Evidences of Widespread Cretaceous Deep Weathering and Its Consequences : A Review

This study highlights the effect of the Cretaceous greenhouse climate on weathering processes. Atmospheric CO2 level was relatively higher in the Cretaceous than it was in both the Jurassic and the Cenozoic. Consequently, temperature and humidity were higher in the Cretaceous than in the Jurassic and the Cenozoic. The interaction among the high levels of atmospheric CO2, extreme global warmth, and humidity in the Cretaceous resulted in widespread deep weathering. Cretaceous palaeo-weathering profiles are observed to occur at higher palaeolatitudes relative to the Jurassic and Cenozoic palaeo-weathering profiles. This implies the upward warming of the Cretaceous palaeolatitude, consistent with palaeotemperature estimates for the Cretaceous. The present thickness of weathering profiles in some selected tropical zones is approximately 200 m. During the greenhouse climatic condition in the Cretaceous, the thickness of weathering profiles at those areas could have been up to 4–5 times the present value. This suggests that many sediments were produced from the Cretaceous weathering events.


Introduction
Weathering in its simplest form refers to the alteration of rocks by the action of water and atmosphere at or near the Earth's surface.When rocks are exposed Earth's hydrosphere and atmosphere many of their constituent minerals, which are igneous or metamorphic in origin, become unstable and eventually alter into clay minerals that are stable on the Earth's surface.The Deep (chemical) weathering involves the aqueous alteration and dissolution of minerals to depths of several metres.It is an important surface process of the Earth, and it relates to many environmental changes caused by interactions and feedback processes among the atmosphere, lithosphere, hydrosphere, and biosphere.Previous studies on the control of chemical weathering have contributed much to our understanding of the links between chemical weathering and its controlling factors, such as tectonics (geological settings and topographical conditions), provenance (or the lithology of the source rock), climate (temperature, precipitation, and runoff), vegetation, and even human activities (West et al., 2005;Gabet and Mudd, 2009;Gislason et al., 2009;Li et al., 2010;Moore et al., 2013).
Weathering rates are controlled by many factors including climate, tectonics, and lithology.Studies on tropical deep weathering (Rabassa, 2010;Rabassa et al., 2011) have suggested that climate is an important factor to consider.The rate of mineral dissolution increases with the increase in temperature.In addition to mineral solubility, temperature controls moisture availability, precipitation, evapotranspiration, and run off, all of which have direct effects on deep weathering (Rasmussen and Brantley, 2011;Goudie and Viles, 2012).Chemical weathering proceeds faster under warm and wet climatic conditions (White and Blum, 1995;Dessert et al., 2003;Gislason et al., 2009).A significant number of field studies (Bluth and Kump, 1994;Summerfield and Hulton, 1994;White and Blum, 1995;Huh et al., 1998;Gaillardet et al., 1999;Riebe et al., 2001Riebe et al., , 2004;;West et al., 2005) have reported trends in the relationship between temperature and weathering on global or regional scales by comparing dissolved load and weathering fluxes or weathering rates at large numbers of field sites under varying climatic conditions.
The link between deep weathering, tectonics, and climate is essential to our understanding of long-term climate variability and global carbon cycles.Chemical weathering is a vital component of the global carbon cycle, and it influences the changes in carbon pools on multiple time scales.The main driver of chemical weathering is carbonic acid, which is formed by the dissolution of CO 2 into water (Cotton et al., 2013).Evidences in the geological record (e.g.De Jong and Schappert, 1972;Rightmire, 1978;Brook et al., 1983) suggests that the concentration of CO 2 in rock profiles could be up to hundreds of times greater than that in the atmosphere.Rock profiles also store three times as much carbon as terrestrial vegetation stores and twice as much carbon as the surface ocean stores (Sigman and Boyle, 2000).Therefore, rock carbon pools can potentially act as important sinks and sources of atmospheric carbon (Trumbore et al., 1996;Knorr et al., 2005).Weathering also acts as the major sink for atmospheric CO 2 over geologic time, and it has been suggested that tectonic uplift may drive CO 2 withdrawal by enhancing weathering (Berner et al., 1983), which consumes CO 2 and removes it from the atmosphere through the formation of HCO 3 − , which is eventually transported to the oceans to be stored as CaCO 3 .The rate of chemical weathering and the rate of mineral dissolution are known to increase with the increase in temperature.Temperature also controls mineral solubility, moisture availability, precipitation, evapotranspiration, and run off, all of which directly affect chemical weathering (Gislason et al., 2009;Beaulieu et al., 2012;Goudie and Viles, 2012).Interplay between the high levels of atmospheric CO 2 and extreme global warmth in the Cretaceous suggests that the Cretaceous was also a period of intense and widespread deep weathering.In this study, we present evidence showing that the interaction among the high levels of atmospheric CO 2 , extreme global warmth, and high humidity in the Cretaceous resulted in intense and widespread deep weathering during the Cretaceous.This study is aimed at highlighting evidence for widespread deep weathering in the Cretaceous caused by interaction among atmospheric CO 2 , global warmth, and humidity.

