge of the shallow reef (Figure S11A). The clarity of annual banding in this slab enables assignment of ages with an ambiguity of about ?3 years over a 50-year-span (Figure S12). To ensure the validity of our age assignment, we dated the 1955 ring. The U-Th age of the ring is 1965?9 (Table 1). This date is about 10 years younger than the 1955 date derived from visual counting of annual bands, but this difference is just within the 1-? errors of the U-Th analysis and band counting. The geometry of the slab reflects a simple HLS history. The rate of rise of the upper surface toward the outer perimeter is about 6.5 mm/y (Figure S12). However, the surface has been eroded slightly, since we see few of the small ridges and troughs commonly associated with the annual non-tectonic vagaries of HLS. Thus, the details of HLS variation over the past few decades are not recoverable from this sample. The most prominent event preserved in the Saibi slab is the HLS unconformity that occurred in about 1962. This was immediately followed by 5 years of unrestricted upward growth. This record of an emergence followed by a larger submergence is reminiscent of the records recovered from many microatolls near the Equator [Natawidjaja et al., 2004] and strongly suggests that the site experienced the same tectonic events in 1962 that are clear in records near the Equator. The outer rings may record a lessening of subsidence rates since about 1990. Alternatively, the low elevation of these rings relative to their predecessors may have resulted from erosion. Malepet Malepet, on the southern northeastern coast of Siberut, is the main port of the island (Figure 3). The Malepet site is on an islet a few hundred meters east of the wharf, about 200m south of the lighthouse (Figure S13). We collected a slab from one head among a population of many dead, modern heads sitting near the outer edge of the intertidal reef. The stockier shape of these microatolls and the shallower depth of their cup-shaped upper surfaces give the first impression that the modern subsidence rate here is lower than at Saibi. Fossil heads are dispersed farther inland on the island, among small stands of mangroves. The upper surfaces of these heads are flatter and intensely eroded. Elevations of their tops are tens of cm higher than the tops of the modern population. Their degree of degradation and variations in the elevations of their tops suggest that they include at least two generations of microatoll. One well-preserved fossil microatoll that we cut (Mp99A3 in Figure S13A) yielded a U-Th age of 4237?16 BP. Since we focus on the modern history in this chapter, we will not describe these mid-Holocene heads further here. The upper surface of the modern head from which Mlp99A1 was cut is well preserved, and its annual rings are clear (Figure S14). Thus, there is no ambiguity in our visual ring count. The U-Th date of the 1993 ring (1994?24) is consistent with the visual count, but too imprecise to be helpful. A least-squares fit to all HLS unconformities recorded since a clear HLS impingement in 1979 yields an average submergence rate of about 6.5 mm/y (Figure S14). Thus our initial impression, based on microatoll morphology, is incorrect -- the rate here is indistinguishable from the 6.5-mm/yr rate at Saibi, 20 km to the northwest. Mapinang The Mapinang site is on the northeast coast of southern Siberut island (Figure 3). The sample site is a narrow shallow reef near the edge of a mangrove swamp within a large bay. We collected two slabs from a representative modern head. One slab (Mpn99B1) was sawed from the outer raised rim of the microatoll, and the other sample (Mpn99B2) was chiseled from the lower central flat of the head (Figure S15). Together these contain an HLS record for the period from the mid-1930s to 1997, the year the head died. Figure S16 shows the relative vertical positions of the two slabs precisely, based on measured elevations of the screws installed along their upper surface. Their relative horizontal positions in the figure are estimated by visual inspection of their positions relative to inner and outer portions of the microatoll. From the map view, it appears that the samples overlap by about 20 cm. Annual banding in both slabs is obscure, hence, the 6-year ambiguity in interior annual band ages for slab Mpn99B1. Stepping across the physical gap between the slab and the chiseled sample adds an additional uncertainty of about ?4 years to the assigned ages of the bands in Mpn99B2. In general, the micro-topography displayed by the Mapinang microatoll resembles that of the microatoll we cut at Saibi (Sa99A1, Figure S12). In the decades prior to the 1960s, the microatoll experienced a lower rate of submergence than in the decades after. The step-like micro-topography of the slab suggests that submergence has been episodic rather than steady – that is, nearly stable periods have alternated with moments of rapid submergence. Flat surfaces between 1955 and 1961, 1970 and 1976 may well represent periods of stable HLS. An outward-sloping surface atop the annual bands of the 1990s suggests slight emergence in that decade. In contrast, free