A Monsoon or Monsoon-like Paleoclimate During the Eocene Paleogene in East Asia

A new paleoclimate classification for deep time

Authors:

Zhang et al

Abstract:

In deep time, climates are mainly classified by climatically sensitive deposits, paleontological evidences, and modeling. However, they only have limited applicability in deep time studies. Here, we propose a new paleoclimate classification based on the widely used Köppen climate classification. The proposed new classification is simple and quantitative, but bridges the gap between modern and deep time climate studies. The new classification is closely related to but differs from that of Köppen by changing some limits. A world map using the new classification shows the same patterns as the world map of the Köppen climate classification. Using the new classification, we are able to solve a long-standing problem about the climates of East Asia during the Eocene. We found that East Asia shared the same climate type (Ca: Subtropical) at all studied locations, supporting the hypothesis of monsoon or monsoon-like climate that prevailed there during the Eocene.

The Asian Monsoon Shifted to the NW From Last Glacial Maximum to Present, may Shift More due to Global Warming

Warming-induced northwestward migration of the East Asian monsoon rain belt from the Last Glacial Maximum to the mid-Holocene

Authors:

Yang et al

Abstract:

Glacial–interglacial changes in the distribution of C3/C4 vegetation on the Chinese Loess Plateau have been related to East Asian summer monsoon intensity and position, and could provide insights into future changes caused by global warming. Here, we present δ13C records of bulk organic matter since the Last Glacial Maximum (LGM) from 21 loess sections across the Loess Plateau. The δ13C values (range: –25‰ to –16‰) increased gradually both from the LGM to the mid-Holocene in each section and from northwest to southeast in each time interval. During the LGM, C4 biomass increased from less than 5% in the northwest to 10–20% in the southeast, while during the mid-Holocene C4 vegetation increased throughout the Plateau, with estimated biomass increasing from 10% to 20% in the northwest to greater than 40% in the southeast. The spatial pattern of C4 biomass in both the LGM and the mid-Holocene closely resembles that of modern warm-season precipitation, and thus can serve as a robust analog for the contemporary East Asian summer monsoon rain belt. Using the 10–20% isolines for C4 biomass in the cold LGM as a reference, we derived a minimum 300-km northwestward migration of the monsoon rain belt for the warm Holocene. Our results strongly support the prediction that Earth's thermal equator will move northward in a warmer world. The southward displacement of the monsoon rain belt and the drying trend observed during the last few decades in northern China will soon reverse as global warming continues.

Evidence of MegaMonsoons From the Carboniferous

Supercritical-flow structures on a Late Carboniferous delta front: Sedimentologic and paleoclimatic significance

Authors:

Ventra et al

Abstract:

Deposits of fluvial systems in highly seasonal tropical climates possess unique architectural and facies characters owing to a flood-prone regime alternating with lengthy periods of ineffective discharge. Distally linked deltaic successions should also feature distinctive attributes, with great potential to preserve the stratigraphic evidence of exceptional discharge events. We describe Late Carboniferous delta-front, valley-confined sandstones from the Pennine Basin (UK), originally deposited at paleoequatorial latitudes during final assembly of the Pangean megacontinent and characterized by giant sedimentary structures with repetitively sigmoidal geometry. Facies traits indicate geologically instantaneous deposition of a large sediment volume from a density current at sustained supercritical-flow conditions, leading to aggradation of cyclic steps, recently identified bedforms developing in high-energy flows and of which this is the first complete outcrop example. The lack of unconformable erosional surfaces and absence of different associated facies point to a single aggradational event during which the structures attained dimensions comparable to those indicated by seismic data sets from which they are remotely detected on modern seafloors. Cyclic-step formation in a deltaic setting suggests that Pangean megamonsoons could have triggered hydrologic events capable of imprinting sedimentologic signatures on shallow-marine deposits.

