Mysterious abrupt carbon-14 increase in coral contributed by a comet

A large and sudden increase in radiocarbon (14C) around AD 773 are documented in coral skeletons from the South China Sea. The 14C increased by ~ 15‰ during winter, and remain elevated for more than 4 months, then increased and dropped down within two months, forming a spike of 45‰ high in late spring, followed by two smaller spikes. The 14C anomalies coincide with an historic comet collision with the Earth's atmosphere on 17 January AD 773. Comas are known to have percent-levels of nitrogen by weight, and are exposed to cosmic radiation in space. Hence they may be expected to contain highly elevated 14C/12C ratios, as compared to the Earth's atmosphere. The significant input of 14C by comets may have contributed to the fluctuation of 14C in the atmosphere throughout the Earth's history, which should be considered carefully to better constrain the cosmic ray fluctuation.

C arbon-14 ( 14 C) is a cosmogenic isotope of C formed on Earth primarily through radiation of atmospheric nitrogen by the reaction: 14 N(n,p) 14 C (refs. [1][2][3][4]. Its abundance in the atmosphere varies with time 5 , which is generally attributed to variations in the earth's magnetic field, solar activity and changes in the carbon cycle 6 . A large and sudden increase in 14 C of ,12% was reported from a tree ring study in Japan to have occurred between AD 774 and AD 775 (hereafter M12) 7 . Their modeling showed that the atmospheric level of 14 C must have jumped over the course of no longer than a year, corresponding to an increase 10 times larger than the average production from Galactic cosmic rays and 20 times larger than that expected over 2 3 11 yr solar cycles. The measured values were shown to be too large for a solar flare or local supernova. Given that no detectable increase in 14 C corresponding to supernovas SN 1006 and SN 1054 were observed 7,8 , it is argued that much higher energies would be required for the M12 event, if it is related to a supernova 7 . Alternative explanations for this mysterious 14 C elevation include a highly energetic radiation burst, e.g., proton storms from giant solar flares 9,10 , a giant cometary impact upon the Sun 11 , or floods of c-rays from supernova explosions 12 . Such high levels of radiation however, might also cause mass extinctions 13 , which are absent following the M12 event. Moreover, it has been argued, based on historical records, that no superflares have occurred in the Sun during the last two millennia 14 .
A simulated carbon cycle model 10 suggested that the strength of the M12 event was significantly overestimated by the previous study 7 . One key issue is the duration of the 14 C input. Based on modeling, it has been proposed that a tree ring record of the event could be explained by a spike in 14 C production that lasted less than 1 year 7 . However, owing in part to the annual resolution of the 14 C data, they could not assess the duration in more detail 7 .
Porites coral with an annual growth rate $ 10 mm/yr has now provided a high temporal-resolution 14  of 45% high. This is followed by two smaller spikes of . 20% over the next 6 months until fall, and then maintained ,15% higher than normal values over the following several months (Fig. 1b). We obtained a 230 Th date of AD 783 6 14 (table S1) at a depth of 2.15 cm, which is 7 annual growth bands above the layer containing the onset of 14 C anomalies at a depth of 16.11 cm and corresponding to an age of AD 776 6 14. When the previously published tree ring spectrum 7 was examined, the 14 C content had actually started to climb in AD 773 (Fig. 1d). There are no other 14 C increases until 200 yrs later 16 . Considering dating errors, the major 14 C increases we observed are also likely to have occurred in AD 773 (Fig. 1a).

