Limited freshwater cap in the Eocene Arctic Ocean

Remains of the freshwater fern Azolla, found in Eocene (~50 Ma ago) sediments in the modern central Arctic Ocean, have been used to suggest that seasonal freshwater caps covered the entire Arctic Ocean during that time, with significant impact on global ocean circulation and climate. However, these records are located on the Lomonosov Ridge, which during the Eocene was a continental fragment barely rifted from Eurasia, separating the smaller Eurasian Basin from the much larger Amerasian Basin to the west. As such, the Lomonosov Ridge does not necessarily record environmental conditions of the broader Arctic Ocean. We tested the hypothesis of freshwater caps by examining sediment records from the western Amerasian Basin. Here we show that in the larger Amerasian Basin the Azolla event is associated with marine microfauna along with allochthonous (terrestrially sourced) organic matter. We propose that Azolla events are related to an increased hydrologic cycle washing terrestrially sourced Azolla, and other organics, into the Arctic Ocean. If freshwater caps did occur, then they were at best restricted to the small Eurasian Basin and would have had a limited impact on Eocene global climate, contrary to current models.


Results
Azolla event in the western Arctic. The late Cretaceous-Cenozoic BMB contains 14 km of sediment infill in a rapidly subsiding deltaic and marine environment. We examined the sediment record from a well named 'Dome Pacific et al. PEX Natsek E-56′ (herein Natsek E-56) (Fig. 1) located in the southwestern part of the BMB (69°45′21.46″N; 139°44′3458″W, Northwest Territories, Canada). Natsek E-56 preserves the early to mid Eocene Taglu Sequence (including the Azolla event; Fig. 2) 16,17 that consists of weakly consolidated silty to pebbly mudstone. The well represents an expanded section characterized by rapid rates of sedimentation that is at least 9 times greater than the Lomonosov Ridge, with 2 km of deposition during the Eocene 17 (Fig. 2). The depositional environment changed significantly across an intra-Taglu unconformity, from outer shelf (mesohaline marine micropalaeontological character) to slope/shelf (with marine dinoflagellate cysts and Azolla present) 18,19 (Table 1,  Figs 2 and 3).
The lower Taglu Sequence contains a well-developed circum-Arctic early Eocene benthic agglutinated foraminiferal assemblage assigned to the Portatrochammina tagluensis Zone (Fig. 2). Foraminifera disappear entirely above the unconformity in the mid Taglu Sequence, replaced by a diverse assemblage of pollen, remains of the freshwater fern Azolla (preserved as megaspores and massulae (Fig. S1)), as well as dinocysts and other microfossils. The Azolla occurrences consist of two major peaks, one in the upper slope sediments and the other in the outer shelf (Fig. 2). The Azolla acmes in Natsek E-56 are coeval with the Azolla event on the Lomonosov Ridge in the earliest mid Eocene (onset 50 Ma) 6 , with similar peaks displayed between both localities. However, our measured average abundance of Azolla plant parts (85 remains per 100 g of dry sediment; maximum 460 remains/100 g of dry sediment at 521 m) (Fig. 3) is significantly lower than the 5-30 × 10 6 remains per 100 g of    Table 1. Summary results for Natsek E-56 of Rock-Eval analyses (pyrolyzed carbon (PC); residual carbon (RC); total organic carbon (TOC); hydrogen index (HI); oxygen index (OI); mineral carbon (MinC)) and carbon isotope organic carbon (δ 13 C org ). The results are further subdivided at the intra-Taglu unconformity separating the lower from mid Taglu Sequence. The average of the results for the entire sequence are in bold. The full Rock-Eval data set for Natsek E-56 are reported elsewhere 32 . HI = Hydrogen index (S2 × 100/TOC), OI = Oxygen index (S3 × 100/TOC). www.nature.com/scientificreports www.nature.com/scientificreports/ S3 = 1.72 mgCO 2 /g, OI = 148.32 mgCO 2 /TOC) (Fig. 3, Table 1). These results are characteristic of lignin and cellulose plants, indicating that the organic matter that occurs in association with the Azolla abundance peaks is composed primarily of terrestrial organics, including plants and woody material.
Redox sensitive elements Mo and U have concentrations consistent with post-Archean average shale (PAAS) 20 , throughout the studied interval of Natsek E-56. These redox sensitive elements are normally enriched in anoxic sediments 21 , thus the average concentrations along with low TOC values is inconsistent with a strongly stratified anoxic environment of the ACEX core. However, the loss of benthic species does indicate low oxygen conditions, suggesting an overall disoxic rather than fully anoxic basin.
Co-occurring Azolla and marine microfauna. We observed a rich assemblage of dinocysts above the mid-Taglu unconformity (1250 m) that indicate Azolla-containing laminae were deposited under slightly less than normal marine shelf conditions (Figs 3, S2, Table S1). Dinocysts included the prevalence of Phthanoperidinium, Cordosphaeridium, Hystrichosphaeridium, Glaphyrocysta, and wetzelielloids, along with the sparsity of offshore genera such as Operculodinium and Nematosphaeropsis, suggesting an inner to outer neritic (shallow marine environment) character 22 . Similar marine dinocyst assemblages were also observed with the Azolla event in the Labrador-Baffin Seaway 23 . At the Lomonosov Ridge, however, only Phthanoperidinium and Senegalinium, low-salinity tolerant genera, were associated with Azolla blooms 22 . In our samples, the co-occurrence of Azolla, which cannot tolerate salinities higher than 1-1.6‰, and marine dinocysts in Natsek E-56 is consistent with ACEX core observations of a mixed fresh/saline water fauna. However, organic matter that co-occurs with both marine fauna and Azolla in Natsek E-56 has an allochthonous origin. We argue then that this is more consistent with terrestrial material (organic matter and Azolla) being washed into a marine environment, rather than the model of Azolla blooming in situ in seasonal freshwater caps 7 . This simpler interpretation is consistent with the lack of evidence for a strongly stratified anoxic water body and alleviates the physical and biological constraints of forming and maintaining seasonal freshwater caps with floating Azolla mats on the Arctic Ocean surface 6 as discussed below.

