A massive input of coarse-grained siliciclastics in the Pyrenean Basin during the PETM: the missing ingredient in a coeval abrupt change in hydrological regime

Abstract

Abstract. The Paleocene–Eocene thermal maximum (PETM) is represented in numerous shallow and deep marine sections of the south–central and western Pyrenees by a 2–4 m thick unit (locally up to 20 m) of clays or marly clays intercalated within a carbonate-dominated succession. This unit records a massive input into the Pyrenean Gulf of fine-grained terrestrial siliciclastics, attributed to an abrupt hydrological change during the PETM. However, the nature of such a change remains controversial. Here we show that, in addition to fine-grained deposits, large volumes of coarse-grained siliciclastics were brought into the basin and were mostly accumulated in incised valleys and in a long-lived deep-sea channel. The occurrence of these coarse-grained deposits has been known for some time, but their correlation with the PETM is reported here for the first time. The bulk of the incised valley deposits in the PETM interval are cross-bedded sands and pebbly sands, almost exclusively made of quartz. The criteria for indicting a relation to the PETM include their stratigraphic position between upper Thanetian and lower Ilerdian marine carbonates, organic carbon isotope data, and a high percentage of kaolinite in the clay matrix. The axially flowing deep-sea channel existed throughout Paleocene times in the Pyrenean Basin, within which coarse-grained calciclastic and siliciclastic turbidites were accumulated. This Paleocene succession is capped by thickly bedded quartz sandstones and pebbly sandstones, probably deposited by hyperpycnal flows, which are here assigned to the PETM based on their stratigraphic position and organic carbon isotopic data. The large and simultaneous increase in coarse- and fine-grained terrestrial siliciclastics delivered to the Pyrenean Gulf during the PETM is attributed to an increased intra-annual humidity gradient. During the PETM a longer and drier summer season facilitated the erosion of landscapes, whereas a dramatic enhancement of precipitation extremes during the wet season led to intensified flood events, with rivers carrying greater volumes of both bed and suspended loads. This scenario argues against the possibility that PETM kaolinites indicate a coeval warm and humid climate in northern Spain. Instead, the kaolinite reflects the erosion of thick Cretaceous lateritic profiles developed on the Hercynian basement.

Orbital forcing of terrestrial hydrology, weathering and carbon sequestration during the Palaeocene-Eocene Thermal Maximum

“…We note that these high detrital mass accumulations rates during the body of the PETM do not coincide with a noticeable increase in grain size, with sediments dominated by clay-grade material throughout. This is consistent with the marked absence of coarser grained turbidite deposition during 35 the PETM within the Zumaia section (Clare et al, 2015), although major increases in coarse grained siliciclastic deposits during the PETM are recorded within deep-water channels within the basin (Pujalte et al, 2015). Total organic carbon MARs are relatively stable before the onset of the event, at ~0.01 g cm -2 ka -1 .…”

“…Abdul Aziz et al, 2008), resulting from the interference of both northern and southern hemisphere precession signals within tropical 30 hydroclimates (Short et al, 1991). This is consistent with a strong hydroclimate control on sediment erosion and transport in the Tremp-Graus terrestrial basins during the PETM (Pujalte et al, 2015;Schmitz and Pujalte, 2007).…”

“…Combined with sediment mixing, this tends to smooth δ 13 C carb profiles, reducing the apparent rate of onset of the event, whilst maintaining an appearance of stratigraphic continuity Kirtland Turner and Ridgwell, 2016). The greatly increased flux of fine-grained detrital sediment from nearby terrestrial 35 basins (Pujalte et al, 2015) into the Zumaia section creates an expanded rather than condensed PETM onset with far greater stratigraphic and temporal fidelity. Once plotted on a common age model the dissolution-controlled lag in time to peak CIE conditions in the deep-ocean bulk δ 13 C is clearly visible, and of the order of 70 ka even in the shallowest and most complete sections (ODP Sites 690 & 1263; …”

“…Situated within the deep-basin of the Pyrenean Gulf, it is the deep-water end of a sediment distribution system that has been mapped in detail over the past two 15 decades (see references and summary within Pujalte et al, 2015). Logged sections span terrestrial alluvial fans and plains, shallow shelf carbonates, incised channel distribution systems and a deep-water basin (Pujalte et al, 2015 and references therein).…”

“…Logged sections span terrestrial alluvial fans and plains, shallow shelf carbonates, incised channel distribution systems and a deep-water basin (Pujalte et al, 2015 and references therein). Within this, the Zumaia section is regarded as the most complete and representative end member of the deep-water depositional system, recording the distal deposition of fluvial-derived fine grained sediment plumes (Pujalte et al, 2015) at middle-lower 20 bathyal (1000 m) paleodepths (Alegret et al, 2009). The aim of this study is to build on the existing detailed depositional framework developed for the region, and to use the Zumaia section as a key recorder of the temporal patterns of climate-driven sediment flux from terrestrial systems to the deep ocean during the PETM.…”