Method
Records of palaeo-weathering profiles (which include Saprolite, kaolinite, bauxite, and laterite) were compiled from the relevant literature covering the Jurassic, Cretaceous, and Cenozoic as presented in Tables 1 -3.Approximate locations of these weathering profiles were then plotted on the reconstructed palaeogeographic maps of the Jurassic and Cretaceous Periods and present day (afterBlakey, 2011).The records of variations in the global atmospheric level of carbon dioxide and palaeo-temperature were also compiled for the same geologic periods from relevant published literatures (e.g.Veizer et al., 2000;Goddéris et al., 2014) and plotted alongside the palaeo-weathering profiles on a palaeo-latitude map.

Palaeoweathering Profiles
Weathering occurs everywhere in the continents, and its intensity depends on geomorphology, climate, and the time span during which the weathering processes have been operational.In areas of high relief, weathering processes may act continually for a long time, but the weathered materials are not always preserved as they are eroded and stripped off the steep hills (Thiry and Simon-Coinçon, 1999).In basinal areas, weathering is only active for a short period of time and palaeoweathering profiles can be preserved between successive sedimentation episodes, but such palaeoprofiles are incipient as they have little time to develop.Mature and deep palaeoweathering profiles are best preserved on the margins of basins and especially passive margins, which are characterized by crustal flexure, and the land surface is relatively stable with regard to the regional base level.At such basin margins, the rate of erosion is low, and sediment deposition does not continue, enabling the development of weathering and providing a protective cover for the weathered materials (Thiry and Simon-Coinçon, 1999).
The products of deep weathering are characteristic of cratonic regions, which have survived in the landscape without being covered by marine sediments for extremely long periods as they have been exposed to long-term subaerial weathering and denudation.Their genesis is related to extremely humid and warm palaeoclimates of "hyper-tropical" nature, with permanently water-saturated soils or perhaps extreme climates, with seasonal and long-term cyclic fluctuations from extremely wet to extremely dry.Deep chemical weathering is the dominant geomorphological process, with the development of enormously deep weathering profiles perhaps up to many hundreds of metres deep.The weathering products are clays and in some cases kaolinite, pure quartz, and other silica-type sands, silcretes, and calcretes (calcium carbonate).
Deep weathering processes accelerate with increased temperatures and availability of water, which explains why deep weathering is more intense in low-latitude areas of high precipitation than in high-latitude areas of low precipitation.At low latitude areas, the average surface temperatures can be as high as 26°C.Locally, the surface temperatures can reach 60°C during the day.Water in such conditions contains six times more free H + ions than in temperate regions, and its action as an agent of chemical weathering is strongly reinforced (Robert, 2008).At such low-latitude areas, weathering profiles of approximately 1.5-m depth are recorded (Robert, 2008).Conversely, only early chemical-weathering products are found in arid and high-latitude areas, where temperatures are low and the lack of precipitation limits the rate of deep weathering.
The upper continental crust consists of plagioclase-and K-feldspar-rich rocks and their weathering products, the clay minerals.The nature of clay mineral assemblages produced during chemical weathering is controlled by the climate, duration of weathering, slope, water-rock ratio, and water chemistry (Weaver, 1989;Fürsich et al., 2005).Plagioclase, K-feldspar, other alkali and alkaline-earth Al silicates and volcanic glass weather clay minerals; feldspars commonly weather kaolinite and illite; and mafic minerals and glass commonly weather smectites, kaolinite, and illite (Nesbitt et al., 1980;Nesbitt and Young, 1989;Weaver, 1989).Chlorite is also formed by the transformation of mafic minerals (Weaver, 1989).Chlorite and illite are formed during the initial stages of deep weathering, while smectite and kaolinite are formed at the advanced stages of weathering (Nesbitt andYoung, 1989, Weaver, 1989;Fürsich et al., 2005).
Kaolinite is produced from intense deep weathering and suggests soil production in the source area under humid-subtropical to tropical climate, supported by a high water-rock ratio and steep slopes with good drainage (Hallam, 1984;Chamley, 1989).Palaeoweathering profiles and palaeosurfaces are important parts of the geological records because they denote fundamental climatic and tectonic events that aid in stratigraphical correlation and division.The development of large-scale palaeosurfaces and extensive palaeoweathering horizons would often have influenced sedimentation in neighbouring marine basins through changes in the rate, nature, and composition of sediment supply (Thiry and Simon-Coinçon, 1999).In the ensuing sections, evidence for deep weathering from the Jurassic to the present is presented with the aim of comparing the spread and number of occurrences.