upward growth in the periods 1962-1972 and 1977-1984 suggests that rapid submergence occurred at the beginning of or during the periods. Despite these apparent variations in the HLS time series, we are not certain that this head has experienced episodic submergence. Moderate erosion, as indicated by a lack of well-preserved concentric rings atop the head, precludes us from making a compelling case. One could argue that differential erosion has created apparent HLS unconformities atop bands that actually grew unrestricted. Hence, a sloping surface with just a few HLS impingements could reflect the interaction between rapid tectonic submergence, natural coral growth upward, and oceanographic fluctuations [Zachariasen et al., 2000]. However, we cannot reject the hypothesis of episodic submergence, especially in light of the apparent emergence of the past decade or so, a phenomenon that is also suggested by the modern microatolls at Saibi and Malepet. A straight-line least-squares fit to the post-1960 HLS data yields an average rate of subsidence of about 7 mm/yr. Libut This site is on the west side of Libut islet, at the mouth of Katurai bay, the long narrow bay on the southeastern coast of Siberut island (Figure 3). A family of modern heads that died during the regional reef death of 1997-1998 occupies the outer half of the shallow reef (Figure S17). In several places, groups of heads have formed large platforms of coalescing microatoll colonies, because radial growth has eliminated the open space between them. The upper surface of most of the microatolls is nearly flat. This gives the false impression that HLS has been stable for many decades. To the contrary, inspection of the internal stratigraphy of the slab we cut indicates that the microatolls actually have the teacup shape indicative of a rapidly submerging site. The interiors of the cups have been filled with successive, thin overgrowths of coral (Figure S18). The slab reveals that the microatoll consists of a continuous outer bowl-shaped perimeter, generally 10 to 20 cm thick, that cradles a family of satellite heads that have grown successively on dead interior flats. The internal stratigraphy of the slab displays rapid, continuous growth of the microatoll’s perimeter since the mid-1950s. Several prominent HLS unconformities on the interior side of the perimeter correlate with major nearly horizontal unconformities across the interior family of satellite heads. These HLS unconformities occurred in 1962, 1968, 1976-77, 1982-85, and 1989. Half of the unconformities that continue across the interior of the head do not continue to the perimeter of the head. This suggests that these interior unconformities are due to a souring of the microenvironment within the confines of the bowl, rather than a regional drop of HLS to a level below the perimeter crest of the head. These HLS unconformities, confined to the interior of microatoll, could be the result of extraordinary heating of poorly circulating water within the interior of the head during lowest tides that dropped nearly to the perimeter crest of the microatoll. The fact that most of these are solitary unconformities rather than series of unconformities that run for many years indicates that this site has not experienced the long periods of HLS stability that we see in many other sites. Instead, Libut appears to have been subsiding at a nearly uniform rate of about 9.5 mm/yr from about 1960 through 1990. Masokut Masokut is one of the larger islets of the archipelago south of Siberut island (Figure 3). The sampling site is on the east coast of the islet. The intertidal reef there is broad and supports large populations of fossil heads and modern, dead heads (Figure S19). The fossil heads occupy parts of the reef nearer the shoreline, whereas the modern heads are concentrated near its outer edge. Our colleague M. Gagan has drilled some of the fossil non-microatoll heads and shown by radiocarbon analysis that they grew in the mid-Holocene (M. Gagan, written communication). A U-Th date from a slab of another fossil microatoll (Ms99A6) yielded a date of death of about AD 1650. Underlying the beach deposits and extending several tens of meters out into the intertidal zone is a thick layer of peat, with remnants of in situ twigs and roots. This carpet of peat is the remains of drowned vegetation that grew on the landward side of the beach. Its presence in the intertidal zone testifies to the ongoing transgression of the shoreline and subsidence of the islet. The peat mat rests on fossil heads and microatolls, a relationship that indicates an earlier episode of uplift. The slab obtained at Masokut, Ms99A7, is from the outer raised rim of a modern microatoll that had been growing near the northern edge of the intertidal reef until its death in 1997 or 1998. The microatoll consists of a tall central mass, surmounted by the outer raised rim. The HLS record from the slab extends back only to about 1970 (Figure S20). The two principal features in the HLS record are free upward growth between about 1969 and 1985 and little net change in HLS between about 1985 and 1992. All but one of the HLS unconformities