Stable Isotopes Of Cenozoic Mummified Fossil Wood Cellulose And Their Paleoclimatic Implications

The Cenozoic is a referential period for predicting climate change in the future,and is the period most closely associated with human survival in geological history.With the massive emissions of greenhouse gases,it is probably that the Earth will experience global warming that had occurred during deep time,such as the"greenhouse Earth"of the late Oligocene and the Miocene:the late Oligocene is the last warm period of the Paleocene,with temperatures?4°C higher and p CO₂?300 ppm higher than today;the Miocene is the first period of the Neogene,with slightly warmer and higher p CO₂ than today.From the late Oligocene to the Miocene,the earth transited from a greenhouse to an icehouse with major paleogeographic changes such as the uplift of the Himalayan–Tibetan Plateau,the retreat of the Tethys Sea,and the form of Arctic and Antarctic ice sheets,which had a dominant impact on the East Asian monsoon.However,there is a lack of reliable terrestrial fossil records for changing climate from the late Oligocene to the Miocene.The modern East Asian monsoon controls precipitation and its seasonal variability in the monsoon region,where more than four billion people rely on accurate predictions of current and future climate.Understanding the evolution of paleoclimate and paleomonsoon will be important for the socio-economic construction of the contemporary monsoon region.In this study,seventy-seven new measurements of oxygen isotope value of?-cellulose(?¹⁸O_cell)for tree rings extracted from forty-three mummified fossil wood from the Nanning Basin,Guangxi during the late Oligocene;four mummified fossil wood from the Guiping Basin,Guangxi during the Miocene;and two modern tree cores(22 tree rings)from the Nanning Basin are used to calculate the mean monthly precipitation of the wet season during the late Oligocene,the Miocene and the present-day.Six hundred and fifty-seven new measurements of high-resolution carbon isotopes(?¹³C_wood)from three mummified fossil wood during the Miocene with published?¹³C_wood of mummified fossil wood in the late Oligocene and modern tree rings from the Nanning Basin are used to compare changes in seasonal precipitation ratio in the late Oligocene,the Miocene and the present-day.By comprehensive analysis of these data,we interpret the evolution for paleoclimate and paleomonsoon during this period,and the main awareness is as follows:1.The tree rings?¹⁸O_cellcould indicate wet season rainfallThe?¹⁸O_cell for tree rings is mainly controlled by the oxygen isotope of meteoric water(?¹⁸O_MW)and relative humidity,and is a reliable proxy for paleoclimatic reconstruction.The?¹⁸O_cell values measured in the Late Oligocene strata of the Nanning Basin,Guangxi(?¹⁸O_cell=22.4±0.5‰),and the Miocene strata of the Guiping Basin,Guangxi(?¹⁸O_cell=24.4±0.8‰)are lower than the?¹⁸O_cell values measured from modern tree rings in the Nanning Basin(?¹⁸O_cell=27.4±1.0‰).During the late Oligocene and Miocene global ice volume was less than present-day,so the?¹⁸O_cell of fossil wood is then corrected by the oxygen isotopes of benthic planktonic foraminifera(corrected?¹⁸O_cell=24.9±0.5‰for the late Oligocene,corrected?¹⁸O_cell=26.6±0.8‰for the Miocene).As no major changes in paleogeography had occurred since the Cenozoic in Guangxi,we exclude the effects of temperature,altitude,and latitude and concluded that the low?¹⁸O_cell values for the mummified fossil wood were caused by heavy rainfall in the late Oligocene and Miocene.We then used?¹⁸O_cell to recover?¹⁸O_MW and calculate the mean monthly precipitation in the Nanning area based on the quantitative relationship between?¹⁸O_MW and mean monthly precipitation in Guangzhou,a region similar to the study site.The calculation results show that the mean monthly precipitation recovered by modern?¹⁸O_cell for the wet season in Nanning(P_wet=230±51 mm)is similar to the records from the Nanning meteorological station(P_wet=219±45 mm,1990–2000),indicating that the tree ring?¹⁸O_cell can be used as a reliable proxy of wet season precipitation.Further,using the corrected?¹⁸O_cell values,we calculate P_wet=349±35 mm for the late Oligocene,60%more than today;and P_wet=268±46 mm for the Miocene,22%more than today.2.High-resolution?¹³C_woodfor tree rings could indicate seasonal precipitation variationWe reconstruct the seasonal precipitation variation(P_s/P_w,i.e.,the ratio of intra-annual summer to winter precipitation)by determining?¹³C_wood of high-resolution sampling Guiping fossil wood in the Miocene.The result shows that the pattern of?¹³C for each tree ring is similar to that in the monsoon region,and the P_s/P_w in the Miocene was 2.5±2.9.Based on the previous studies on fossil wood in late Oligocene and modern tree rings from Nanning,the P_s/P_w was 5.1±6.0 for the late Oligocene and 5.3±10.6 for the present-day,which both are higher than the result reconstructed by the Miocene fossil wood.The above results indicate that the precipitation pattern in the study area since the Cenozoic was dominated by summer(P_s/P_w>1).Seasonal precipitation variation reflects the monsoon intensity,thus the East Asian monsoon underwent a strong–weak–strong evolution during the late Oligocene–Miocene–Modern.3.The tree rings?¹⁸O_cell and?¹⁸C_woodcould recover the interannual precipitationBased on the above results of the?¹³C_wood and?¹⁸O_cell from the Nanning and Guiping,combined with Monte Carlo error propagation,we calculate the summer precipitation is1148 mm(95%C.I.=646–1650 mm),the winter precipitation is 275±234 mm and the annual precipitation at Nanning is 1425±234 mm for modern Nanning within 95%confidence intervals,which is similar with the data from the Nanning meteorological station(P_s=1106±214 mm,P_w=285±105 mm,P_total=1467±271 mm;1980 to 2020);the winter precipitation in the late Oligocene and the Miocene was 317±235mm and 591±315mm,11%and 107%more than present-day;and the annual precipitation in the late Oligocene and the Miocene was 2226±356mm and 2029±367mm,52%and 38%more than present-day.The above suggests that the precipitation for the wet season and annual since the late Oligocene–Miocene–modern have been decreasing.The reason for this may be due to the rain shadow effect caused by the uplift of the Tibetan Plateau,but is more likely to be the result of global cooling since the Oligocene.Temperature does not change the intensity of the monsoon,but it affects the amount of precipitation in the monsoon region.The change of East Asian monsoon intensity(i.e.,strong–weak–strong)during this period is consistent with previous studies related to the orbital forcing.We suggest that the Asian paleomonsoon was mainly driven by the movement of the Intertropical Convergence Zone(ITCZ)before the uplift of the Tibetan Plateau,but with the uplift of the Tibetan Plateau during the Miocene,the previous paleomonsoon pattern gradually became'extinct',and the East Asian monsoon is mainly controlled by the uplifted topography of the Tibetan Plateau in recent days.And winter precipitation in the Miocene was much higher than in the Late Oligocene and present-day,perhaps due to weaker winter monsoon in the face of high annual precipitation amount.Based on the mummified fossil wood found in the Guangxi,we first reconstruct seasonal precipitation variability quantitatively in this area during the Miocene,and provide the first quantitative estimate of intra-annual precipitation for the late Oligocene and the Miocene in this region,filling a key gap in the late Oligocene to Miocene paleo–precipitation record and providing essential evidence for assessing the relationship between regional topographic uplift,global temperature change,and the East Asian monsoon.As both the late Oligocene and the Miocene were warmer than the present-day,we hypothesize that monsoonal rainfall will increase significantly over monsoon regions as p CO₂ rises.