Abstract
When geologists collect sea-level proxy data (remains of organisms that grew within tight elevation ranges) to reconstruct former relative sea level (RSL), samples are commonly collected at sites spread across hundreds of meters or more. A fundamental assumption is that sea level itself is level across the site: if different generations of an RSL proxy, hundreds of meters apart, are found at different elevations, then the elevation differences are interpreted as changes in RSL over time. But what if, instead, the differences were caused by spatial variability in sea level, rather than changes in RSL over time? This possibility is generally not tested for in such studies.
At a site in Ilocos Region, Philippines, we simultaneously deployed portable pressure-sensor tide gauges at two ends of our site, 2.1 km apart, for 8 days. Although the full tidal range is recorded by each gauge, sea level was consistently higher at the northern gauge by at least a few centimeters, and the difference between the gauges increased by ~5 cm after the onset of a wind event. The northern end of the site is a renowned surfing beach exposed to the northwesterly waves, whereas the southern end of the site is protected.
We also collected different generations of coral microatoll slabs at this site. Elevations of coeval corals in each generation allow us to analyze sea-level variability over spatial gradients. One particular generation includes a pair of coeval corals which are 360 m apart. Despite the fact that both corals grew at the same time, the northern coral yields an RSL reconstruction that is consistently 2–13 cm higher than the southern coral. This suggests a general concern that spatial differences in sea level recorded even over short distances may map into inferred RSL change.
Both sets of evidence imply that sea level is persistently higher in the north than in the south. We suggest that this could be due to greater wave setup in the north. Work at this site and our exploration of this issue are ongoing.