Radical copolymerization of donor−acceptor (D-A) monomer pairs has served as a versatile platform for the development of alternating copolymers. However, due to the use of conventional radical polymerization, the resulting copolymers have generally been limited to nondegradable vinyl polymers. By combining radical D-A copolymerization with radical ring-opening polymerization (rROP), we have synthesized an alternating copolymer with a high incorporation of degradable backbone units. Copolymerization of N-ethyl maleimide (NEtMI) with the cyclic ketene acetal (CKA) 2-methylene-4-phenyl-1,3-dioxolane (MPDL) was demonstrated to proceed in an alternating fashion, and controlled polymerization was achieved using reversible addition−fragmentation chain transfer (RAFT) polymerization. Spontaneous copolymerization, in the absence of an exogenous initiating source, occurred when the mixture of monomers was heated, presumably due to the large electron disparity between the comonomers. Chain-extension with styrene afforded well-defined P(MPDL-alt-NEtMI)-bpolystyrene copolymers, and degradation of the homopolymers and block copolymers showed complete breakdown of the alternating copolymer.

Methane is produced and consumed via numerous microbial and chemical reactions in atmospheric, hydrothermal, and magmatic reactions. The stable isotopic composition of methane has been used extensively for decades to constrain the source of methane in the environment. A recently introduced isotopic parameter used to study the formation temperature and formational conditions of methane is the measurement of molecules of methane with multiple rare, heavy isotopes ("clumped") such as 13 CH 3 D and 12 CH 2 D 2 . In order to place methaneclumped isotope measurements into a thermodynamic reference frame that allows calculations of clumped isotope-based temperatures (geothermometry) and comparison between laboratories, all past studies have calibrated their measurements using a combination of experiment and theory based on the temperature dependence of clumped isotopologue distributions for isotopically equilibrated systems. These have previously been performed at relatively high temperatures (>150 °C). Given that many natural occurrences of methane form below these temperatures, previous calibrations require extrapolation when calculating clumped isotope-based temperatures outside of this calibration range. We provide a new experimental calibration of the relative equilibrium abundances of 13 CH 3 D and 12 CH 2 D 2 from 1 to 500 °C using a combination of γ-Al 2 O 3 -and Ni-based catalysts and compare them to new theoretical computations using Path Integral Monte Carlo (PIMC) methods and find 1:1 agreement (within ±1 standard error) for the observed temperature dependence of clumping between experiment and theory over this range. This demonstrates that measurements, experiments, and theory agree from 1 to 500 °C, providing confidence in the overall approaches. Polynomial fits to PIMC computations, which are considered the most rigorous theoretical approach available, are given as follows (valid T ≥ 270 K): Δ 13 CH 3 D ≅ 1000 × ln(K 13 CH 3 D ) = (1.47348 × 10 19 )/T 7 − (2.08648 × 10 17 )/T 6 + (1.19810 × 10 15 )/T 5 − (3.54757 × 10 12 )/T 4 + (5.54476 × 10 9 )/T 3 − (3.49294 × 10 6 )/T 2 + (8.89370 × 10 2 )/T and Δ 12 CH 2 D 2 ≅ 1000 × ln(8/3K 12 CH 2 D 2 ) = −(9.67634 × 10 15 )/T 6 + (1.71917 × 10 14 )/T 5 − (1.24819 × 10 12 )/T 4 + (4.30283 × 10 9 )/T 3 − (4.48660 × 10 6 )/T 2 + (1.86258 × 10 3 )/T. We additionally compare PIMC computations to those performed utilizing traditional approaches that are the basis of most previous calibrations (Bigeleisen, Mayer, and Urey model, BMU) and discuss the potential sources of error in the BMU model relative to PIMC computations.

Carbonate mud represents one of the most important geochemical archives for reconstructing ancient climatic, environmental, and evolutionary change from the rock record. Mud also represents a major sink in the global carbon cycle. Yet, there remains no consensus about how and where carbonate mud is formed. Here, we present stable isotope and trace-element data from carbonate constituents in the Bahamas, including ooids, corals, foraminifera, and algae. We use geochemical fingerprinting to demonstrate that carbonate mud cannot be sourced from the abrasion and mixture of any combination of these macroscopic grains. Instead, an inverse Bayesian mixing model requires the presence of an additional aragonite source. We posit that this source represents a direct seawater precipitate. We use geological and geochemical data to show that “whitings” are unlikely to be the dominant source of this precipitate and, instead, present a model for mud precipitation on the bank margins that can explain the geographical distribution, clumped-isotope thermometry, and stable isotope signature of carbonate mud. Next, we address the enigma of why mud and ooids are so abundant in the Bahamas, yet so rare in the rest of the world: Mediterranean outflow feeds the Bahamas with the most alkaline waters in the modern ocean (>99.7th-percentile). Such high alkalinity appears to be a prerequisite for the nonskeletal carbonate factory because, when Mediterranean outflow was reduced in the Miocene, Bahamian carbonate export ceased for 3-million-years. Finally, we show how shutting off and turning on the shallow carbonate factory can send ripples through the global climate system.