Is the trace fossil Macaronichnus an indicator of temperate to cold waters? Exploring the paradox of its occurrence in tropical coasts

Abstract

The trace fossil Macaronichnus is reported for the first time from low-latitude, tropical settings based on its occurrence in nearshore deposits in the Upper Oligocene–Lower Miocene Naricual Formation and in the Middle to Upper Miocene Urumaco Formation in northern Venezuela. Macaronichnus is an intrastratal trace fossil attributed to the deposit-feeding of worms in high-energy, sandy shallow-marine environments. The majority of its occurrences are from Mesozoic to Cenozoic high- to intermediate-latitude shorelines. The opheliid polychaetes Ophelia limacina and Euzonus mucronata make structures identical to those described from the fossil record in modern intertidal and shallow subtidal sediments of northwestern United States, western Canada, and Japan. Macaronichnus shows a geographical and environmental distribution in the fossil record similar to that of its modern producers and has been proposed as an indicator of high to intermediate latitudes. Accordingly, its presence in the Neogene of Venezuela is highly anomalous and seems to challenge its paleoclimatic value. However, this occurrence may be related to seasonal coastal upwelling of nutrient-rich cold waters. Such oceanographic conditions were prevalent in the northern coast of South America, at least from the Late Oligocene to the Early Pliocene prior to the final closure of the strait of Panama. This study underscores the value of Macaronichnus because its presence in the tropics may indicate upwelling conditions, providing high-resolution information in paleoceanographic reconstructions.