Evidence for Jurassic Deep Weathering
Evidences of Jurassic deep weathering and the details of their locations are presented in Table 1 and Fig. 1.Saprolite, kaolinite, bauxite, and laterite are the common products of Jurassic deep weathering.These deep weathering-products are dependent on the climate because they are positively correlated with temperature and humidity (Pedro, 1968;Thomas, 1994;Tardy and Roquin, 1998).The Jurassic palaeogeographic description is characterized by the existence of two major continental blocks: Laurasia and Gondwana (Scotese, 2001;Blakey, 2011).Laurasia, comprising North America, Europe, and Asia, was located in the northern hemisphere extending from above the equator to the northern polar areas, while Gondwana, which had rifted into Western Gondwana (comprising South America and Africa) Eastern Gondwana (comprising Australia, Antarctica, and India), was mainly situated between the subtropical-tropical belt and the southern polar region (Fig. 1).Jurassic deep-weathering profiles occur in both Laurasia and Gondwana.There are reports in the geological records of Jurassic deep weathering occurring at high latitudes of approximately 52° N.An example is southern Sweden, as presented in Table 1 and Fig. 1.Ahlberg et al. (2003) explained that deep weathering affected basement rocks of southern Sweden from the Late Triassic through the Cretaceous.These deep-weathering events were caused by long-term warmth and humidity, in conjunction with the decomposition of abundant organic matter and subsequent infiltration of acidic water in the region, which is consistent with the assertion that the processes leading to the greenhouse climatic condition prevailing globally in the Cretaceous were initiated in the Jurassic or even the Triassic.Jurassic deep-weathering events have also been reported in the Yilgarn Craton in Western Australia (Cornelius et al., 2008).During the Jurassic, Australia was still connected to India and Antarctica in Eastern Gondwana.The approximate palaeolatitude of the Yilgarn Craton in Western Australia during the Jurassic was 55° S, which implies that warm climatic conditions extended to palaeolatitude 55° S during the Cretaceous.2; Fig. 2).This implies the upward warming of the Cretaceous palaeolatitude, which is consistent with palaeotemperature estimates for the early Cretaceous (Huber et al., 1995;Friedrich et al., 2012).
There are documented evidences for numerous mid-Cretaceous deep-weathering events in the geological records (Table 2; Fig. 3).Mid-Cretaceous deep-weathering events are more widespread than early Cretaceous deep-weathering events (Figs. 2 and 3), which is consistent with the assertion that the Earth's highest temperature of approximately 28°C in the Phanerozoic occurred in the middle of the Cretaceous (Veizer et al., 2000;Scotese, 2001).
Evidence for Late Cretaceous deep weathering is found in North America, South America, Europe, and Asia (Table 2; Fig. 4).Although the Earth was still generally warm in the Late Cretaceous, the temperature had started dropping, especially around the polar regions.This was in response to the drop in the atmospheric level of CO 2 caused by a decline in the volcanic activities associated with the disintegration of the Cretaceous continents.This explains why Late Cretaceous deep weathering is only restricted to the tropical regions, where temperatures were relatively higher than at the polar region at that time.Evidences of Late Cretaceous deep weathering are restricted between palaeolatitudes 55° N and 50° S (Fig. 4).     2 3.1.3Evidence for Cenozoic Deep Weathering Evidences of Cenozoic deep weathering and the details of their locations are presented in Table 3.7 and Fig. 3.12.The final phase in the breakup of Pangea occurred during the Cenozoic as North America and Greenland rifted away from Europe.India, which started moving northward after separating from Antarctica and Australia in the Cretaceous, collided with Southeast Asia, while Australia also moved northwards, resulting in the present positions of these continents (Fig. 3.12).Cenozoic palaeoweathering profiles occur between palaeolatitudes 60° N and 30° S (Figs 3.12), which is consistent with palaeolatitudes having tropical climatic conditions at present.No Cenozoic palaeoweathering profiles have been reported at the polar regions, where cold climatic conditions prevail at present.At present, bauxite forms only under the hot and humid climatic conditions occurring between latitudes 30° N and 30° S (Tardy et al., 1990), while saprolite formation currently occurs primarily between latitudes 30° N and 30° S (Bárdossy, 1982).This suggests positive correlation between temperature and weathering processes.

