20
September - 2017
Wednesday
SUBSCRIBE TO NEWS

Ongoing Scientific Research

Dr. Gina Moseley

Since the early days of exploration in Mulu, scientific research has been a key element of the expeditions (Smart and Willis 1982, Farrant et al. 2007).  Of particular interest over the last 30 years, has been the geomorphic and chronological evolution of the caves, spearheaded by Prof. Pete Smart and later by Dr. Andy Farrant for his Ph.D thesis.  The most recent expedition welcomed Gina Moseley onto the team who was up to date with new research methods and technology that could be applied to the vast river caves of the Gunung Mulu National Park.

Collecting sediment samples | photo © Dave Clucas

Caves are useful in landscape evolution studies because they often mark the level of previous water tables and when dated, yield estimates of fluvial incision and tectonic uplift rates.  Entrenchment of the Melinau and Melinau Paku Rivers within the National Park has created a base level for cave development thus, as incision has proceeded over the Quaternary, a stacked vertical sequence of cave passages extending up to 500 m above the present Clearwater resurgence has formed.  During wet interglacial periods, increased landslips and sediment production result in aggradation of the Melinau and Melinau Paku Rivers and the development of large alluvial fans of which their remnants are present as terraces (Rose 1982).  Large volumes of coarse gravels derived from the adjacent sandstone uplands are also transported into the caves and deposited at resurgence level, preventing incision and focusing erosion laterally on the cave walls to create morphologically distinctive notches (Smart et al. 1985).  If the alluvial fan extends so far as to block the resurgence, ponding occurs within the passages and finely laminated muds are deposited.  During drier glacial climates, incision of the Melinau and Melinau Paku dominates processes and the local water table drops.  If the base level falls faster than the rate of incision, upper cave passages are abandoned and groundwaters are diverted to new lower phreatic routes.  Once abandoned, the passages become suitable for formation of speleothems (stalagmites and stalactites) thus cave deposits typically comprise coarse gravels overlain by finely laminated muds and a capping speleothem layer (Smart et al. 1985).

Farrant et al. (1995) were successful in obtaining a chronology for the development of the lower 120 m of cave passage using alpha spectrometric dating of speleothems which has an upper age limit of ~350 thousand years (ka).  Above 120 m, the speleothems were too old to be dated using the methods available at the time.  Farrant et al. (1995) thus used the record of palaeomagnetic reversals preserved within cave sediments to develop a chronology extending back to 850 ka, however, above 186 m the record was too sparse for reliable interpretation.  The upper ~300 m of cave development from the early Quaternary therefore remained undated.  In addition, Farrant et al. (1995) demonstrated that in the 0-700 ka range, the chronology for formation of cave wall notches and thus aggradational events is closely associated with 11 major interglacial peaks.  Palaeoclimatic fluctuations and associated changes in hydrology are therefore a major driving force for karst evolution and speleogenesis.

The discovery of Whiterock Cave in recent years opened up a new avenue for research in the higher levels of the Clearwater system which, when combined with newly available dating techniques offered promise to establish a complete chronological framework for cave development within the Park.  The project aims to extend the sediment palaeomagnetic record and apply two newly available techniques: uranium-lead (U-Pb) dating of speleothems and aluminium-26/beryllium-10 (26Al-10Be) cosmogenic burial dating of quartz gravels.  Intercomparison of the three methods will allow confirmation of the reliability of the derived chronology.

Preparing speleothems in the lab. Photo © Robbie Shone

Cosmogenic 26Al-10Be burial dating of quartz gravels has proven successful in the New River valley caves, Virginia (Granger et al. 1997) and Mammoth Cave, Kentucky (Granger et al. 2001), for dating caves over an extended (5 Ma) timescale (Granger and Muzikar 2001). Quartz accumulates 26Al and 10Be from secondary cosmic rays within the top few metres of the earth’s surface as it is exhumed within hill slopes and transported through river systems.  Subsequent burial of the quartz deep (>25 m) within cave systems ceases nuclide production, and the 26Al and 10Be products decay exponentially.  Because 26Al decays roughly twice as fast as 10Be, the inherited nuclide ratio (N26/N10) decreases exponentially over time and the ratio of 26Al/10Be in buried gravels may be used to determine burial ages.

Uranium-lead dating of speleothem calcite relies on measurement of the cumulative amount of radiogenic lead which occurs in speleothem calcite through time due to uranium series decay. The technique has been shown to be applicable in areas with uraniferous groundwaters (and thus speleothems) and low levels of natural lead (Richards et al. 1998).

