1.3 Thesis Objectives
1.3.1 Detecting eustatic and tectonic signals with carbon isotopes
Spanish Pyrenees
Cyclic sedimentation refers to the organised repetition of distinctive lithological units (Duff et al. (1967)). These units are found in a large range of depositional environments and with a wide order of amplitude and time-scale (Einsele et al.
(1992), Figure 1.1). This observed periodicity suggests a specific mechanism capable of creating the cyclicity (Schwarzacher (2007)) but this mechanism can be difficult to assess (Castelltort and Van Den Driessche (2003), Schwarzacher (2007), Simpson and Castelltort (2012)). Indeed, sedimentary systems are complex environments where climate, tectonics and the biosphere interact creating numerous possibilities to create cyclicity (Schwarzacher (2007)). Apart from the influence of Milankovitch cycles — change of Earth’s rotation around its axis and around the sun influencing Earth’s climate — which are predictable and calculated back in time (Laskar et al.
(2011)), these cyclicities with periods of a thousand to a million years, occur over longer time-scale than what we are able to observe making direct comparison with recent deposits difficult (Schwarzacher (2007), Romans et al. (2016)).
The potential of stratigraphic cycles to be recorded in the sedimentary record, is a function of accommodation (i.e., space available for the deposition of sediments) and sediment supply (Cross (1988), Gardner (1995), Mitchum et al. (1977), Posamentier and Vail (1988), Catuneanu et al. (2009), Figure 1.6). However, virtually all possible driving factors able to influence the amplitude of sediment supply and/or accommodation space with different magnitudes and frequency are able to generate such cycles (Castelltort and Van Den Driessche (2003), Schwarzacher (2007)).
Accommodation on one hand is controlled by rise and fall of the sea level as well as subsidence, i.e., the gradual downward settling of Earth’s surface. Sediment supply on the other hand is controlled by topography (tectonics) and climate (Figure 1.2). For instance, an increase in uplift rates, affecting the topographic relief will provide more sediment to the system (e.g., Burbank et al. (2003)). Climate
change such as the one occurring during the EECO will increase precipitation favouring erosion and transport of sediment towards the basin (e.g., Slotnick et al.
(2012)), creating a similar signal in the stratigraphic record than a increase in uplift. Moreover, climate and tectonic forcings can occur on similare time-scale making them difficult to untangle (Romans et al. (2016), Figure 1.1). Despite the increase in geochemical tools to analyse the sedimentary record (i.e., clumped isotopes (Eiler (2007), cosmogenic nuclide (Covault et al. (2011)), defining the origin of stratigraphic cycles is still a challenge. Answering this question is central for geology because when sediments get lithified and become part of the geological record; direct observation and quantification of the driving factors are not available any more. Therefore, by understanding the processes taking place at intermediate time-scales, one can try to predict Earth’s surface responses to long-term changes.
EUSTATISM
SUBSIDENCE
SEDIMENT FLUX
available accommodation space
Figure 1.6: Factors controlling the stratigraphic record. From Lafont (1994).
In the Ainsa basin, in the South Pyrenean foreland basin, cyclic deposition is observed under the form of sandy sediment gravity systems separated by important thickness of hemipelagic marls. The origin of the cyclic deposition of these sediment gravity flow systems from the Hecho Group is a long standing question (e.g., Mutti et al. (1988), Heard et al. (2008), Cantalejo and Pickering (2014, 2015), Weltje and Van Ansen (1996), Nijman (1998), Pickering and Bayliss (2009), Payros et al.
(2009)). The controls put forward to explain this repetitive deposition pattern are either upstream driven (i.e., tectonic or climatic) or dowstream driven (i.e., sea-level fluctuations).
To understand the origin of cyclicity in the Ainsa basin, the following question need to be addressed: What is the respective role of tectonics and climate in controlling the sediment flux to the Ainsa basin? What proxy can be used to confront the sediment gravity flow occurrence to tectonic/-climatic forcings?
Introduction 15
1.3.2 Alluvial record of an early Eocene hyperthermal within the Castissent Formation, South Pyrenees
We know that climate has varied through time at different time-scale and amplitude.
For instance in more recent times, the last ice age which started 2.5 million years ago is characterized by extensive ice sheet in the northern hemisphere (Lorens et al. (2004)). The glaciers have been advancing and retreating during this ice age during periods known as glacial-interglacial cycles with a periodicity paced with Milankovitch cycles of 100, 41 and 23 kyr (Sigman and Boyle (2000), Hays et al. (1976)). Past climatic events have shaped today’s landscapes by carving mountains and lakes such as the Geneva lake (Wildi et al. (1999)). Moreover, they have influenced today’s living organisms evolution and distribution (Hewitt (2000)) and are therefore important focus points of past-climate studies.