Discussion
Palaeogeographic studies (Scotese, 2001;Golonka, 2007;Blakey, 2011) show that, before the Jurassic period, all the continents of the Earth were sutured together into one supergiant continent, Pangea.The disintegration of this supergiant continent started in the early Jurassic and subsequently resulted in two giant continents: Laurasia and Gondwana.The disintegration of Laurasia opened up what eventually became the Gulf of Mexico and the Atlantic Ocean.Closely associated with Pangea's breakup and the subsequent disintegration of continents are a series of volcanic activities that emitted large volumes of CO 2 into the atmosphere (Larson, 1991).It is estimated that during the Cretaceous, the global atmospheric level of CO 2 was significantly higher than that at present (Berner and Kothavala, 2001;Bice and Norris, 2002;Bice et al., 2003;Bata et al., 2015).As the Earth's surface temperature is significantly affected by greenhouse gases (mainly CO 2 ), the enhanced tectonic activity during the Cretaceous, which emitted large quantities of CO 2 into the atmosphere, resulted in a corresponding rise in global temperatures, leading to a greenhouse climatic condition in the Cretaceous (Huber et al., 2002;Jenkyns et al., 2004).There are evidences in the geological record (Royer et al., 2004;Royer, 2008Royer, , 2010) ) for co-variance in the global temperature with atmospheric CO 2 during the Phanerozoic (Fig. 6).These evidences suggest that the extreme global warmth during the Cretaceous was a direct consequence of the high level of atmospheric CO 2 that prevailed globally at that time (Skelton, 2003;Wang et al., 2014).
The results presented in this study (Figs 1 to 6; Tables 1 to 3) show the occurrence of deep weathering at a relatively higher palaeolatitude in the Cretaceous than in the Jurassic and Cenozoic.Since the rate of weathering is known to increase with temperature (e.g.Bluth and Kump, 1994;Summerfield and Hulton, 1994;White and Blum, 1995;Huh et al., 1998;Gaillardet et al., 1999;Riebe et al., 2001Riebe et al., , 2004;;West et al., 2005;Rabassa, 2010;Rabassa et al., 2011), the occurrence of palaeo-weathering profiles at a relatively higher palaeolatitude in the Cretaceous implies the upward warming of the Cretaceous palaeolatitude, which is consistent with the assertion that the Cretaceous was the warmest period in the Phanerozoic.
The mid-Cretaceous (Albian) was the warmest part of the Cretaceous, with surface temperatures reaching 28°C (Veizer et al., 2000;Huber et al., 2002;Jenkyns et al., 2004).At that time, it is believed that tropical to subtropical conditions extended perhaps as far south as 70° S.This is attributed to a unique ocean-current circulation at that time, which resulted in a large transfer of heat from the equatorial zones to the poles (Frakes, 1979).During the Cretaceous, the equatorial zone was heated much more intensely than at present not only because of the increase in the atmospheric level of CO 2 , which resulted in rising global temperatures at that time, but also because the huge extension of the Pacific Ocean at low latitudes at that time would have lowered the global albedo, strongly increasing the heat capacity of the oceans and thereby influencing the Earth's heat reservoir at that time (Rabassa, 2014).
Higher temperatures occurring during the Cretaceous forced higher evaporation rates, increasing the water content of the atmosphere and the global greenhouse effect.This implies that a much higher precipitation rate must have prevailed over the continents under very warm Cretaceous climatic conditions.Thus, the precipitation during the Cretaceous would have been higher and more intense than at present.Such exceedingly high precipitation rates would have generated huge volumes of water as surface runoff, and soil infiltration during the Cretaceous would have perhaps occurred down to very deep levels of several hundreds of metres because of the hydrostatic pressure of the hyper-saturated soils throughout the year.This would have generated extremely intense weathering processes and very thick weathering mantles with huge weathering profiles.
At present, because of the climatic conditions within the wet equatorial regions, the weathering layer reaches maximum depths of up to 200 m, with precipitations on the order of 2,100 mm/year and the minimum mean annual temperatures above 15°C (Rabassa, 2014).Typical present-day weathering profiles within the wet equatorial regions have approximately 100-m-thick kaolinitic zones with another 100 m of montmorillonitebeidellite-hydromica underneath an additional 50 m of gruss (Rabassa, 2014).Such expected weathering layers would be much larger in thickness under a hypertropical climate than under the present conditions.The Cretaceous climatic condition can be considered hyper-tropical, with no present analogues.Thus, if the weathering front today is at approximately 200 m in selected tropical zones, it could have been up to 4-5 times the present value during the Cretaceous, which implies that the thickness of the weathering layer during the Cretaceous would have perhaps been up to 1,000 m.

