The botanical provenance and taphonomy of Late Cretaceous Chatham amber, Chatham Islands, New Zealand

© 2018 Fossil resin (amber) has been recently reported as common, but small, sedimentary components throughout the lower Upper Cretaceous (Cenomanian; 99–94 Ma) strata of the Tupuangi Formation, Chatham Islands, eastern Zealandia. From these deposits, resin has also been identified and obtained from well-preserved, coalified specimens of the conifer fossil Protodammara reimatamoriori Mays and Cantrill, 2018. Here, we employed attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) to both dispersed and in situ amber specimens. These resulted in very similar chemical signatures, indicating that these fossils are likely from the same or closely-related botanical sources. The FTIR data are typical of a conifer source within the ‘cupressaceous resins’ category of Tappert et al. (2011). Carbon-13 nuclear magnetic resonance spectroscopy (13C NMR) facilitated the probable identification of these ambers as ‘Class Ib’ (sensu Anderson et al. 1992). Based on these spectral data sets, the likely botanical sources of the amber were either Araucariaceae or Cupressaceae; both of these conifer families were common and widespread in the Southern Hemisphere during the Cretaceous. However, the morphology and anatomy of P. reimatamoriori support an affinity to the latter family, thus indicating that the Cretaceous amber of the Chatham Islands was generally produced by members of the Cupressaceae. Comparing the FTIR data to the published spectra of modern resins, we also identify a band ratio which may aid in distinguishing between the FTIR spectra of Araucariaceae and Cupressaceae, and outline the limitations to this approach. A high concentration of ester bonds in Chatham amber specimens, which exceeds typical Cupressaceae resins, is probably caused by taphonomic alteration via thermal maturation. The source of thermal alteration was likely pre-burial wildfires, conditions for which P. reimatamoriori was adapted to as part of its life cycle. A comparison of ambers of the Chatham Islands with modern resins and amber from various localities in Australasia reveals that, taphonomic influences aside, Chatham amber has a unique signature, suggesting that members of the basal Cupressaceae (e.g., Protodammara) were not major contributors to other documented Australasian amber deposits. The closest analogy to Chatham amber deposits appears to be the Upper Cretaceous Raritan Formation, USA, which is characterised by its rich amber, charcoal and Cupressaceae fossil assemblages. This study further supports the hypotheses that the early Late Cretaceous south polar forests were dominated by Cupressaceae, and regularly disturbed by wildfires.