The Eocene is considered as a green-house period, where ice caps are not present or only ephemeral (e.g., Zachos et al. (2001), Pekar et al. (2005)). However, other orbitally paced climatic events know as ”hyperthermal events” occur during the Paleocene and Eocene periods (e.g., Lauretano et al. (2016), Figure 1.5).
Hyperthermal events are defined in marine settings and measured on whole-rock as well as foraminifera by a carbon and oxygen isotope excursion larger than one standard deviation together with an increase in iron (Fe) content in the whole-rock (Kirtland-Turner et al. (2014)). These hyperthermals represent rapid global warming events of 1 to 3 degrees (Stap et al. (2010)). Their origin is still debated but these events are caused by a rapid injection of isotropically light carbon (i.e., depleted in 13C, giving a negative δ13C signal, see δ13C inset) potentially linked to changes in ocean ventilation and oxidation of organic matter in abyssal oceans (Sexton et al. (2011)). These Eocene hyperthermal events can show similar features (after Foster et al. (2018), Figure 1.7):
∗ A rapid warming episode with a duration of about 40 kyr (and 200 kyr for the PETM).
∗ A reduction in oceanic oxygen content which can lead to ocean anoxia and increase carbon burial
∗ A increase in atmospheric CO2 and ocean acidification leading to carbonate dissolution in marine environments
∗ An increased hydrological cycle and increase seasonality
∗ An increase of continental erosion/weathering rates
Figure 1.7: Earth surface feedback to a hyperthermal event. CIE: Carbon isotope excursion. CCD: carbonate compensation depth
Studying hyperthermal events is important in several ways. Because these events happen at a global scale, they can be useful correlation points and critical fea-tures for studies addressing environmental signal propagation in source-to-sink systems (e.g., Duller et al. (2019)). Moreover, studying Earth’s behaviour to past hyperthermal events is one of the only ways to test and to provide fundamental information to climatic models modelling today’s climate change (Foster et al.
(2018)). Finally, hyperthermal events, and particularly important ones such as the PETM, are important moments of mammalian turnover and have the potential to influence the evolution of life (Gingerich (2006), Chew (2009)). These knowledge gains and today’s climatic circumstances, explain why hyperthermal studies have flourished since 30 years with more than 600 scientific paper on the PETM alone (McInerney and Wing (2011)).
Major events such as the PETM event have proven to be detectable in both marine and continental environments (e.g.; Abels et al., 2016; Koch et al., 1992) but the signal preservation potential of smaller scale climatic events in continental environments is less straightforward (Foreman and Straub (2017)).
Since the first studies that applied sequence stratigraphy concepts to continental deposits, the preservation of environmental signals in the continental stratigraphic record has been considered incomplete, especially during falling sea-level (Shanley and McCabe (1994), Wright and Marriott (1993)). Floodplain deposits are consid-ered as discontinuous in nature due to non-continuous flood, avulsion, and channel migration sedimentation processes and the irregular depositional thickness relative
Introduction 17 to the position of the channel (Turner et al. (2015)). This potential incompleteness of the sedimentary record (Barrell (1917), Sadler (1981)) and the capacity of a sedimentary section to document a continuous paleoclimatic signal have probably led many workers to prefer the deep-marine records for paleo-climatic studies.
Yet, continental strata, which form in direct contact to environmental conditions occurring at the time of their formation, might preserve important clues to the way the Earth system reacted to abrupt warming (Sheldon and Tabor (2009)).
However, the extent to which fluvial successions can provide complete and faithful archives of past climatic events, especially those with the smallest magnitudes, is still largely unknown (Foreman and Straub (2017), Trampush et al. (2017), Straub and Foreman (2018)).
The South Pyrenean foreland basin deposits in the Tremp-Graus basin encompass the Eocene period where numerous hyperthermal events occur. Among the South Pyrenean successions, the Castissent fluvial Formation is an important depositional event occurring at ca. 50 Ma which deposited, besides thick coarse sand channels, lateral important paleosol successions. Moreover, several studies give important time-constraints over this period, allowing a good calibration with global climatic records.
From this setup arise the following questions: Can the continental environment record small-scale hyperthermal events? If yes, is it possible to point out which hyperthermal event is recorded? What is the geochemical signature and climatic impact of a small-scale hyperthermal event in a continental environment?