The Boron Isotopic Composition of Elephant Dung: Inputs to the Global Boron Budget

A survey of boron in kerogen showed isotopically light δ¹¹B values (0 to -50%) that are distinctly different from most mineral and natural water B reservoirs. Diagenesis releases this isotopically light B into pore fluids when hydrocarbons are generated, thus enriching oilfield brines in ¹⁰B. This observation suggests that borated biomolecules (BM) are primarily tetrahedral favoring ¹⁰B, whereas ¹¹B is preferred in trigonal coordination. Plants, with optimal concentrations up to 100ppm, contribute more B than animal remains to sediment. Elephants are one of the largest herbivores on earth, consuming 200 - 250 kg of plant material/day and producing 50 kg of manure/day. They are inefficient at digestion, thus the manure contains >50% undigested plant material. Dung samples are therefore ideal for studying the δ¹¹B of both the food input and digested output of a significant B supply to sedimentary systems. Horse and rabbit manure were studied for comparison to evaluate B isotope variations in the food supply and potential vital effects on the output. B-content and isotopic composition of dung plant material and digested fractions were measured in the solid state by secondary ion mass spectrometry. The digests were rinsed in 1.8% mannitol, a B-complexing agent, to remove surface adsorbed-B, then air dried and Au-coated for charge compensation. Results showed that the elephant diet contains 3-13 ppm B, with an average δ¹¹B of -20 ± 0.8% (1σ), while rabbit food had 88 ppm B with a δ¹¹B of -50 ± 1.3 %. The digested fraction of the elephant dung contains 4-10ppm B with average δ¹¹B values of -12 ± 1.2%. In comparison, horse manure with 11-21 ppm B has a δ¹¹B of -10.7 ± 0.5% and rabbit manure contains 2-3 ppm B with a δ¹¹B of -8.8 ± 1%. Boron isotope compositions of these manures are indistinguishable (within error). Clearly plant material is a major contributor of isotopically light B to sediments. The herbivores studied fractionate their total B intake in favor of ¹¹B, thus increasing the δ¹¹B of the (solid) digested material relative to the food source. This would not affect the overall BM input to the sediment because the dung contains the undigested plants. If we assume that the average B isotopic composition of dung, ~10ppm B at -20%, represents an average BM in sediment, and that the mass of sediments (1E24 g) is comparable to the mass of seawater with an average 5ppm B at +40%, then it is clear that BM plays a major role in balancing the global B budget. Note: This research was NOT funded by taxpayer dollars. The Phoenix Zoo kindly approved the proposal to sample elephant dung for this study and their support is greatly appreciated.