The Miocene Neogene Monsoonal Forests of Southwest China

Late Miocene vegetation dynamics under monsoonal climate in southwestern China

Authors:

Li et al

Abstract:

To better understand vegetation dynamics and the Asian monsoonal climate in the Neogene, we reconstructed the vegetational succession and climate of Wenshan basin in southwestern China during the late Miocene (11.62-5.33 Ma). We used newly available palynological data and the results of a quantitative bioclimatic analysis. The late Miocene palynoflora in Wenshan basin resembles that of modern evergreen broadleaf vegetation in subtropical East Asia. Based on pollen elements and reconstructed palaeoclimates in 72 samples, we found the composition of the vegetation in the Wenshan basin fluctuated during the late Miocene and was likely driven by natural climate variability. Quantitative estimates suggest that the mean annual temperature (MAT) ranged from 16.6 to 17.5°C; the mean annual precipitation (MAP) was ~ 1500 mm; and the monsoon intensity index (MSI) ranged from 11.3 to 17.1. Our results indicate the Wenshan basin experienced a warm, wet, and temperate to subtropical climate. By comparison with other late Miocene sites in Yunnan, we show that temperatures differed slightly from region to region and, overall, annual and seasonal precipitation levels were higher in the late Miocene than at present, but with a weaker monsoon intensity than in the Wenshan basin today.

The Effects of Tibetan Plateau Uplift on the Asian Monsoon

The impact of regional uplift of the Tibetan Plateau on the Asian monsoon climate

Authors:

Zhang et al

Abstract:

The climatic effect of Tibetan Plateau (TP) uplift is a hot topic in paleoclimate research. In this paper, the results of a set of numerical simulations are reported to analyze the impact of regional TP uplift on the Asian monsoon climate. The focus is on the impact of the regional uplift in the west and east parts on South Asian summer monsoon, as well as the impact of regional uplift on East Asian winter monsoon. The results show that the uplifts of the Himalaya and the central-southern TP have little effect on summer precipitation increasing over South Asia. Whereas, the further uplifts of the TP in the west and east parts, including the uplifts of both the eastern and western TP, have important effects on the strengthening of the South Asian summer monsoon. The uplift of the eastern TP leads to low-level cyclonic circulation anomalies appearing in the Bay of Bengal, accompanied by anomalous upward movement, and in turn precipitation markedly increases in the Bay of Bengal. Whereas, the uplift of the western TP produces low-level cyclonic circulation anomalies around the uplifted region and intensifies the moisture transport from the tropical ocean, and thus precipitation further increases in South Asia, especially in the northern Arabian Sea. The strengthened summer monsoon in South Asia is coupled with the intensification in diabatic heating and moisture convergence. For the East Asian winter monsoon, the intensification of winter monsoon winds is related more to the regional uplifts than the uplift of the whole TP. The uplifts of the central-southern TP, the northern TP and other northern topography have greater effects on the strengthening of the winter monsoon winds.

Seasonal and Regional Records of Pliestocene Quarternary Asian Monsoons

Temperature and leaf wax δ2H records demonstrate seasonal and regional controls on Asian monsoon proxies

Authors:

Thomas et al

Abstract:

Orbital-scale precipitation isotope records can elucidate climate forcing mechanisms and provide benchmarks for climate model validation. The ability to differentiate the influence of temperature, seasonality, and vapor transport history on precipitation isotope proxies is critical to both objectives. We present a 300 k.y. leaf wax hydrogen isotope record from the South China Sea with the effects of local condensation temperature removed (δ2Hwax–T). δ2Hwax–T reflects annually integrated precipitation δ2H in the Pearl River catchment of southeast China. Depleted δ2Hwax–T lags minimum precession (Pmin) by 1.0 ± 0.7 k.y., reflecting the influence of maximum summer insolation and minimum winter insolation, with a minor influence of global ice volume, which lags Pmin by 3.3 ± 0.4 k.y. In contrast, annually integrated cave δ18O minima in central China, 1000 km north of our site, lag Pmin by 2.7 ± 0.3 k.y., in phase with ice volume minima. This phase indicates that precipitation δ18O in central China is more strongly influenced by ice volume forcing at the precession band, with a lesser influence of Northern Hemisphere insolation. Our new δ2Hwax–T data demonstrate that precipitation isotopes in Asia have strong regional variability. Interpreting water isotope records within the context of regionally varying temperature, seasonality, and sensitivity to changing glacial boundary conditions is imperative to understanding Asian hydroclimatic change.

