INDONESIAN OCEAN
Chapter 3. Paleogeodetic records from microatolls above the central Sumatran subduction zone
3.4. Paleogeodetic and paleoseismic sites
3.4.6. Memong site
3.4.6.2. HLS history from a large head, H14
The two largest heads at the site display similar morphologies. In each case, a wide and higher outer rim flanks a gently outward-sloping core. H21 (Fig. 3.29 and 3.31a) is almost circular, and its outer rim is nearly continuous. It has a diameter of about 3.5 m, which suggests that its record of HLS extends well into the 19th century.
However, we did not slab this head.
Head H14 has an oval shape in plan view (Fig. 3.31b). Its long axis is also 3.5 m, but the raised outer rim is not contiguous around the entire perimeter. The center of H14 consists of an elevated, hemispherical core. Its elevated nature suggests a significant emergence event early in the growth history of the microatoll. Except atop this central core, the upper surfaces of H14 still display concentric ridges, which indicate that bioerosion has been quite modest.
We have collected two cross sectional slabs (Mm99A1 and Mm00A1) that span the entire long axis of H14 (Fig. 3.31b). These provide an opportunity to compare the HLS history of each radius.
The north radius
Slab Mm00A1 reveals a continuous HLS history from the middle of the 19th century to 1997 (Fig. 3.32a). Its clear annual banding leaves little ambiguity in the time series. A band that we assigned to 1865 by ring counting was U-Th dated to AD 1860±6 (Table 3.1), confirming that the precision of our visual counting of the bands is no worse than a few years over more than a century of growth.
The principal features of the HLS record from the north radius are a central dome that formed prior to about 1875, a broad middle flat that displays a very low rate of emergence between about 1885 and 1935, and an outer raised rim.
The central dome is eroded on top, as evidenced by the truncation of several annual bands. The youngest preserved annual band atop the central dome formed in about 1865. A die-down is evident in the stratigraphy at the beginning of 1874. This die- down is about 15 cm below the eroded top of the central hemisphere. Thus, this amount is a minimum value for an emergence that exposed the dome. We are unsure when in the period between about 1865 and 1874 the emergence of the central dome occurred because nearly a decade of growth has been removed by erosion. It is conceivable that it occurred in 1874. On the other extreme, the majority of the emergence may have occurred in about 1865, with an additional increment of a few centimeters in 1874. We would like to believe that emergence occurred during the great earthquake of 1861 [Newcomb and McCann, 1987], but both the visual ring counting and the U-Th date support an emergence event or episode several years later. Determination of the δ13C record across the bands of the 1860s and 1870s might resolve this, since changes in light intensity have been shown to be associated with change in water depth at another of our sites [Mike Gagan, personal comm., 1999]. Furthermore, the great Indian drought of 1876 might show up as a spike in δ18O values. Unfortunately, these analyses are beyond the scope of our current efforts.
A submergence event or episode enabled upward growth of about10 cm between 1875 and 1885. From 1885 to 1935, steady emergence occurred at a rate of just ~0.9 mm/yr.
Another submergence event or episode began in 1935. The evidence for this is upward growth that commenced sometime between about 1935 and 1939. Slight erosion of the upper edges of the growth bands obscures a precise determination of the year of initiation of this upward growth. But steeply inclined growth undulations in the annual bands from 1935 to 1955 indicate that strong upward growth began in 1935. By 1975, at the latest, the HLS of the head was about 11 cm above its level in 1935. Growth was restricted to about this level until the death of the head. When we mapped the head in mid-1997, the outer edge was completely covered with living polyps. Substantial erosion of this perimeter occurred between 1997 and our visit in 2000.
The south radius
Slab Mm99A1 from the south radius of Head H14 tells a story that is similar to the one told by the north radius. However, this section is not as useful, because the HLS record between 1923 and 1965 is missing (Fig. 3.32b). This missing section precludes a visual determination of ages for those bands that formed prior to 1965. Instead, we must rely on one U-Th date and on a comparison of the morphologies of the north and south radii.
Emergence of the central hemisphere occurred after the growth of the ring dated 1864±3. This is consistent with emergence during the great earthquake of 1861, but appears to be significantly earlier than the 1865 to 1874 date range for the emergence suggested by analysis of the northern radius (Mm00A1). Furthermore, the emergence event seen in the 1874 band of the north radius is absent in the south radius. The annual- band ages shown on the south radius cross section assume that the emergence of the inner
hemisphere dates to the great earthquake of 1861. But in our analysis of this radius in Fig. 3.32c, we assume that the slow, steady emergence begins in 1885, as it does on the north radius. Clearly, if we wish to resolve what happened at the Memong site in the 1860s and 1870s, we will need to resolve this chronological conflict.
Despite the discrepancy in their age of emergence, the general growth pattern and gross morphology of the north and south cross sections resemble each other. Assuming that the visible HLS clips of the early emergence recorded on both sections are contemporaneous, their HLS histories appear to be identical in pattern, magnitude and relative elevation (Fig. 3.32c).
Disentangling the 1935 and 1962 events
The slow, persistent half-century of emergence evident in both radii ended in about 1935. The 1935 ring of the north radius marks the beginning of several decades of unhindered upward growth (Fig. 3.32a and c). This upward growth continued until 1975.
The coral record, alone, only indicates that the submergence occurred at a rate of no less than a few millimeters per year, because this was the upward growth rate of the coral head in the decades following 1935. We suspect, though, that the 11 cm difference in elevation of the tops of the 1935 and 1975 growth bands is evidence primarily of sudden submergence during the 1935 event. If, as we have seen elsewhere in the region, there was also a rise of HLS in 1962, it is hidden in the record of this period during which the head was still striving to grow upward to HLS. Thus, any disturbance of 1962 can not be separated from that of 1935 in the Memong record. As a temporary and tentative solution, we assume that the magnitude of the 1935 disturbance is 7 cm, the vertical
distance between the 1935 surface and the 1962 surface on the north radius. We assume that the magnitude of submergence in 1962 equals the 4 cm vertical difference between the 1962 and 1975 bands (Fig. 3.32c).