Discussion
The coral 14 C spectrum shown in Fig. 1 is difficult to be explained using normal production pathways from Galactic cosmic rays. The abrupt 14 C increase by ,45% within two weeks (Fig. 1b) requires a radiation intensity 100 times stronger than the previous estimation for M12. Since the residence time of carbon dioxide in the atmosphere is 5-15 years 17,18 , 14 C spikes in coral suggest highly uneven distribution. It is well established that a comet collided with the Earth's atmosphere from constellation Orion (or Shen in traditional Chinese astronomy) on 17 January AD 773, the 7th year of Emperor Dai Zong of the Tang Dynasty. The phenomenon (hereafter Dai7) lasted less than one day and had an accompanying coma that stretched across the whole sky 19,20 . ''Dust rain'' in the daytime before the ''comet'' implies that a considerable amount of cometary material was added to the atmosphere assuming these two events are associated. Celestial observations were especially significant to the emperors of ancient China, especially in the Tang Dynasty, and these were carefully recorded. This event was recorded in several different official archives in China 19,20 , included by royal celestial officers in Chang'an (now Xi'an), the capital city of the Tang dynasty (34u169N, 108u549E).
It is quite possible that Dai7 resulted in the M12 global abrupt 14 C increases recorded in tree rings and corals. Comas are known to have percent levels of nitrogen by weight (in the forms of NH 3 , NH 2 , NH, etc) 21,22 , and are heavily exposed, as compared to nitrogen within the earth's atmosphere because of lacking a magnetic field protection 23 . Considering that meteorite usually has 14 C and 10 Be about two orders of magnitude higher than those of rocks from the Earth's surface [24][25][26] , it is reasonable to propose that coma and comet may be expected to have 14 C/ 12 C ratios several orders of magnitude higher than that of the Earth's atmosphere 23 . Generally, 14 C occurs in very low concentrations in the Earth's atmosphere, i.e., no more than one part per trillion of the total carbon content of the atmosphere 27 . The total amount of preindustrial 14 C in the atmosphere was ,150 metric tonnes. Assuming an average 14 C/ 12 C ratio of 1 3 10 26 in the Dai7 comet, ,150 million metric tonnes of C from the Dai7 event would double the 14 C content of the Earth's atmosphere. Assuming a C abundance of 10% in the comet, a total of ,30-150 million metric tonnes of materials would then be required to explain the 14 C anomalies. This is only about 1-3% of the estimated total mass-loss of Haley's Comet in 1910 (ref. 28). With the considerable uncertainties surrounding the dispersal of cometary material throughout the atmosphere and shallow oceans, such a process seems commensurate with the observed 14 C increases (Fig. 1).
The coma 14 C would have been dispersed into the Earth atmosphere heterogeneously (Fig. 2). Because the coma is far better exposed to cosmic radiations than the nucleus, it should have a much higher 14 C/ 12 C ratio. A considerable proportion of the coma with its higher 14 C/ 12 C content is probably scattered and absorbed into the outer atmosphere. The bulk of the cometary material with 14 C/ 12 C values that are much lower than that of the coma, but still considerably higher than the Earth's atmosphere, may be expected to descend into the troposphere and become incorporated into corals and trees. Four months later, the high 14 C/ 12 C material captured in the outer atmosphere (stratosphere) mixes downward into the troposphere, a process facilitated by summer storms, and is absorbed by corals, resulting in their high and fluctuating 14 C spikes in coral (Fig. 1b). After another six months, the enriched 14 C material becomes well mixed and imparts elevated 14 C levels to the whole atmosphere (Fig. 1b).
Consistent with the 14 C increase, there was a 30% increase in the decadal 10 Be flux record in Dome Fuji from AD 755 to 785 (refs. 7,16,29), which has been attributed to a burst of high energy c-rays 12 . We were not able to obtain 10 Be data in this study. Nevertheless, 10 Be is another cosmogenic isotope formed through spallation of nitrogen 12 , 14 N(n,p 1 a) 10 Be, or oxygen, which often co-varies with 14 C. The increase in 10 Be, can also be interpreted by the Dai7 event. The comet with abundant oxygen and nitrogen, could likewise produce high amounts of 10 Be under exposure to cosmic radiation.
As an alternative, short radiation bursts, e.g., the merger of two magnetized neutron stars, can produce a spinning black hole and launch a relativistic energy jet as observed in short c-ray bursts 30 that might also explain the brief input of 14 C and 10 Be (ref. 12). This could conceivably produce an interaction between the short c-ray burst and the magnetic field of the Earth which might appear to be a comet. However, the c-ray burst is fast and interacts with the entire magnetic field of the earth in seconds; therefore it is not easily explained as having ''entered from the constellation of Shen (Orion)'' 19,20 . It is also difficult to explain the 'dust rain'' beforehand, unless the dust rain was only a coincidence.
It has long been recognized that 14 C and 10 Be in the Earth's atmosphere varied dramatically throughout the history of the Earth 5,16,31 , which has previously been solely attributed to cosmic radiations [1][2][3][4]13 . The coincidence of Dai7 and the 14 C, 10 Be spikes in tree rings and coral suggests that comets might also contributed significant amount of 14 C to the Earth's atmosphere episodically.  Methods Coral core. A 1.2 -m long core of fossil Porites coral XDH was drilled from Xiaodonghai Reef in the northern South China Sea in 1997. Slabs of 7 mm in thickness, were sectioned, washed with ultrapure water, and dried for X-ray images. X-ray diffraction analysis shows our coral samples are 100% aragonite and scanning electron microscopy image indicates the absence of secondary aragonite around the coral part having the 14 C spike. The subsamples were crushed and homogenized one by one in an agate mortar.
Measurements. Sample XDH-2 at depth of 2.15 cm was dated by 230 Th techniques 32 in the High-Precision Mass Spectrometry and Environment Change Laboratory (HISPEC), at National Taiwan University, on a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) (Table S1).
Carbon-14 sample preparation was carried out in the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry. About 8-9 mg coral sample power was weighed and put in a special reaction quartz tube reacted with purified H 3 PO 4 for more than 24 hours at room temperature after being kept continuously in a 1.0 3 10 23 torr vacuum system for at least 4 hours. CO 2 from the reaction tube is purified and then transferred to a tube and graphitized 33 . The graphite samples were analyzed in the AMS laboratory at Peking University 34 , the standards used during the analysis are NIST OXI and OXII, the analytic precision for our samples are better than 3% and 5% for half-annual and biweekly samples, respectively. d 18 O measurements from the same biweekly subsamples were carried out using MAT-252 mass spectrometry equipped with Kiel II micro carbonate automatic sample input device at the Institute of Earth Environment, Chinese Academy of Sciences. The results are expressed in the delta (d) notation relative to the Vienna Pee-Dee Belemnite (V-PDB) standard. The analytical error of the laboratory standard is approximately 6 0.2% for d 18 O (ref. 35).