We report a 2000-year Antarctic ice-core record of stable carbon isotope measurements in atmospheric methane (δ ¹³ CH ₄ ). Large δ ¹³ CH ₄ variations indicate that the methane budget varied unexpectedly during the late preindustrial Holocene (circa 0 to 1700 A.D.). During the first thousand years (0 to 1000 A.D.), δ ¹³ CH ₄ was at least 2 per mil enriched compared to expected values, and during the following 700 years, an about 2 per mil depletion occurred. Our modeled methane source partitioning implies that biomass burning emissions were high from 0 to 1000 A.D. but reduced by almost ∼40% over the next 700 years. We suggest that both human activities and natural climate change influenced preindustrial biomass burning emissions and that these emissions have been previously understated in late preindustrial Holocene methane budget research.

An indoor experiment involving 10 rumen-cannulated Romney sheep was conducted in May and June 1998 at AgResearch Grasslands, Palmerston North, New Zealand, under restricted feeding conditions, in order to test the hypothesis that animal factors, in particular rumen fractional outflow rate (FOR) and rumen volume, have an influence on the between-sheep variation in methane (CH₄) emission. Sheep were fed 2-hourly on chaffed lucerne hay. Following an acclimatization period of 21 days, the experiment lasted 16 days. Energy and nitrogen (N) balances were measured on days 1–6. Cr-EDTA marker was continuously infused into the rumen from day 9 to 16, and rumen contents emptied and sampled on days 13 and 16. Particulate and fluid FOR were estimated using feed lignin and Cr-EDTA, respectively. Daily CH₄ production was measured by the sulphur hexafluoride tracer technique on days 2, 5, 6, 12 and 15 of the experiment.CH₄ production (g/day) was positively correlated with the pool size of organic matter (OM) in the rumen (OM pool, g) (r=0·84, P=0·002), OM intake (OMI, g/day) (r=0·67, P=0·04), and the rumen fill (g, wet digesta) (r=0·76, P=0·01). Multiple regression analysis showed that CH₄ production was best predicted (R²=0·88) as a function of OM pool and the molar % of butyrate; however, OM pool alone accounted for a large proportion (R²=0·71) of the variation in CH₄ production.CH₄ yield (% gross energy intake, % GEI) was negatively correlated with the particulate FOR (%/h) (r=−0·75, P=0·01) and buffering capacity of rumen fluid (mmol HCl) (r=−0·72, P=0·02), but positively correlated with the digestibility of cellulose (r=0·66, P=0·04). Multiple regression analysis showed that CH₄ yield was best predicted as a function of particulate FOR, OMI (g/kg liveweight^0·75) and the molar % of butyrate (R²=0·88). Particulate FOR alone explained a large proportion (R²=0·57) of the variation in CH₄ yield. Particulate FOR was negatively correlated with rumen fill (r=−0·69, P=0·03) and digestibility of cellulose (r=−0·65, P=0·04).These results suggest that sheep with lower rumen particulate FOR (i.e. longer rumen retention times) had larger rumen fills and higher fibre digestibilities and CH₄ yields. If rumen particulate FOR is to be used as a tool for CH₄ mitigation, the repeatability of its relationship to CH₄ emission must be assessed, preferably under grazing conditions.