There Were Asian Monsoons During the Eocene Paleogene

Asian monsoons in a late Eocene greenhouse world

Authors:

Licht et al

Abstract:

The strong present-day Asian monsoons are thought to have originated between 25 and 22 million years (Myr) ago, driven by Tibetan–Himalayan uplift. However, the existence of older Asian monsoons and their response to enhanced greenhouse conditions such as those in the Eocene period (55–34 Myr ago) are unknown because of the paucity of well-dated records. Here we show late Eocene climate records revealing marked monsoon-like patterns in rainfall and wind south and north of the Tibetan–Himalayan orogen. This is indicated by low oxygen isotope values with strong seasonality in gastropod shells and mammal teeth from Myanmar, and by aeolian dust deposition in northwest China. Our climate simulations support modern-like Eocene monsoonal rainfall and show that a reinforced hydrological cycle responding to enhanced greenhouse conditions counterbalanced the negative effect of lower Tibetan relief on precipitation. These strong monsoons later weakened with the global shift to icehouse conditions 34 Myr ago.

More Contrarian Data: Cool Equitorial Temperatures From Early Eocene Paleogene India

Cool equatorial terrestrial temperatures and the South Asian monsoon in the Early Eocene: Evidence from the Gurha Mine, Rajasthan, India

Authors:

Shukla et al

Abstract:

Early Eocene (~ 55–52 Ma) laminated lacustrine sediments overlying lignites in the Gurha Mine (27.87398°N, 72.86709°E), Rajasthan, India, yield a diversity of fossil leaves, flowers, fruits, seeds and insects. CLAMP (multivariate foliar physiognomic) analysis of two horizons separated by an estimated several tens of thousands of years of deposition indicates cool equatorial (~ 10°N) temperatures and a pronounced monsoon signature. A lower assemblage consisting of 54 leaf morphotypes and an upper assemblage of 57 leaf forms yielded mean annual temperatures (MAT) of 24.7 and 23.9 °C, respectively. The uncertainty (± 2.82 °C) means these temperature regimes are identical despite few similarities in the morphotypes between the two assemblages.The mean annual range of temperature (MART) was approximately 9.7 °C for both assemblages. When corrected for evapotranspirational cooling these temperature regimes are similar to those experienced today at 10°N on the west coast of India and surprisingly cool for the tropics at a time of extreme global warmth. Growth was year round. The tropical to paratropical fossil floras also suggest a moist regime (80% annual relative humidity) and high mean annual precipitation of ~ 1800 mm for both assemblages but with a pronounced wet/dry seasonality indicative of a pronounced monsoonal regime. The lower assemblage has a stronger monsoon index (11.8) than the upper assemblage (8.8). The two assemblages seem to have been deposited less than 100 ka apart. This suggests that not only a pronounced South Asian monsoon existed when India and Asia first made contact, but also a variation in monsoon strength existed that cannot be ascribed to tectonic drivers.

Will Global Warming Impact the Indian Monsoon?

Global warming and South Indian monsoon rainfall—lessons from the Mid-Miocene

Authors:

Reuter et al

Abstract:

Precipitation over India is driven by the Indian monsoon. Although changes in this atmospheric circulation are caused by the differential seasonal diabatic heating of Asia and the Indo-Pacific Ocean, it is so far unknown how global warming influences the monsoon rainfalls regionally. Herein, we present a Miocene pollen flora as the first direct proxy for monsoon over southern India during the Middle Miocene Climate Optimum. To identify climatic key parameters, such as mean annual temperature, warmest month temperature, coldest month temperature, mean annual precipitation, mean precipitation during the driest month, mean precipitation during the wettest month and mean precipitation during the warmest month the Coexistence Approach is applied. Irrespective of a ~ 3–4 °C higher global temperature during the Middle Miocene Climate Optimum, the results indicate a modern-like monsoonal precipitation pattern contrasting marine proxies which point to a strong decline of Indian monsoon in the Himalaya at this time. Therefore, the strength of monsoon rainfall in tropical India appears neither to be related to global warming nor to be linked with the atmospheric conditions over the Tibetan Plateau. For the future it implies that increased global warming does not necessarily entail changes in the South Indian monsoon rainfall.

