INDONESIAN OCEAN
Chapter 3. Paleogeodetic records from microatolls above the central Sumatran subduction zone
3.4. Paleogeodetic and paleoseismic sites
3.4.8. Tanjung Anjing site
The tops of the fossil heads are about 20 to 70 cm above those of the modern heads. Most of their upper surfaces have been flattened by bioerosion. Among the few exceptions is the 90 cm in diameter head DC#2, which still preserves a non-microatoll hemispherical form. In contrast, the remnant morphology of a few heads including DC#6 (Tf99A6) suggests that they appear to be microatolls. The C-14 analysis of the coral drill cores yields ages ranges from about 2000 to 6000 B.P, thus confirming that they are all Mid-Holocene heads (Table 3.2).
Sample code Elevation above mean value of modern head's crests
(cm)
C-14 Dates uncorrected (year BP)
Tf99A2 56.7 -6110 ±70 Tf99A3 20.7 -2120 ±60 Tf99A4 69.5 -3940 ±70 Tf99A5 44.4 -4320 ±70 Tf99A7 55.8 -4260 ±70
Table 3.2 C-14 absolute age for the Mid-Holocene heads (from M. Gagan, written. comm., 2000.)
The site is a 60–100 m wide and 100 m long intertidal reef flat with numerous modern Porites heads (Fig. 3.36a). The cup-shape morphology of these microatolls is a testament to the dominance of submergence at the site (Fig. 3.36b).
All of the microatolls were dead when we visited in mid-2000. But the concordance of perimeter crest elevations suggests that they are of the same generation (Fig. 3.36c). A few Goniastrea heads (i.e., H4 and H5) display perimeter elevations 10 cm higher than that of the Porites heads, but it is typical for this genus to have HLSs about this much higher than those of Porites. A small cluster of highly degraded flat- topped heads at the edge of the mangrove swamp rise to elevations about 25 cm higher than the elevations of the modern Porites heads (Fig. 3.36c). These are probably mid- Holocene heads, but we did not sample them.
3.4.8.2. HLS history from a modern Porites head
Microatoll H11 is the best representative of the population of modern heads (Fig.
3.36a). Its regular shape and 2 m diameter (Fig. 3.36b) enable reconstruction of an HLS history for much of the 20th century. The basic HLS record is of near stability punctuated by large emergence and submergence during both the 1935 and 1962 events.
Analytical problems
Interpretation of the HLS record is difficult, however. This stems from ambiguities in dating the annual bands and from rather intense bioerosion of the head.
All of the modern heads appear to have suffered significant grazing and boring by predators, so we were unable to sample an unaffected head.
Furthermore, the annual growth bands are poorly developed. This yields unusually large uncertainties in age assignments from visual ring counting. We assume an outer perimeter age of 1997. Working inward, we find that the 1962 band is uncertain by 1 to 4 years, and the 1935 band is uncertain by 3 to 7 years.
To help overcome these problems, we took the unusual step of preparing two thin slabs from the thick sawed slab, rather than preparing just the customary single thin slab (Fig. 3.37a and b). We cut the thick slab from the south, so one of the thin slabs is from the west side and the other is from the east side of the thicker slab. The west-side cut is contiguous through the decade of the 1950s and early 1960 to mid-1960s, whereas the east-side slab is not. The east-side slab is contiguous through the1960s, 1970s, 1980s, and 1990s, whereas the west-side slab is not. Together they give us a significantly more complete record.
We also dated an unusually large number of samples by U-Th, to constrain the ages of the annual bands better (Fig. 3.37a and b). Several of these ages, however, are also problematic or of limited use. A large uncertainty in the date of the 1959 band (1939±23) renders the analysis consistent with the visual counting, but not useful. The date for the 1941 band (1936±4) is marginally at odds with the ring count, but shows that our counting is not far off.
The three remaining dates are consistent with each other, but are incompatible with our ring counting. The two U-Th analyses of the 1938 band yield dates of 1923±5 and 1928±2 (Fig. 3.37a,b), several years older than our estimate. Last, a U-Th analysis of the 1913 band yields a date of 1902±5, also several years older than our estimate.
These three U-Th analyses suggest that our pick for the 1935 ring is off by about a decade. At the risk of letting our stratigraphic prejudices lead us into a blunder of observation, we choose here, tentatively, to accept our visual counting and ignore the U- Th analyses. To do otherwise would force us to assign a date in the mid-1920s to the largest submergence event in the record, which at all neighboring sites appears to have occurred in 1935.
The 1935 event and before
H11 grew freely upward and outward from about 1902 until at least the early 1920s (Fig. 3.37a and b). This growth appears in Fig. 3.37c as the small climbing triangles on the left side of the plot. HLS is poorly constrained during this period prior to initial HLS impingement.
Growth during most of the 1920s and early 1930s is unclear, because many of these annual bands were removed by erosion. This erosion occurred before 1940, because the void that represents the missing material was filled with new growth that post-date 1940 (Fig. 3.37a). Unfortunately, it is not at all clear that the head reached HLS prior to 1935. The upper surfaces of the annual bands that formed in the late 1920s and early 1930s may be eroded rather than limited by HLS impingement.
The 1935 event begins in both slabs as a die-down of at least 10 cm. This suggests that the site initially raised at least this amount. This emergence was followed by substantial submergence, which allowed the head to grow freely upward once again.
Free upward growth continued until the next HLS impingement, in about 1948. This impingement is about 26 cm above the HLS impingement of 1935 (Figs. 3.37a, b and c).
HLS history between 1935 and 1962
The microatoll poorly constrains HLS during its free upward growth from 1935 to 1948. However, from 1948 to 1961, HLS impingements are common. HLS elevation does not change appreciably during this period, although the west-side cut shows a slight rise in the latter half of this period. The high degree of bioerosion on this head leads us to doubt the significance of this observation. The least-squares average rate of submergence during the period 1948 to 1961 is ~0.7 mm/yr (Fig. 3.37c).
The 1962 event
In 1962 (or 1961), HLS dropped 3 to 8 cm (Fig. 3.37a, b and c). This represents a minor emergence event. Quite soon thereafter, the microatoll submerged, which allowed free upward growth from 1962 until the early 1970s. The net effect of these events is submergence of about 10 cm.
HLS history between 1962 and 1997
Once the microatoll had reached the new HLS level, in the early 1970s, it was constrained to grow outward and inward. This growth continued until the death of the head, we presume in 1997. Although the net change in HLS for this quarter century is about nil, there is a record of initial submergence that is followed by emergence. For the first fifteen years of this period, the microatoll submerged at about 3.6 mm/yr. For the last decade, it emerged at 3.5 mm/yr (Fig. 3.37c).
3.4.8.3. Summary
Two submergence events dominate the HLS history at Tanjung Anjing. Both consist of an initial emergence event followed by a much larger episode of submergence.
In the case of the 1935 event, the emergence/submergence pair was about 10 cm and about 26 cm in magnitude. In 1962 the emergence and submergence were about 8 cm and 13 cm respectively.
HLS behavior prior to 1935 is indeterminate, because bioerosion has obscured any HLS clips from that period. HLS from 1948 to 1961 is nearly stable (~0.7 mm/yr).
From the early 1970s to the mid-1980s HLS rises at ~3.6 mm/yr. From the mid-1980s to 1997, it falls at ~3.5 mm/yr.