Enhanced Terrestrial Runoff during Oceanic Anoxic Event 2 on the North 1 Carolina Coastal Plain , USA 2

8 A global increase in the strength of the hydrologic cycle drove an increase in flux of terrigenous 9 sediments into the ocean during the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2) and was an 10 important mechanism driving nutrient enrichment and thus organic carbon burial. This global change is 11 primarily known from isotopic records, but global average data don’t tell us anything about changes at 12 any particular location; such reconstructions of local terrigenous flux can help us understand the role of 13 regional shifts in precipitation in driving these global trends. The North Atlantic basin was one of the 14 epicenters of enhanced organic carbon burial during OAE2, and so constraining terrigenous flux is 15 particularly important in this region; however, few local records exist. Here, we present two new OAE2 16 records from the Atlantic Coastal Plain of North Carolina, USA, recognized with calcareous 17 nannoplankton biostratigraphy and organic carbon isotopes. We use carbon/nitrogen ratios to constrain 18 the relative contribution of marine and terrestrial organic matter; in both cores we find elevated 19 contribution from vascular plants beginning just before OAE2 and continuing through the event, 20 indicating a locally strengthened hydrologic cycle. Terrigenous flux decreased during the brief change in 21 carbon isotope values known as the Plenus carbon isotope excursion, and then increase and remain 22 elevated through the latter part of OAE2. TOC values reveal relatively low organic carbon burial in the 23 inner shelf, in contrast to black shales known from the open ocean. Organic carbon content on the shelf 24 appears to increase in the offshore direction, highlighting the need for cores from the middle and outer 25 shelf. 26 https://doi.org/10.5194/cp-2021-25 Preprint. Discussion started: 15 March 2021 c © Author(s) 2021. CC BY 4.0 License.


Introduction 27
The Cretaceous was characterized by intermittent periods of enhanced organic carbon burial 28 linked to widespread black shale deposition and anoxia, termed Oceanic Anoxic Events (OAEs; 29 Schlanger and Jenkyns, 1976;Jenkyns 2010). Although OAEs were originally defined by the widespread 30 occurrence of black shales (Schlanger and Jenkyns, 1976) they were soon found to be associated with  Carbon isotopes reveal global changes in organic carbon burial rates but don't tell us anything 41 about where that organic matter was buried. This is important because local organic matter enrichment 42 can vary significantly in both timing (e.g., Tsikos et al., 2004) and magnitude (e.g., Owens  burial, the hydrologic cycle did not increase uniformly, but instead some regions experienced a greater 47 sample was run in duplicate and a check standard was run in triplicate every twentieth sample to ensure 183 the accuracy of the results. The elemental results were calibrated against a known sulfanilamide standard 184 and the precision of the results is +/-0.1% or better, and variation of duplicate samples was within range 185 of this uncertainty. The carbon isotope results were calibrated against four known reference standards 186 which cover the range of isotopic signatures expected in organic material (-15‰ to -35‰), and duplicates 187 and check standards were run at the same interval as above. All of the isotopic results are reported in per 188 mil (‰) relative to VPDB and the precision of the results is +/-0.1‰ or better.  Overall this interval represents a fining upward sequence from sand to sandy silt to silty clay; the sandy 247 clay contains thin discrete beds of coarser material, include shell hash, possibly indicating deposition 248 above storm wave base before deepening to uniform silty clay representing deposition on the shelf below 249 storm wave base at the top of the informal marine member. The overlying contact with the Collins Creek Formation is marked by a readily observable unconformity. indicating that foraminifera were present in these sections but that they were subsequently dissolved, 299 either in situ or in the 17 years since the cores were drilled. This may be due in part to the relatively foraminifera in more distal coastal plain cores (e.g., Valentine, 1982Valentine, , 1984Zarra, 1989;Gohn, 1992) 304 bodes well for future micropaleontological studies in this region. 305

Carbon Isotopes 307
Organic carbon isotope (δ 13 C) data ( Figure 6 termination of the OAE2 carbon isotope excursion roughly corresponds with the Cenomanian-Turonian 317 boundary (e.g., Kennedy et al., 2005) and has been used to define that level in our cores.

Organic Carbon/Nitrogen Ratios 326
The ratio of total organic carbon to total nitrogen is a common proxy for the relative contributions 327 of algae and land plants to sedimentary organic matter (e.g., Meyers, 1994Meyers, , 1997Meyers, , 2003. Due to 328 differences in their composition (e.g., the abundance of cellulose in land plants) vascular plants tend to 329 have C/N ratios of 20 or greater, while algae have C/N ratios of 4-10 (Meyers, 1994). Changes in C/N 330 ratio in marine settings therefore reflect changes in the relative contribution of terrigenous organic matter 331 to offshore areas. C/N can thus be used to reconstruct changes in the hydrologic cycle, with increased C/N 332 ratios indicating a higher flux of terrestrial organic matter due to enhanced weathering (Meyers, 2003). 333 Sediments with low TOC (<0.3 wt%) can cause problems for C/N interpretations because in such settings 334 the proportion of inorganic nitrogen can be high enough to artificially depress the data, suggesting more 335 marine organic matter than is really there (Meyers, 1997); our data is consistently above 0.5 wt% TOC so 336 this is not a concern (see section 4.3.2, above).

OAE2 on the eastern North American shelf 381
The Smith Elementary School and Hope Plantation cores represent the second and third records 382 of OAE2 on the US Atlantic Coastal Plain. As such, they provide important insight into a surprisingly 383 understudied region. In the modern ocean, about 85% of organic carbon burial occurs along continental 384 margins (e.g., Burdige, 2007). A survey of all known OAE2 localities with a complete carbon isotope 385 excursion and TOC data by Owens et al. (2018) found that there is a significant amount of "missing" 386 organic carbon when reconstructed organic carbon burial is compared to "expected" carbon burial based 387 on carbon isotope data. This was based on 170 sites which, with some extrapolation, represent just 13% 388 of total Cenomanian-Turonian global ocean area, which meant that similar values had to be assumed for 389 the rest of the seafloor (Owens et al., 2018). OAE2 is perhaps the best studied event of the Cretaceous,

415
Sedimentation rate also plays an important role in organic carbon accumulation. While we don't 416 have dry bulk density measurements from these cores to calculate mass accumulation rates, we can 417 approximate using reasonable values for organic-rich silicilastic rocks (2.4 g/cm 3 , following Owens et al., 418 2018). We can determine the average sedimentation rate during the event using the observed thickness of 419 the OAE2 carbon isotope excursion in each core and the orbitally-tuned duration of OAE2 at the Global 420 constant sedimentation rate on the shelf during OAE2 is almost certainly an oversimplification but it is 422 sufficient for our purpose of comparing general trends between these cores. Using these values we find 423 organic carbon mass accumulation rates (OC MAR) during OAE2 average 0.05 g/cm 2 /kyr at Hope 424 Plantation, 0.04 g/cm 2 /kyr at Smith Elementary School, 0.06 g/cm 2 /kyr at Bass River, and 0.11 g/cm 2 /kyr 425 at Spinks. For comparison, the same method indicates OC MAR rates of 0.29 g/cm 2 /kyr at DSDP Site 603 426 and 2.84 g/cm 2 /kyr at Tarfaya (Owens et al., 2018). Owens et al. (2018) found an average OC MAR on 427