in this record occurred between 1985 and 1997. The best-preserved unconformities are those of 1992 and 1997. The older of the two was preserved soon after its formation by overgrowth of the 1993-1997 bands. Although the bands that grew from 1985 to 1988 appear to be eroded, they and the 1989-1992 bands indicate that HLS rose at a low rate during their growth. Free upward growth occurred from 1992 until the death of the head during the devastation of the Mentawai reefs in late 1997 or early 1998. This suggests a submergence event in or beginning in 1992 at least 70 mm in magnitude. This event most likely occurred before mid-1994, because the strong El Niño-Southern Oscillation/ Indian Ocean Dipole (ENSO/IOD) event of that year [Saji et al., 1999] would likely have appeared as an HLS unconformity had submergence not occurred before then. The geometry and stratigraphy of the outer raised rim demonstrates the occurrence of either a large submergence event or the beginning of a period of rapid submergence in about 1969. The outer raised rim grew radially outward and inward from a small attachment on the inner head (Figure S20a). The first identifiable annual band of the outer raised rim grew in 1970 ± 2. This is contemporaneous, or nearly so, with the initiation of rapid upward growth of outer raised rims at Beuasak, about 20 km to the north. Unabated upward growth continued 300 mm and 15 years after the initiation of this event. This is about 100 mm larger than the magnitude of unrestricted growth that occurred during the same event at Beuasak. Pulau Panjang Pulau Panjang is an elongate islet a few kilometers north of the northern tip of Sipora island (Figure 3). We collected a slab from a cup-shaped microatoll on a broad shallow reef near the southern tip of the island (Figure S21). We surveyed only a few of the modern microatolls that populate the wide reef platform. None of the microatolls had living corallites along their perimeters. We assume that these microatolls died during the widespread 1997 ecological disaster. Numerous snags standing on the intertidal flat, between the beach and the main concentration of modern microatolls, attest to a submerging coastline. The Pj03A1 slab was cut from a deep, cup-shaped microatoll, which has a diameter of about 2 meters. The sample includes the shoulder of the microatoll’s core and its outer raised rim. The outer raised rim is unusually long and narrow (Figure S22). Some unfavorable environmental condition kept the corallites from re-establishing themselves and thriving in the interior of the “tea cup.” The shoulder of the microatoll’s core displays the oldest annual bands, which grew upward without restriction until about 1934. After HLS unconformities formed in about 1934, the corallites resumed nearly unimpeded upward growth, forming the thin outer raised rim, the wall of the teacup. Minor HLS unconformities, mostly on the interior side of the raised rim, prevented substantial growth into the bowl of the teacup. This slab shows that the rate of submergence is nearly equal to the upward growth rate of the corallites. The least-squares fit to the HLS unconformities yields an average submergence rate of about 8.5 mm/yr. Initial rates of subsidence (from mid-2004 to early 2006) determined from the Sumatran GPS Array station, PPNJ, nearby are even higher than this – about 23 mm/yr (Table 3). Tuapejat Tuapejat, the administrative capital of the Mentawai islands, is a small town on the northern tip of Sipora island, just a few kilometers south of the Pulau Panjang site (Figure 3). In the middle of the mangrove-lined bay next to the town is a small, treacherous shallow populated by a few dead microatolls (Figure S23). The microatolls have the teacup morphology indicative of rapid submergence. Our slab, Tp00A1, is a radial cut from the perimeter of the outer raised rim to the center of the lower inner flat (Figures S23B and S24). Unfortunately, we misplaced the older portion of the slab. Thus, we have had to estimate the morphology of this piece and its position relative to the outer raised rim from our field sketch and surveyed elevations across the head. The internal stratigraphy of the raised perimeter records a period of unfettered upward growth from 1969 until 1994, and a period of HLS impingement between 1994 and the microatoll’s death in 1997. The magnitude of free upward growth is about 30 cm, similar to that at Masokut, but enduring a decade longer. The record of the microatoll’s last five years of growth, from 1994 to 1997 is obscured by the erosive activity of burrowing clams, but appears to have been clipped by the regionally prominent HLS unconformities of 1994 and 1997. The presence of a circular raised outer rim on this coral head indicates that the top of the interior head died prior to the initiation of growth of the rim. If the top of the head died because it had reached the level of lowest low tide in or just before 1969, then we can calculate an average submergence rate of about 10.7 mm/y for the period 1969-1994. The lack of HLS unconformities between 1969 and 1994 indicates that this rate was greater than the growth rate of the microatoll.