Evidence of a Summer Monsoon in Eocene Paleogene Antarctica

An early–middle Eocene Antarctic summer monsoon: Evidence of ‘fossil climates’

Authors:

Jacques et al

Abstract:

A warmer and mostly ice-free South polar region prevailed during the early–middle Eocene, indicative of a low latitudinal temperature gradient. Climatic models mostly fail to reconstruct such a low gradient, demonstrating our poor understanding of the mechanisms involved in heat transfer. Here we describe a new phenomenon that shaped the southern high latitude climate during the early–middle Eocene: the Antarctic summer monsoon. Our palaeoclimatic reconstruction is based on 25 morphotypes of fossil dicotyledonous leaves from the early–middle Eocene fossil leaf assemblage of Fossil Hill from King George Island, the Antarctic Peninsula. We use a novel CLAMP (Climate Leaf Analysis Multivariate Program) calibration which includes new climatic parameters that allow us to characterise better the seasonality in precipitation. Our reconstruction indicates a warm humid temperate climate with strong seasonality in temperature and precipitation. Seasonality in precipitation indicates a rainfall rate of 6.4 ± 1.30 mm/day during summer (summer daily rate of precipitation; SDR) and a summer precipitation representing more than 60.3 ± 8.28% of annual rainfall (ratio of summer precipitation; RSP), which fulfils the definition of a summer monsoon in the modern world. This implies a seasonal alternation of high- and low-pressure systems over Antarctica during the early–middle Eocene. Such a climate regime would have impacted upon global atmospheric circulation and heat transfer. This climatic regime presents a challenge for climatic models and their ability to reconstruct accurately palaeoclimates at high southern latitudes and thereby understand latitudinal heat transfer in a ‘greenhouse Earth’ regime.

The Indian Monsoon Through the Quaternary

A glimpse of the Quaternary monsoon history from India and adjoining seas

Authors:

Saraswat et al

Abstract:

The evolution of the monsoon is briefly summarized taking into account previous studies conducted on both terrestrial and marine records recovered from the Indian subcontinent and the adjoining seas. While the initiation of the monsoon is debated, it seems clear that a major intensification of the summer monsoon occurred at ~ 8.2 Ma. A seasonal monsoon circulation with distinct summer and winter monsoon phases was established at ~ 2.8 Ma. In the Late Quaternary, the summer monsoon weakens during glacial periods as compared to interglacial periods. The centennial to sub-centennial scale changes in the monsoon during both glacial and interglacial periods suggest strong links between high latitude processes and the tropical monsoon and its role in modulating northern hemispheric climate. The glacial terminations are marked by weak monsoon activity. The summer monsoon weakened during the Middle Holocene after the Early Holocene optimum. A change in the monsoon intensity though alters the salinity of the seas surrounding India, the quantitative paleosalinity reconstructions still have large associated uncertainties and an effort should be made to lower them.

Evidence From Gulf of Mexico Deep Core Suggests Monsoonal Cycle was Intensified During Holocene Thermal Maximum

PALEOENVIRONMENTAL AND PALEOCLIMATIC IMPLICATIONS OF ENHANCED HOLOCENE DISCHARGE FROM THE MISSISSIPPI RIVER BASED ON THE SEDIMENTOLOGY AND GEOCHEMISTRY OF A DEEP CORE (JPC-26) FROM THE GULF OF MEXICO

Authors:

TRIPSANAS et al

Abstract:

Several meltwater floods initiated by the Laurentide Ice Sheet (LIS) drained to the Gulf of Mexico (GOM) through the Mississippi River during the last deglaciation (10–21 cal ka [calibrated kiloannum]). Such floods have been efficiently captured in the geochemical record of the Jumbo Piston Core 26 (JPC-26) from the northwestern GOM as distinct peaks in the content distribution of Si, Al, and Fe due to their common occurrence throughout North America. On the other hand, peaks in the content distribution of Ti, K, Zr, and V adequately describe the sediment source of each flooding event due to their clustered areal distribution in North America. The presence of three distinct peaks in the distribution of Si, Al, and Fe at 8.5, 5.2–7.6, and 2.9–3.5 cal ka indicates that events of enhanced Mississippi River discharge (EMRD) occurred during the Holocene as well. The geochemical signature of these flooding events suggests that their origin and sediment source is variable, depending on the retreating state of the LIS and the prevailing precipitation patterns in North America during these time periods. A comparison of the 8.5 and 2.9–3.5 cal ka EMRD events with the oxygen isotope and gas (CH4) records of the Greenland Ice Sheet Project 2 (GISP-2) ice core denotes that these events might be linked to short global warming episodes associated with periods of increased solar irradiance. The 5.2–7.6 cal ka EMRD event is associated with the Holocene Thermal Maximum, which resulted in the intensification of the North American monsoonal circulation.