Constraints on forming a freshwater lens and floating mats.
Modern Azolla ferns only float freely on the surface of quiescent freshwater, such as ponds and canals in tropical, subtropical, and warm temperate regions 6,24 . They are not found on larger water bodies exposed to significant wave action that breaks up mats; it would thus be particularly challenging for a floating fern to cover the ~11.4 × 10 6 km 2 Arctic Ocean 25 . Given the size and annual modern freshwater flux (~8500 km 3 ) 26 the Arctic Ocean could only develop a seasonal freshwater cap of ~0.75 m. Even if increased Eocene cyclone activity 27 led to river flow that was a maximum of twice as high as modern (based on estimates of doubling rainfall 28 , but not accounting for increased evapotranspiration) the annual freshwater cap would not exceed 1.5 m, well within the fair-weather mixing zone and storm wave base down to 5 m 29 . The same increased frequency and intensity of cyclones in the Eocene Arctic and northward shift in storm tracks would have further exacerbated mixing 26,30 , making formation and maintenance of a freshwater cap in the central Arctic Ocean highly improbable. After annual die-off, due to lack of winter sunlight, Arctic Ocean mats would require entirely vegetative reproduction in tolerable bottom water salinities not found in marine environments. Thus, even with a freshwater cap, Azolla would not have germinated in the Arctic Ocean.

Discussion
We do not attempt here to re-interpret ACEX data, and allow that the interpretation of a seasonal freshwater lens from that location may still be valid. However, based on the differences between the BMB and Lomonosov Ridge records, it is possible that a subaerial ridge, or submerged sill, separated the Arctic Ocean into two distinctive bodies of water creating differing depositional environments. The high Azolla accumulation observed in the ACEX core could at best be a localized phenomenon restricted to the newly formed Eurasia Basin, that did not represent overall dominant Arctic Ocean conditions. The presence of Azolla along with marine microfauna and terrestrial organic matter in the BMB more likely reflects the change to warmer Eocene temperatures and an increased hydrological system leading to enhanced mass transport of terrestrial material, including freshwater fern debris, from freshwater bodies surrounding the Amerasian Basin. If correct, our alternative interpretation has significant implications for models of the transition from a global greenhouse climate towards the modern icehouse being triggered by Arctic Ocean wide growth of Azolla 9,31 . Factoring the size of the basin, it was suggested that atmospheric carbon dioxide as high as 2500 to 3500 ppm was reduced by half after the Azolla Event 30 , and that growth of Azolla contributed to at least 40% of carbon drawdown via net carbon fixation and subsequent sequestration 29 . Our results suggest Azolla growth was significantly more limited than these models and do not support such levels of CO 2 drawdown. Other mechanisms should be explored to explain amelioration of Eocene hothouse conditions.

Methods
Cuttings and duplicate samples (n = 195 and n = 5, respectively) were collected from the Natsek E-56 well between 229-2226 m and prepared for analysis at the Geological Survey of Canada (Calgary). The handpicked samples were washed lightly with tap water to remove drilling mud then oven dried at ~35 °C for ~24 hours before being powdered. Aliquots of the powdered samples were subjected to the below analysis. Additional details are published elsewhere 32 .
Rock-Eval 6 pyrolysis was conducted at the Geological Survey of Canada (Calgary) on a Rock-Eval 6 Turbo device following the Basic Method 33,34 . Total organic carbon (TOC) content and Rock-Eval parameters were determined on bulk ground samples to provide information on organic carbon source, hydrocarbon source-rock potential, presence of hydrocarbons, and thermal maturity. Detailed data are presented elsewhere 32 .