Summary and Conclusion
This study provides a historical account of Cretaceous Earth, outlining the climatological conditions that prevailed at that time which led to widespread deep weathering.Specifically, this study demonstrated the following: i) During the 80-million-year Cretaceous period, the CO 2 content of the global atmosphere drastically increased in response to volcanism associated with the disintegration of the former continents.This caused both atmospheric and oceanic temperatures to rise and vary over a wide range.The extreme global warmth of the Cretaceous caused most of the polar ice caps to melt, which resulted in the expansion of the volume of seawater.This further resulted in a significant rise in the global sea level at that time.
ii) During the Cretaceous, the interaction among the high levels of atmospheric CO 2 , extreme global warmth, and high humidity resulted in intense and widespread deep weathering.
iii) A comparison between Jurassic, Cretaceous, and Cenozoic palaeoweathering profiles shows that deep weathering was relatively more intense and widespread in the Cretaceous than in the Jurassic and Cenozoic.Cretaceous palaeoweathering profiles were also observed at higher palaeolatitudes, implying the upward warming of the Cretaceous palaeolatitude, which is consistent with palaeotemperature estimates for the Cretaceous.
iv) The Cretaceous climate condition was hyper-tropical, with no present analogues.Thus, if the weathering front today is at approximately 200 m in selected tropical zones, it could have been up to 4-5 times the present value during the Cretaceous.This suggests that the thickness of the weathering layer during the Cretaceous would have perhaps been up to 1,000 m.

Figure 1 .
Figure 1.Reconstructed Jurassic palaeogeographic world map (modified from Blakey, 2011) showing estimated locations of Jurassic palaeoweathering profiles.Details of the locations of the Jurassic palaeoweathering profiles are presented in Table 1

Figure 2 .
Figure 2. Reconstructed early Cretaceous palaeogeographic world map (modified from Blakey, 2011) showing estimated locations of early Cretaceous palaeoweathering profiles.Details of the locations of the early Cretaceous palaeoweathering profiles are presented in Table 2

Figure 3 .
Figure 3. Reconstructed mid-Cretaceous palaeogeographic world map (modified from Blakey, 2011) showing estimated locations of mid-Cretaceous palaeoweathering profiles.Details of the locations of the mid-Cretaceous palaeoweathering profiles are presented in Table 2

Figure 4 .
Figure 4. Reconstructed Late Cretaceous palaeogeographic world map (modified from Blakey, 2011) showing estimated locations of Late Cretaceous palaeoweathering profiles.Details of the locations of the Late Cretaceous palaeoweathering profiles are presented in Table 2

Figure 5 .
Figure 5. Present-day world palaeogeographic map (modified from Blakey, 2011) showing estimated locations of Cenozoic palaeoweathering profiles.Details of the locations of the Cenozoic palaeoweathering profiles are presented in Table3

Table 1 .
Evidence for Jurassic deep weathering

Table 2 .
Evidence for Cretaceous deep weathering