Indian Monsoon to Become More Erratic with Global Warming

The Indian monsoon is a complex system which is likely to change under future global warming. While it is in the very nature of weather to vary, the question is how much and whether we can deal with it. Extreme rainfall, for example, bears the risk of flooding, and crop failure. Computer simulations with a comprehensive set of 20 state-of-the-art climate models now consistently show that Indian monsoon daily variability might increase, according to a study just published by scientists of the Potsdam Institute for Climate Impact Research.

"Increased variability – this rather technical term translates into potentially severe impacts on people who cannot afford additional loss," says Anders Levermann, one of the study's authors and co-chair of PIK's research domain Sustainable Solutions. "The fact that all these different models agree is a clear message that adaptation measures can be built on." Even if seasonal mean precipitation would remain unchanged, impacts could be substantial, Levermann points out. "Focusing on the average is not always useful. If rainfall comes in a spell and is followed by a drought, this can be devastating even if the average is normal. This requires the right kind of adaptation measures that account for this variability – such as intelligent insurance schemes, for example."

"Limiting global warming is key, adaptation cannot replace but rather complement it"

The strongest change of 13 to 50 percent is found in a scenario in which greenhouse gases continue to be emitted unabated. However, even if global warming would be limited to the internationally acknowledged threshold of 2 degrees Celsius of global warming, this would bear the risk of additional day-to-day variability between 8 and 24 percent above the pre-industrial level, according to the analysis. "So limiting global warming is key to reduce day-to-day monsoon variability, adaptation cannot replace but rather complement it," says Levermann.

The researchers focused on the ten models with the most realistic monsoon pattern – a conservative approach, as these ten models yield generally lower rates of change. The other ten models showed higher rates of change. "This is not about exact percentages. It is the clear trend that conveys the message," says Arathy Menon, lead-author of the study. The scientists used the latest ensemble of climate models, prepared for the 5th assessement report of the International Panel on Climate Change. All of them show increased variability.

"This is a robust indicator"

Taking into account all 20 models, the spread of results reduces when the scientists looked at the rainfall changes per degree of global warming independent of the exact time path of the warming. The consistent result is that 4 to 12 percent variability change of daily monsoon rainfall in India are to be expected per degree Celsius of warming. "This is a robust indicator," says Menon.

Super Drought Research Suggests Stronger Monsoon in the Southwest US With Global Warming?

Multidecadal to multicentury scale collapses of Northern Hemisphere monsoons over the past millennium

Authors:

1. Yemane Asmerom (a)
2. Victor J. Polyak (a)
3. Jessica B. T. Rasmussen (b)
4. Stephen J. Burns (c)
5. Matthew Lachniet (d)

Affiliations:

a. Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131;

b. Leander Independent School District, Leander, TX 78646;

c. Department of Geosciences, University of Massachusetts, Amherst, MA 01003; and

d. Department of Geoscience, University of Nevada, Las Vegas, NV 89154

Abstract:

Late Holocene climate in western North America was punctuated by periods of extended aridity called megadroughts. These droughts have been linked to cool eastern tropical Pacific sea surface temperatures (SSTs). Here, we show both short-term and long-term climate variability over the last 1,500 y from annual band thickness and stable isotope speleothem data. Several megadroughts are evident, including a multicentury one, AD 1350–1650, herein referred to as Super Drought, which corresponds to the coldest period of the Little Ice Age. Synchronicity between southwestern North American, Chinese, and West African monsoon precipitation suggests the megadroughts were hemispheric in scale. Northern Hemisphere monsoon strength over the last millennium is positively correlated with Northern Hemisphere temperature and North Atlantic SST. The megadroughts are associated with cooler than average SST and Northern Hemisphere temperatures. Furthermore, the megadroughts, including the Super Drought, coincide with solar insolation minima, suggesting that solar forcing of sea surface and atmospheric temperatures may generate variations in the strength of Northern Hemisphere monsoons. Our findings seem to suggest stronger (wetter) Northern Hemisphere monsoons with increased warming.

That's interesting when combined with the projection of the monsoon being delayed under global warming. A stronger, but later monsoon could be...not so good.  Oh my green chiles!