Speleothem samples. Photo © Robbie Shone

In the field, the scientific assault was conducted in conjunction with exploration trips, with researchers and explorers sharing supplies underground whilst pursuing their own objectives in their respective teams.  Samples suitable for palaeomagnetic analysis were obtained from clean, vertical, finely laminated, undisturbed sedimentary sequences and duplicated six times within a horizon to test for reliability.  Each sample was orientated using a spirit level and compass and shall be analysed using a Molspin Magnetometer.  Using the procedure of Granger et al. (2001), 100 to 200 g of clear white quartz gravel were sampled from distinctive former river beds.  Though it is possible to analyse sands, these were avoided because of the ease in which they can be remobilised.  Speleothems were abundant throughout the cave unlike quartz gravels and sediments thus a full suite of samples covering the full elevation range were collected. The quartz, sediment and speleothem samples collected in 2009 are shown in Table 1. Permission to collect samples was granted by the Forest Department, Sarawak, permit number 99/2008.

Upon return to the UK, the Natural Environment Research Council Cosmogenic Isotope Facility (NERC CIAF) granted support for a pilot analysis of six quartz samples to the value of £7905.  Samples submitted include two surface samples (MR-09-1, MR-09-2), two low elevation samples (LC-09-12-2, WR-09-02-1) and two high elevation samples (WR-09-18-1, HOTM-09-26-1).  Moseley visited the NERC CIAF for training in chemistry procedures and mass spectrometric methods.  Initial results appear promising though at present it would be premature to adopt a specific interpretation.  In addition, preparation of speleothem samples for dating is underway.  All samples have been sawn in half and appear to have a dense macrocrystalline structure suitable for dating.  Funding for analytical costs of U-Pb dating has been provided by the British Cave Research Association and Quaternary Research Association.

Dr Gina Moseley at work in the lab. Photo © Robbie Shone

Members of the expedition also undertook sampling for the US based researchers, Dr. Kim Cobb and Dr. Nele Meckler.  Four gour pool sites were sampled for pool water, calcite and bedrock in the northern extremities of the Whiterock Cave.  Petrographic and stable isotope analysis of the samples will be used into research for palaeoclimate reconstruction of the tropical Pacific region.

The base of the 2009 expedition within the Melinau Gorge area resulted in research focusing on the caves of northern Api and southern Benarat.  However, in order to fully understand the geomorphic evolution of the karst landscape and sequence of speleogenesis in the National Park, our studies will need to expand to other areas of Mulu, including the cave passages of the Southern Hills, Benarat and the remainder of Api.

References

Farrant, A., Kirby, M. and Smart, P.L. 2007 Cave and Karst Science, 34(2), 51-60

Farrant, A. R., Smart, P. L., Whitaker, F. F. and Tarling, D. H. 1995. Long-term Quaternary uplift rates inferred from limestone caves in Sarawak, Malaysia. Geology, 23(4), 357-360

Granger, D. E., Fabel, D. and Palmer, A. N. 2001. Pliocene-Pleistocene incision of the Green River, Kentucky, determined from radioactive decay of cosmogenic 26Al and 10Be in Mammoth Cave sediments. GSA Bulletin, 113(7), 825-836

Granger, D. E., Kirchner, J. W. and Finkel, R. C. 1997. Quaternary downcutting rate of the New River, Virginia, measured from differential decay of cosmogenic 26Al and 10Be in cave-deposited alluvium. Geology, 25(2), 107-110

Granger, D. E. and Muzikar, P. F. 2001. Dating sediment burial with in situ-produced cosmogenic nuclides: theory, techniques, and limitations. Earth and Planetary Science Letters, 188(1-2), 269-281

Richards, D. A., Bottrell, S. H., Cliff, R. A., Ströhle, K. and Rowe, P. J. 1998. U-Pb dating of a speleothem of Quaternary age. Geochimica et Cosmochimica Acta, 62(23-24), 3683-3688

Rose, J. 1982. The Melinau River and its terraces. Cave Science, 9(2), 113-127

Smart P.L. and Willis R.G. (Compilers). 1982. Mulu ’80 Expedition. Cave Science, 9(2), 55–164

Smart, P.L., Bull, P.A., Rose, J., Laverty, M., Friederich, H. and Noel, M. 1985. Surface and underground fluvial activity in the Gunung Mulu National Park, Sarawak. In: Douglas, I. and Spencer, T. (Editors), Environmental Change and Tropical Geomorphology. London: Allen and Unwin. 123–148