Paleotemperature reconstructions of the end-Cretaceous interval document local and global climate trends, some driven by greenhouse gas emissions from Deccan Traps volcanism and associated feedbacks. Here, we present a new clumped-isotope-based paleotemperature record derived from fossil bivalves from the Maastrichtian type region in southeastern Netherlands and northeastern Belgium. Clumped isotope data document a mean temperature of 20.4±3.8 ∘C, consistent with other Maastrichtian temperature estimates, and an average seawater δ18O value of 0.2±0.8 ‰ VSMOW for the region during the latest Cretaceous (67.1–66.0 Ma). A notable temperature increase at ∼66.4 Ma is interpreted to be a regional manifestation of the globally defined Late Maastrichtian Warming Event, linking Deccan Traps volcanic CO2 emissions to climate change in the Maastricht region. Fluctuating seawater δ18O values coinciding with temperature changes suggest alternating influences of warm, salty southern-sourced waters and cooler, fresher northern-sourced waters from the Arctic Ocean. This new paleotemperature record contributes to the understanding of regional and global climate response to large-scale volcanism and ocean circulation changes leading up to a catastrophic mass extinction.

3 AKNOWLEDGEMENTS 4 INTRODUCTION 5 1. TYPES OF ESTUARIES 1.1 River-dominated estuaries 1.2 Marine-dominated estuaries 2. ESTUARINE CARBON CYCLE 2.1 Carbon sources 2.2 Atmospheric flux 2.3 Continental shelf flux 3. PROJECT SUMMARY METHODS AND MATERIALS 11 1. FIELD METHODS 1.1 Study area 1.2 Sampling techniques 2. LAB METHODS 2.1 Analytical techniques 2.2 Computational techniques RESULTS 15 1. SALINITY 2. DISSOLVED OXYGEN 3. TOTAL ALKALINITY 4. CARBON 4.1 Particulate organic carbon 4.2 Carbon to nitrogen ratio 4.3 Carbon dioxide 4.4 Dissolved inorganic carbon 4.5 Air–sea CO2 flux DISCUSSION 21 1. ORGANIC CARBON 1.1 Sources of POC in the water column 1.2 Mixing of POC throughout the system 1.3 Spatial anomalies 1.4 Seasonal anomalies 2. INORGANIC CARBON 2.1 Sources of CO2 in the water column 2.2 Sources of TA and DIC 2.3 Mixing of DIC throughout the system 3. AIR–SEA CO2 FLUX 3.1 Site-specific fluxes and trends 3.2 Total system flux CONCLUSIONS 33 REFERENCES 34